The time the x86 emulator team found code so bad they fixed it during emulation

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ppaulmooreparks 2 days ago 171 commentsRead Article on devblogs.microsoft.com

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psanchez•2 days ago

This reminds me of a story from 15 years ago, where I was developing a technology to download games on demand by hooking into the OS calls.

There was a particular game that was superslow when this tech was applied. Original game loading took around 15-20 seconds, whereas once the tech was applied it took easily 3-5 min, even with all data already downloaded.

When I started digging into it, I realized the reason was the game was using something like

   fread(data, 1, 65536, fptr);

instead of

   fread(data, 65536, 1, fptr);

Which basically expanded back in the day to 65k reads of 1 byte for several MB file. Each fread translated to 65k reads of ReadFile Windows API. Since my code was hooking on ReadFile system call, and my call was heavier than ReadFile, the game loading felt really slow. Unusable. It would have not been fun for players.

The easy fix was to swap arguments for certain calls. The long fix required to use an internal cache to account for these cases so that the hooked ReadFile was faster when data was already in disk.

Funny thing is that as we started rolling out the tech and applying it to more and more games we realized lots of games did this. We went for the cache fix and games ended up loading faster than before. Honestly, games could have load all the data in a couple of seconds by just swapping the args. I'm guessing developers did this on purpose so that games seemed like they were loading a lot of stuff, although you never know.

Taniwha•2 days ago

I used to be a graphics card/chip architect for macs in the early/mid 90s - our chips were the fastest, but some programs were resistant because they did stupid stuff: pagemaker invalidated the font cache every time it went thru its main loop, quark with ATM did an n*2 thing every time it wrote text etc etc. We had special hardware to accelerate text drawing and it did nothing because the software pissed it away. We considered creating a plugin that fixed all these things, it would have been hard to maintain, in the end we travelled around to the people who made these apps and talked them through their problems

To be fair excel would erase places white that it wanted to write up to 9 times before it drew any black pixels, we made that very fast! we didn't tell them :-)

At the time 24-bit framebuffers were so slow that before we built graphics acceleration hardware people would switch back to 8-bit to get stuff done, making 24-bit/true colour your daily driver was a big step forward.

nxobject•2 days ago

Does that make you the first in a long tradition of GPU developers going to blockbuster app devs to say "hey, you should be doing this instead?"

PS – I am looking through the NuBus cards that I have... did you work for SuperMac or RasterOps?

Taniwha•2 days ago

I was probably not the first to have to do that, we knew what apps our customers used, making them better was the whole point of the operation

I did the architectural design for the SuperMac cards. I figured out what needed to be accelerated, dropping code into people's machines to see where the cycles were going. Others did the physical design for the first 2 cards, I did the design of the chip in the Thunder and later cards (designed the data paths and state machines and a full simulation, someone else actually laid the gates)

If your card has a SQD01 on it it's my work. It peaks at 1.5Gb/s on solid fills

urbandw311er•2 days ago

This is a horrible and yet not unexpected insight into the internals of Excel

Taniwha•2 days ago

To be fair this was Excell 25 years ago, may no longer be true.

One of the other bugs (the Quark/ATM one) was also because of the programmers were worried about writing over stuff that hadn't been completely erased, the Quark guys wrote a string with 2 spaces at the end through a box that masked the end of the string, the ATM font renderer saw it couldn't fit the text so it split it in half and tried again so it drew N/2 N/4 N/8 ... strings. It spent all it's time in the 68k's multiply instructions figuring out how wide the strings (and substrings) were, our fancy 24-bit character rendering hardware was an afterthought

Xirdus•1 day ago

There's a good chance it was Excel's workaround for some other GPU's buggy behavior.

bathtub365•2 days ago

In all of the software you’ve written, are you aware of how many on-screen pixels you’ve overdrawn?

trelbutate•2 days ago

> To be fair excel would erase places white that it wanted to write up to 9 times before it drew any black pixels

I feel like I'm having a stroke trying to read this, what does it mean??

PaulHoule•2 days ago

I remember when 24 bit color was exotic and aspirational and you had to settle for 16.

spauldo•1 day ago

I swear if Sun Microsystems was still around, their machines would still ship with 8-bit pseudocolor and you'd have to pay an extra $3k for 24-bit.

projektfu•2 days ago

I got the extra vram in my LC to allow for 24-bit color but it was dog slow. The 16 bit data path didn't help. If I wanted it, I'd get things done in 8 bit or mono until it was ready, then switch to 24 bit for the final look.

saltcured•1 day ago

Yeah, even in Linux we were doing these things with X Windows bit depths.

