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Someone’s Been Messing with My Subnormals

moyix.blogspot.com

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Re: Someone’s Been Messing with My Subnormals

#41

Post by jerf »

yjftsjthsd-h wrote: post #27
jeffbee wrote: post #16

Earlier quoted context omitted.

ChromeOS is sort of the successor to Gentoo. The images are built with profile-guided, link-time, and post-link optimization, and they are targeted to the specific CPU in a given Chromebook. Every other Linux leaves a large amount of performance on the table by targeting a common denominator CPU that's 20 years old and not having PGO.

It's not a successor, it's a derivative. And yes, if you're only targeting specific known hardware than you can and probably should optimize for it, but most distributions fully intend to be usable on very nearly any x86(_64) hardware so they can't do that.

It's also a bit less relevant when everything is so fast. I used Gentoo on a cheap-for-the-time Pentium 133MHz. Gentoo was basically the difference between a modestly pleasant system and an unusably slow system if I tried to run a standard still-compiled-for-386 distro on it.

I've long since stopped worrying about it because on the systems I run, which are not top-of-the-line but aren't RPis either, it's not worth worrying about anymore for most programs. At most maybe you should target the one particular program you use that could use a boost.

Re: Someone’s Been Messing with My Subnormals

#42

Post by Night_Thastus »

moyix wrote: post #24
garaetjjte wrote: post #20

> it turns out that when you use -Ofast, -fno-fast-math does not, in fact, disable fast math. lol. lmao. What about -fno-unsafe-math-optimizations?

Nope, it still links in crtfastmath: $ gcc -Ofast -fno-unsafe-math-optimizations -fpic -shared foo.c -o foo.so $ objdump -j .text --disassemble=set_fast_math foo.so foo.so: file format elf64-x86-64 Disassembly of section .text: 0000000000001040 : 1040: f3 0f 1e fa endbr64 1044: 0f ae 5c 24 fc stmxcsr -0x4(%rsp) 1049: 81 4c 24 fc 40 80 00 orl $0x8040,-0x4(%rsp) 1050: 00 1051: 0f ae 54 24 fc ldmxcsr -0x4(%rsp) 1056: c3…

Ouch. Two flags that should reasonably stop this, and neither do. This feels a bit like the time I was told "No, -wAll does not in fact add all warnings".

Re: Someone’s Been Messing with My Subnormals

#43

Post by fweimer »

nsajko wrote: post #21

-Ofast isn't a good name for the option, but in GCC's defense the manual is pretty clear about all this, and there's no excuse for blindly turning on compiler options - they literally change the semantics of your code.

What's missing is that it also affects linking, and results in this strange action-at-a-distance. Maybe disabling the linker part with -shared would be a reasonable compromise.

Re: Someone’s Been Messing with My Subnormals

#44

Post by becurious »

cesarb wrote: post #32

At a previous company I worked at, we had an issue with our software (Windows-based, written in a proprietary language) randomly crashing. After some debugging, we found that this happened whenever the user made some specific actions, but only if, in that session, the user had previously printed something or opened a file picker. The culprit was either a printer driver or a shell extension which, when loaded, changed…

Had this exact same problem. It was a specific color inkjet driver doing this, my guess is to enable dithering or something similar. It’s one of those things that infects everything in the code base because the way you print with GDI is to progressively draw parts of the page - so you have to call in and out of code that talks to the printer DC. We also had to render one item using Direct3D retained mode and that added to the fp control word complexity. Things seemed to be more robust on NT based OSes.

Re: Someone’s Been Messing with My Subnormals

#45

Post by ack_complete »

cesarb wrote: post #32

At a previous company I worked at, we had an issue with our software (Windows-based, written in a proprietary language) randomly crashing. After some debugging, we found that this happened whenever the user made some specific actions, but only if, in that session, the user had previously printed something or opened a file picker. The culprit was either a printer driver or a shell extension which, when loaded, changed…

Had to deal with this same issue when I had a program supporting plugins, DLLs compiled with Delphi would turn on all the floating point traps. Took a while to track down what was causing FP faults in comctl32.dll. It got so bad that I had to put in a popup dialog that would name and shame the offending DLL so the authors would fix their broken plugins. It's an ABI violation in Windows since the ABI specifically defines FPU exceptions as masked, so this was more egregious than just turning on FTZ/DAZ (which Intel-compiled DLLs did).

Many of these same DLLs would also hijack SetUnhandledExceptionFilter() for their custom exception support, which would also result in hard fastfail crashes when they failed to unhook properly. Ended up having to hotpatch SetUnhandledExceptionFilter() Detours-style to prevent my crash reporting filter from being overridden. Years later, Microsoft revealed that Office had done the same thing for the same reasons.

The new version of this problem is DLLs that use AVX instructions and then don't execute a VZEROALL/VZEROUPPER instruction before returning. This is more sinister as it doesn't cause a failure, it just causes SSE2 code to run up to four times slower in the thread.

Re: Someone’s Been Messing with My Subnormals

#46

Post by Const-me »

That thread-local MXCSR register is particularly entertaining in a thread pool environment, such as OpenMP. OSes carefully preserve that piece of thread state across context switches.

I tend to avoid touching that value, even when it means extra instructions like roundpd for specific rounding mode, or shuffles to avoid division by 0 in the unused lanes.

Re: Someone’s Been Messing with My Subnormals

#48

Post by nsajko »

mbauman wrote: post #26
nsajko wrote: post #21

-Ofast isn't a good name for the option, but in GCC's defense the manual is pretty clear about all this, and there's no excuse for blindly turning on compiler options - they literally change the semantics of your code.

I wholeheartedly disagree. -Ofast Disregard strict standards compliance. ... There's strict standards compliance and then there's the crazy grab bag of code changes that is `-ffast-math`. Further, I'd say gevent can defensibly say that -ffast-math is okay for them given what the manual says: -ffast-math ... it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/spec…

Well, how would you improve the docs? Both documentation entries seem reasonable to me.

That said, I don't see why the -Ofast option even needs to exist, except backwards compatibility, as -ffast-math and the others can (and should IMO) be specified explicitly.

Re: Someone’s Been Messing with My Subnormals

#49

Post by nsajko »

fweimer wrote: post #43
nsajko wrote: post #21

-Ofast isn't a good name for the option, but in GCC's defense the manual is pretty clear about all this, and there's no excuse for blindly turning on compiler options - they literally change the semantics of your code.

What's missing is that it also affects linking, and results in this strange action-at-a-distance. Maybe disabling the linker part with -shared would be a reasonable compromise.

You're wrong, both the doc entry for -Ofast and the one for -ffast-math say that they can result in incorrect programs. Programs are produced by linking, so I don't see what other way to interpret this is possible.

Re: Someone’s Been Messing with My Subnormals

#50

Post by leni536 »

TazeTSchnitzel wrote: post #33

Global state is the root of so many evils! FPU rounding mode, FPU flush-to-zero mode, C locale, errno, and probably some other things should all be eliminated. The functionality should still exist but not as global flags.

At least many of those are thread-local. But not C locale, it is truly horrible.
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