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

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ppaulmooreparks about 4 hours ago 61 commentsRead Article on devblogs.microsoft.com

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ashdnazg•4 minutes 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.

psanchez•about 2 hours 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•about 2 hours 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.

urbandw311er•about 1 hour ago

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

bathtub365•6 minutes ago

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

Taniwha•22 minutes 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

Someone•35 minutes 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)

somenameforme•about 1 hour 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•30 minutes 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•1 minute 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•16 minutes ago

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

DonHopkins•24 minutes 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.

lukan•37 minutes 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.

dlcarrier•about 3 hours 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.

Cthulhu_•about 1 hour 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•40 minutes 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...

easyThrowaway•about 1 hour 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•42 minutes 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•21 minutes 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•about 3 hours 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•about 3 hours 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•about 1 hour 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•about 2 hours 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•about 1 hour 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•about 2 hours 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•about 2 hours 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•about 2 hours 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?

dlcarrier•42 minutes ago

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

limflick•about 2 hours ago

> to rename the game executable to hl2.exe

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

hurtigioll•about 2 hours 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•about 2 hours 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•about 2 hours ago

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

AHTERIX5000•about 1 hour 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•about 2 hours 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.

selcuka•about 3 hours 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•about 3 hours ago

Ahh... Good old funrollloops...

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

PhilipRoman•about 1 hour ago

Right up there with fun, safe math optimizations

kazinator•about 2 hours 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•8 minutes 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).

classichasclass•about 3 hours ago

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

jeffbee•about 3 hours 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•about 1 hour ago

speaking of which, what became of it?

hbbio•13 minutes ago

Acquired by a patent monetization business...

notorandit•about 3 hours 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.

electroglyph•about 3 hours ago

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

m1r•about 3 hours ago

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

MadnessASAP•about 3 hours ago

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

McGlockenshire•about 3 hours ago

> offensive program

Agreed.

notorandit•about 3 hours ago

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

Ah, yes. Microsoft's!

selcuka•about 3 hours 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").

ant6n•about 2 hours 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?

yieldcrv•about 3 hours ago

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

solidity sweating profusely