8 bit psuedo color, so the color palette switched with every focus-follows-mouse window boundary crossing. 16 bit direct color with banding but no more palette psychedlia.

This was equal parts to make it faster and to allow for higher framebuffer resolutions with limited VRAM.

dhosek•2 days ago

16 bits? Luxury. I had 6 colors when I was a kid and was happy to have them.

jjuran•1 day ago

In my 68K Mac emulator running on modern (or even decade-old) hardware, performance in the traditional sense is less of a concern, but other issues arise. The big ones include CPU-burning loops that wait for a length of time or for an interrupt-decremented counter to reach zero, as well as invalid memory accesses (which I've made crash — no NULL deref for you).

> We considered creating a plugin that fixed all these things, it would have been hard to maintain, in the end we travelled around to the people who made these apps and talked them through their problems

Since talking to developers is no longer an option, I actually do write "Such-and-such Tune-up" extensions that patch applications dynamically to make them run better (or at all) in Advanced Mac Substitute, or even Mac OS itself.

Taniwha•1 day ago

Yeah those low core global system variables (including a readable/writeable 0) at fixed addresses were very much a thing, they were a bad design decision made for the original Macs with almost no memory, and made running more than one app (switcher/multifinder) a difficult transition back in the day. Someone wasn't planning ahead

I also worked on the original A/UX port for the Mac II, some hardware (like the IWM) required tiny buzzy loops, we ran into one bug where using the floppy caused ADB to freeze, but only on the release machines, not the prototypes all our engineers had, turned out there was hardware that made access to the VIA faster by pulling the clock in for 1 cycle, if you sat in a loop reading the timer in the via to measure a sector time for the IWM in too tight a loop it upped the output clock from the VIA to the ADB chip and over clocked it ....

xattt•2 days ago

What would have been the purpose of stupid code like that?

Was it a workaround for things that didn’t fully complete on one iteration, so the devs kept hammering away at it until it worked?

phire•2 days ago

They were most likely just bugs. Quite possibly really stupid bugs.

Not every bug results in the program doing the wrong thing, they often just make the program do the right thing very slowly.

And nobody notices, since it still produces the right result.

kazinator•1 day ago

It's not necessarily stupid code in the game, but something the C library is doing that it probably shouldn't.

If the stream is buffered, then all operations, including fread, are supposed to go through the buffer.

All three of these should issue buffer-sized reads to the operating system:

1. A loop which calls getc(stream) 65536 times.

2. fread(buf, 1, 65536, stream)

3. fread(buf, 65536, 1, stream)

The more direct behavior of fread should only kick in if the stream is configured as unbuffered.

I would say that the way low-level reads are issued to the host operating system is a "visible effect" of the program, so I suspect this may actually be a matter of conformance. I.e. it's not okay to issue those reads however the stream library wants as long as the data is read.

Xirdus•1 day ago

Reminds me of the "community patch" to GTA Online from a few years ago. The game was plagued by 10+ minute loading times. The situation remained for years and only got worse with time. Some hacker figured out that the game spent 80% of loading time reading the in-game store listing file. The file was tens of megabytes IIRC, and it literally used the Schlemiel the Painter's Algorithm - for each entry, start reading from the beginning byte after byte. The hacker made a tiny patch that made it remember where it found the last entry. This cut the total loading time by 80%, from over 10 minutes to less than 3.

Edit: removed incorrect information.

exrook•1 day ago

This is not quite an accurate telling of rockstar's reaction, there were actually receptive to it and paid out $10k for the discovery. Though it's an understandable mistake given rockstar's hostile history with the gta modding scene.

See the original post and discussion for the whole story:

https://nee.lv/2021/02/28/How-I-cut-GTA-Online-loading-times... https://news.ycombinator.com/item?id=26296339

Xirdus•1 day ago

That's not how I remember these events when they were playing out. I distincly remember social media posts warning about the dangers of modifying game files, plus refusal to acknowledge the issue. Note there were 2 full weeks between the blog post and the update mentioning the bounty. I'm pretty sure the massive community outrage in between has played a role in it. But I don't have any sources and I was wrong about at least one thing (lack of attribution), so I'm okay assuming I'm wrong about everything else too.

Someone•2 days ago

> Which basically expanded back in the day to 65k reads of 1 byte for several MB file. Each fread translated to 65k reads of ReadFile Windows API

What software did that that badly? If the code asks for (up to) 65,536 single byte items, why would you split that into 65,536 calls?

Also, that change changes behavior. The old call could read anything from zero to 65,536 bytes, the new one only can read zero or 65,536 bytes.

(Reading the source of a few implementations, I think most implementations will fill the output buffer with partial objects if the input doesn’t supply an integral number of them, but the return value of fread cannot signal that to the caller)

tom_•1 day ago

The standard says that fread calls fgetc multiple times for each object:

> For each object, size calls are made to the fgetc function and the results stored, in the order read, in an array of unsigned char exactly overlaying the object

(wording unchanged since C99)

If the file is unbuffered, depending on how the implementation handles buffering, and how it interprets the standard, then perhaps it does end up hitting a path where there's 1 ReadFile call per byte...

I don't know how most implementations get around this. Presumably it's valid to interpret "calls are made" as "behaving as if calls are made", meaning fread can copy data out of the FILE's buffer directly, or make calls directly to whatever routine fgetc defers to, rather than calling fgetc N times literally. Looks like glibc's fread does this.

Someone•about 22 hours ago

> The standard says that fread calls fgetc multiple times for each object:

>> For each object, size calls are made to the fgetc function and the results stored, in the order read, in an array of unsigned char exactly overlaying the object

Aha! That phrase led me to https://man7.org/linux/man-pages/man3/fread.3p.html. I consulted https://man7.org/linux/man-pages/man3/fread.3.html and https://man.openbsd.org/fread.3. Neither mentions that.

Now, I checked https://cplusplus.com/reference/cstdio/fread/. It doesn’t mention it, either.

⇒ this appears to be POSIX-specific.

Finally, if somebody implements fread as “For each object, size calls are made to the fgetc function”, it doesn’t matter whether you ask for 1 object of size 65,536 or 65,536 objects of size 1; both would call fgetc 65,536 times.

klodolph•1 day ago

I think it’s pretty rare for files to be unbuffered like that. AFAIK it’s mostly stderr that ends up unbuffered, at least on Unix-like systems.

micampe•2 days ago

A long time ago I worked with someone who read 1 byte at a time from a socket because they insisted data was cached so the kernel was going to batch it magically somehow. It took me days to convince them to measure it.

vidarh•2 days ago

I used to make it a general rule to start all my optimisation of any network code by running strace and look for excessive read's and write's, because you'd be shocked how many did stuff like that if they didn't know the length of a string, or to read the length first, instead of reading into a buffer.

I had to convince people with benchmarks regularly that, yes, you could write the handful of lines to do proper user-space buffering and trivially run rings around any code that did extra context switches, because a lot of people didn't realise the cost difference between system calls and calling their own functions.

This included, by the way, the MySQL client library, at one point, which would do small read for length fields instead of larger non-blocking reads into a buffer all the time

quietbritishjim•2 days ago

That's different: you're talking about the application code, like OP.

But I think the parent comment's point is that the issue is in the implementation of fread itself in the standard library. It's perfectly reasonable for an application to pass it 1, 65536 (i.e. one byte, up to 65536 times) and expect it not to issue 65536 separate OS calls.

macintux•2 days ago

I assumed it was a simple mistake: easy to forget what order the two integers are sent.

mort96•2 days ago

Wait, is that wrong? I always call fread as:

    fread(data, 1, sizeof(buffer), f);

with the rationale that I'm interested in reading sizeof(buffer) individual bytes. The buffer size is incidental, not the size of the items I'm trying to read from the file; "read one item whose size is sizeof(buffer)" seems semantically wrong.

Is this just the case of Windows having a bad stdlib fread implementation 15 years ago or is my thinking here actually wrong?

chadgpt3•2 days ago

It's not wrong. Guy just wrote a bad implementation of fread and blamed everyone else.

DarkUranium•2 days ago

He didn't write it.

The C runtime authors did (presumably Microsoft, if it's MSVCRT).

He's hooking into ReadFile, a layer below the stdlib. By the time it reaches the hook, it's already split.

projektfu•2 days ago

fread should be buffered, but different values may cause buffering at different rates. Perhaps it didn't generate 65535 calls to ReadFile but it generated 16 or 64.

fsfod•2 days ago

Part of Windows Explorer actually does tons of tiny 4 byte ReadFile calls in to its tracking database like file when you delete a file. If you deleting lots of files this quickly adds up.

pbhjpbhj•2 days ago

Is this why Windows takes so long to delete things?? Presumably those reads aren't done when using del from a console as that always seems a bit faster.

jonathanlydall•1 day ago

Its slowness is also a function of security software or any other file system "filters" (I believe they're called) are installed.

For example, I run TortoiseGit which has a caching feature which is supposed to make it faster at showing what to commit. Disabling it increases the number of items I can delete per second in my Windows Explorer from about 1000 to about 3000 while making not making TortoiseGit operations meaningfully slower (that I can tell).

This is a Dev Drive [0] on my machine, it would probably be slower on my C: drive which has full Windows Defender real time file scanning.

[0]: https://learn.microsoft.com/windows/dev-drive/

Asmod4n•1 day ago

Windows Explorers zip implementation also seams to do 1 byte reads by the speed is has compared to every other zip implementation.

somenameforme•2 days ago

Doesn't that break anything relying on the return value? fread gives you the number of objects read as a return. So I think a pretty typical thing would be to fread and then parse that number of characters, and that'd just break?

jcul•2 days ago

I've seen a lot of code that just assumes fread / fwrite succeeded without bothering to check the return value...

But in this case if the code was calling fread 65536 times in a loop and getting 64KiB each time it wouldn't be good either!

Sounds like the parent comment had to fix this with the internal cache thing to speed up the small freads. I think they meant the easy fix would have been swapping the args in the original / caller code.

account42•2 days ago

There are no small freads in the story, whatever implements those freads supposedly split them up into many calls. But that sound more like a problem of that implementation than the fread callers as size == 1 is correct when you are reading a bag of bytes.

koolala•2 days ago

I think they turned it from a tiny file read to a tiny ram read.

DonHopkins•2 days ago

The type of programmer who swaps the args to fread tends to be the type of programmer who doesn't bother to check the return value, fortunately.

Edit: mort96: So did you check the return value or not?

mort96•2 days ago

If I have a buffer of bytes, and I intend to treat the content of that buffer as individual bytes, what is semantically wrong with "read 65k 1-byte-sized items into this buffer"? Wouldn't it be a bit unnatural to express it as "read one item whose size is 65k"?

account42•2 days ago

But the args aren't necessarily swapped just because they end up in a slow case in some implementation.

lukan•2 days ago

"I'm guessing developers did this on purpose so that games seemed like they were loading a lot of stuff"

I really hope that was not the case and rather think incompetence or to deal with obscure legacy problems, but the gamer in me gets enraged at the thought someone would artificially increase loading times.

Dwedit•1 day ago

Is this actually real? I thought fread just multiplied the two numbers together to compute a total size. Meanwhile, the Win32 API call ReadFile actually does do a separate system call if you call it multiple times.

gwbas1c•1 day ago

> The long fix required to use an internal cache to account for these cases

That's because the OS does the same thing too. It's the right fix, when I implemented something similar, we implemented caching right away.

chadgpt3•2 days ago

Why does your fread to anything other than multiplying the two arguments?

Sesse__•2 days ago

The idea of having two arguments to fread() is presumably to be able to do something else than all-or-nothing when there's a short read.

chadgpt3•2 days ago

Yes, it divides the bytes read by the element size to get the return value.

Which is the obvious reason you'd pass an element size of 1: you want to know how many bytes were read.

dfox•2 days ago

The most important fix in SP1 for Office 2007 was fixing exactly that in Excel. Doing ridiculous amount of 4 byte reads made it basically unusable on network filesystems.

dlcarrier•2 days ago

SimCity had a read-after-free bug that Microsoft patched in Windows 95. That was a lot easier for customers than having Maxis fix it, which could have required exchanging copies of the game.

oceansky•2 days ago

There's also the opposite effect, a windows security update broke GTA San Andreas because it relied on undefined behavior.

https://silentsblog.com/2025/04/23/gta-san-andreas-win11-24h...

icase•2 days ago

in this dark age of agents writing code that gets debugged by other agents, i love reading stuff like this: stories of human intuition fixing human mistakes. thanks for a fascinating read.

Cthulhu_•2 days ago

It feels like graphics drivers do / did this a lot too. At the very least they make specific optimizations for specific games, probably by tweaking settings and features that the game developers didn't optimize properly themselves.

kalleboo•2 days ago

Famously if you renamed Quake 3 to "Quack" 3, it would slow down on the ATI Radeon 8500 https://web.archive.org/web/20091016055550/https://hardocp.c...

account42•2 days ago

That's a case of the driver cheating but there are also lots of cases where the game is just full of bugs that the driver has to work around in order to not be blamed for them.

SyzygyRhythm•2 days ago

There are many, many, cases like this, including correctness fixes. One recent example I remember had a shader that computed: x = a / b * b

The optimizer was allowed, but not obligated, to transform that into: x = a

However, in this case, b was sometimes 0. And if so, the unoptimized version computed: x = a / 0 * 0 = Inf * 0 = NaN

So badness ensued if the that particular path didn't get optimized, which could happen under various circumstances. We had to add some code to ensure that transformation always happened on that game.

DarkUranium•2 days ago

I'm curious, what's the ratio of:

- deciding to inform the game developer & wait for reply vs not waiting for reply vs just fixing it yourself without informing the developer; and

- if informed: developer actually fixing it vs only saying they would fix it vs no reply whatsoever (not counting automated "thank you for your inquiry" replies, in cases where you don't already have more direct channels to the dev than email)

I've always kind of wondered this because in a way, it's kind of weird that it's fixed for them, at least for new releases / games actively being developed.

(Full disclosure: I'm a game developer myself, with a very high interest in engine plumbing & dev [including graphics], though finding a job for the latter is easier said than done.)

rbits•1 day ago

Yep. I know the Minecraft optimisation mod Sodium has encountered some issues because Nvidia drivers try to optimise the game in ways that can cause issues for them

easyThrowaway•2 days ago

The most interesting part is that IIRC they shipped the entire Windows 3.11 memory allocator to make it work.

I have very little understanding on how allocation works at OS level, but I'm surprised there are no wrappers like dgVoodoo or dxWrapper specifically for this kind of issues. There are quite a bunch of old Windows games (Need for Speed 1-4 for a start) that refuse to run on modern OSes due to rather...bold memory management strategies.

rincebrain•2 days ago

Apparently the recollection of the fix was that they deferred actually freeing memory for a while if they detected it was SimCity running. [1]

[1] - https://www.joelonsoftware.com/2000/05/24/strategy-letter-ii...

DonHopkins•2 days ago

A story I heard at Sun, which may be apocryphal but was fucking hilarious enough to be a repeatable rumor, was that a release of an early operating system in BETA was determined to be solid and tested and ready to release and ship to customers, so they simply changed the version string from something like "SunOS2.1BETA" to "SunOS2.1FCS" (First Customer Ship), and recompiled. But the change from a 12 character version to an 11 character version threw off the alignment of some important data structures somewhere in the kernel, and the entire OS ran MUCH SLOWER because of 68k unaligned memory accesses!

hodgehog11•2 days ago

I think we're starting to see more of this sort of thing happening now with Proton and Wine gaining prominence in the Linux community. Some games (Elden Ring comes to mind) have bad enough PC ports when they come out that the compatibility layer can incorporate a hotfix to improve performance, while users of the software on the original platform still had to suffer.

Gigachad•2 days ago

Fairly sure GPU drivers do the same thing where they include a ton of per game tweaks to make them run faster. It does feel like a fragile way of doing things where an external component that should be agnostic to the software running ends up including a handful of junk trying to fix stuff that should have been fixed by the consumer of the driver.

zoenolan•2 days ago

The big one I remember was many applications, not just games assuming the buffer swap was performed by a blit into the display buffer, not an framebuffer pointer update. They relied on the previous frames data still being in the back buffer. For those applications you were forced to blit the buffer, not swap the pointer and take a performance hit.

I also remember a media player being called out by name in the code for doing invalid operations, needing a work around and code to detect it was running just to function.

Guvante•2 days ago

It goes the other way too, sometimes you trigger some optimization silliness in the driver and the game needs to adapt to avoid it.

rickdeckard•2 days ago

then the driver gets updated and the game either continues to optimize (wrong) or branches out into code that was written before that driver came out and generally wasn't that well tested, and the circle continues...

It's the life of a (game) developer...

anilakar•2 days ago

GPU driver packages are already a huge collection of workarounds for bad game engine coding.

An Nvidia employee once told me that one of the easiest ways to squeeze out a few extra frames on your old machine is to rename the game executable to hl2.exe.

st_goliath•2 days ago

> GPU driver packages are already a huge collection of workarounds for bad game engine coding.

And of course, browser engines also do the same things for certain websites:

https://github.com/WebKit/WebKit/blob/main/Source/WebCore/pa...

necovek•2 days ago

I can see how it can modify GPU driver behavior, but I cannot see how it would get you better performance with everything else the same?

What it should do is ensure some things not relevant to Half-Life 2 were not done, thus getting better performance for this game in particular, but there is no guarantee that same optimizations work for other applications or games, so one should not expect an overall improvement.

Unless they are doing some silly things like dropping quality, but that's the "everything else the same" point.

If not, why not have this enabled as default behavior instead?

sfink•2 days ago

In general, because it's a flag that says to do things in an incorrect but faster way. It's like -ffast-math. The applications for which it's intended don't do anything where the incorrectness matters. Some random application falsely labeled hl2.exe may or may not.

> What it should do is ensure some things not relevant to Half-Life 2 were not done, thus getting better performance for this game in particular, but there is no guarantee that same optimizations work for other applications or games, so one should not expect an overall improvement.

I can't quite parse this. Yes, there is no guarantee that the optimizations will work for another game, which is precisely why you can expect an improvement with hl2. With non-hl2, you may get an improvement, you may not, and you may get incorrect behavior.

Everything else is not the same, but hl2 doesn't use the stuff that's different.

dlcarrier•2 days ago

I wouldn't be surprised if it made other games on the Source engine faster, but everything else slower.

limflick•2 days ago

> to rename the game executable to hl2.exe

This seems genuinely unbelievable. Does anyone have a technical explanation for this?

hurtigioll•2 days ago

gpu drivers detect games, among other thing by looking at executable names

then driver "optimizes" behavior, sometimes dishonestly (reducing precision), sometimes honestly (working around game engine stupidity)

proton_9•2 days ago

This sounds like a really interesting story, would like to read more on why half life 2 specifically? the game itself was pretty well optimized and ran on really low end hardware even back in the day.

db48x•2 days ago

Because everyone reported performance metrics using it as a benchmark. Higher number = more sales.

AHTERIX5000•2 days ago

Yep, someone needs to do the same workarounds Windows drivers do but on Linux and the translation layer is a good spot for them, look at https://github.com/HansKristian-Work/vkd3d-proton/blob/938d7... for example

harrall•2 days ago

A big portion of GPU driver updates are actually just that, same with Windows updates.

Windows 95 patched a bug in SimCity just to get it to work.

kazinator•2 days ago

> Anyway, my colleague found that there was one program that needed to allocate around 64KB of memory on the stack and initialize it. The standard way of doing this is to perform a stack probe to ensure that 64KB of memory is available, then subtracting 65536 from the stack pointer, and then initializing the memory in a small, tight loop.

Actually, the standard way of allocating 64 kB of memory on the stack is to just assume you can do it, subtract 64k from the stack pointer, and hope for the best.

Most stack allocations in the wild are not checked.

i_don_t_know•2 days ago

IIRC you have to probe every page of the stack on Windows. You cannot just subtract a value from ESP/RSP. If you don't probe every page in order, you get a page fault or some other exception (I don't remember which one).

NobodyNada•1 day ago

The reason for this is to ensure stack overflows are detected. The OS places a guard page above the top of the stack, which will cause a segfault if accessed. That way stack overflows are guaranteed to crash rather than stomping on valid memory that belongs to something else. However, if a stack frame is larger than a page (say, because it includes a large buffer), then it is possible for the program to "jump over" the guard page and access memory beyond.

In order to protect against this, the compiler inserts some dummy reads or writes as needed to ensure every page is touched in order from bottom to top. This ensures the guard page is hit before the application has a chance to write to memory beyond it.

Here's an example: https://godbolt.org/z/oTbzTczM6

justsid•1 day ago

How else would the OS know your read/write 16 pages away from the current stack pointer is in fact an attempt to increase the stack and not just really bad pointer arithmetic and a bug? How many pages should the runtime let you skip before its just a segfault?

selcuka•2 days ago

To be fair it is possible that the developer enabled a special "unroll all loops, no matter what" optimisation flag during compilation.

I agree it would be stupid for a compiler to even support such a flag, but those were the 1980s/90s.

cyberax•2 days ago

Ahh... Good old funrollloops...

https://www.shlomifish.org/humour/by-others/funroll-loops/Ge...

PhilipRoman•2 days ago

Right up there with fun, safe math optimizations

account42•2 days ago

At least these actually make things faster usually.

lozf•1 day ago

Heh, "funrollloops" reminds me of recompiling FreeBSD 4 on my thinkpad back in the early aughts. The word made me imagine some sort of processed breakfast cereal with too many additives.

ack_complete•1 day ago

Doesn't require any special flags, just hitting optimizer limits can do it with MSVC.

https://www.reddit.com/r/cpp/comments/1i36ahd/is_this_an_msv...

ashdnazg•2 days ago

I worked on a transpiler from Nand2tetris assembly to WebAssembly, and had some really annoying memory corruption bug that I just couldn't solve.

That is, until I checked the program I used for testing (which I didn't write), and found the following code:

  dealloc(this)
  return this->field

With the original allocator, this worked fine, since the deallocation didn't touch the memory.

My allocator, however, overwrote the field during the deallocation with bookkeeping stuff, which meant the returned value was not what the programmer intended and after a short while the program crashed.

Unlike TFA, I had the luxury of just fixing the test program.

wazoox•2 days ago

IIRC, one of the similar old story from Raymond Chen is about SimCity 2000, that did a similar trick (free memory, then start immediately using it) that worked just fine under DOS, but was a big no-no starting with Windows 95. The game was so common that Windows had to include a special rule to make it run...

andikleen2•1 day ago

Dave Jones used to have a series of "Why user space sucks" Linux kernel conference talks with many such examples, usually with dumb and redundant system calls.

However as someone who looks a lot at instruction traces I could probably write on e on why Linux kernel code sucks too. One of my current pet peeves is the way Linux walks bitmasks of CPU bits, which is a reasonably common operation. Due to a chain of unfortunate changes and decisions it currently needs 16+ instructions to find the next bit for something which the x86 instruction set has a single instruction. Of course that is so big that it is even outlined, adding even more overhead.

jeffbee•2 days ago

People from Transmeta told me stories about how their translators were full of special case optimizations to fix horrors they discovered in Microsoft Windows itself.

wolfi1•2 days ago

speaking of which, what became of it?

hbbio•2 days ago

Acquired by a patent monetization business...

zimmund•1 day ago

I can't stop thinking about all the unoptimized code we have around. As processors (and memory) over the last 2-3 decades improved faster than we needed to fix the inefficiencies we created, we silently accepted that we don't need efficiency everywhere. So maybe a compiler, an emulator or some critical piece of code were created with this in mind, but the average app or website just waste resources left and right and pray for the best.

With more and more code being written with AI (which has notoriously inefficient solutions to simple problems), I expect this issue to become more prevalent. I just hope we optimize at the source of the problem (AI and humans using it) and not on platforms (compiler and engine/kernel heuristics)

smallstepforman•1 day ago

Half the compute and reduce memory by factor of x4 and in a decade we’ll have double the performance we have now.

I do old school embedded, the amount of desktop bloat is insane. Any function I really need to refactor, I can reduce size and improve performance. And there are better engineers out there that are more efficient than me.

classichasclass•2 days ago

Betting Alpha was the native architecture in question. It seemed to have the best support.

projektfu•1 day ago

Yeah, but I thought DEC wrote the FX!32 translator for the Alpha. Perhaps Raymond was talking about those people and didn't want to mention that they weren't Microsoft people.

cranx•2 days ago

Loop unrolling is a basic compiler optimization and depending on the machine language and processor instruction set should be faster taking into account all the house keeping required to execute a conditional, jump, move register values etc. This article is missing the analysis of why. If someone didn’t “like” it and was offended then that seems like an equally silly reason. On the surface 256k to init less does seem silly, but what if it was faster?

ryukoposting•1 day ago

A few things to consider.

In this case we're talking about a tight initialization loop with probably a single instruction in the body. The HW optimizations necessary to make a loop like this perform equally to the unrolled form are so rudimentary that they're taken for granted on basically any CPU, even 30 years ago. Seriously, we're talking about optimizations I made in an "intro to Verilog" class as an undergrad, and I'm not even a HW engineer.

It also depends how often this code is being hit. Does the code run once while the program loads? Nobody will notice a 2 microsecond improvement in loading times. Does the code run in a timing-sensitive hot path, like a game loop or a GUI rendering thread? Well now optimization matters. But again, consider the HW argument above.

Also remember that, back then, storage wasn't cheap. 256K of code is 18% of a 1.44MB floppy, and 35% of a 720K floppy.

ant6n•2 days ago

Arguably more of an optimization, rather than a fix. Looks like un-unrolling a loop, or better, rolling a loop. Or rolling straight line code?

senfiaj•2 days ago

Yeah, but after a certain point the win is negligible. Huge code can also increase cache misses which will slow down things.

zftnb666•about 22 hours ago

"It works" doesn't mean it works. It means it hasn't failed spectacularly yet.

0xdecrypt•2 days ago

256 KB of code to zero 64 KB of memory is the kind of optimization that makes you question every life choice that led to it.

rasz•1 day ago

I blame Intel. It took them 33 years (ERMSB) to finally standardize REP MOVSB as _the_ fast path. Another 10 years passed and someone discovered https://lock.cmpxchg8b.com/reptar.html

electroglyph•2 days ago

heh, when Raymond Chen dunks on the MSVC team =)

mkl•2 days ago

There's no indication it was MSVC, and there are lots of compilers (and used to be more).

m1r•2 days ago

Couldn't they just turn the optimization off for this loop?

MadnessASAP•2 days ago

They didn't have the code for the offensive program, they were creating the emulator to run it on a different architecture.

McGlockenshire•2 days ago

> offensive program

Agreed.

notorandit•2 days ago

Which optimizer replaces a 64k loop with 64k instructions?

Ah, yes. Microsoft's!

selcuka•2 days ago

There is no indication that the compiler that produced the code was Microsoft's. Actually the article hints otherwise ("[...] whatever compiler was used to compile this code").

notorandit•2 days ago

Who has been validating that approach to solve their own optimization target?

notorandit•2 days ago

> they fixed it during emulation

It means the fix was applied to run during the emulation loop execution, not that the fix was found and applied while the emulation loop was running.

Which would have made it an emulation code escape.

pantulis•2 days ago

I was just curious and checked The Old New Thing archive... yes I've been reading Raymond Chen's stories for as long as I remember but hey, it's been 23 years of delivering consistently solid stories about Windows.

yieldcrv•2 days ago

> All in all, it took this program 256 kilobytes of code to initialize 64 kilobytes of data.

solidity sweating profusely

canucker2016•1 day ago

I was looking through the compiler docs about memory allocation and I found the section about the debug version of the CRT which could fill the allocated memory with a non-zero canary value to help detect uninitialized memory (assuming you weren't calling calloc - which zero-init's allocated memory).

But there wasn't any similar programmatic debugging aid for detecting uninitialized stack memory.

Going further down the rabbit hole, I discovered the _chkstk function.

The MS C compiler would emit a call to _chkstk on function entry to ensure that stack memory had been paged in. But further reading noted that _chkstk was only emitted if the function allocated a lot of stack memory. And there was source code! MS included the assembly language source code for _chkstk in the CRT source code, installed with compiler.

I needed _chkstk to be emitted for every function not only for functions that allocated >= 4KB of stack variables.

Curses, foiled again.

Then, while perusing the list of compiler command line switches, I see "/Ge".

  /Ge (Enable Stack Probes)

  Activates stack probes for every function call that requires storage for local variables.

Ahhhhh! The grey, storm clouds parted and the sun rays bathed shone down on me in their warmth.

I had all the pieces I needed to fill uninitialized stack memory with a non-zero canary value so I could make detection of uninitialized stack variables more reliable.

_stkfil was born

Modifying _chkstk was easy. I needed to write to every byte of stack in a stack page instead of reading only 4 bytes and skipping to the next page of stack.

While I was mucking in the bowels of modifying _chkstk, I added a 4-byte global variable to hold my canary value. Let the app override what value to use.

In debug builds, _stkfil helped find a couple of bugs, but soon all the stray uninited stack vars were gone and the code was forgotten.

Then I read about InitAll in https://www.microsoft.com/en-us/msrc/blog/2020/05/solving-un...

  InitAll - Automatic Initialization

  In addition to the previously mentioned approaches, Microsoft is now using a feature known as InitAll which performs automatic compile-time initialization of stack variables.

  This section documents how Windows is using this technology and the rationale for why.

  Current Windows Settings

  The following types are automatically initialized:

  - Scalars (arrays, pointers, floats)
  - Arrays of pointers
  - Structures (plain-old-data structures)

  The following are not automatically initialized:

  - Volatile variables
  - Arrays of anything other than pointers (i.e. array of int, array of structures, etc.)
  - Classes that are not plain-old-data

  For optimized retail builds, the fill pattern is zero. For floats the fill pattern is 0.0.

  For CHK builds or developer builds (i.e. unoptimized retail builds), the fill pattern is 0xE2. For floats the fill pattern is 1.0.

canucker2016•1 day ago

And Android 11+ has been doing similar userspace stack-init thing - https://android-developers.googleblog.com/2020/06/system-har...