Shared library compiled with -ffast-math modifies FPU state

[simonbyrne]

On linux x86_64

touch foo.c
clang -Ofast -fpic -shared foo.c -o foo.so
objdump --disassemble foo.so

gives:

Disassembly of section .text:

00000000000004f0 <set_fast_math>:
 4f0:	0f ae 5c 24 fc       	stmxcsr -0x4(%rsp)
 4f5:	81 4c 24 fc 40 80 00 	orl    $0x8040,-0x4(%rsp)
 4fc:	00
 4fd:	0f ae 54 24 fc       	ldmxcsr -0x4(%rsp)
 502:	c3                   	retq
 503:	66 2e 0f 1f 84 00 00 	nopw   %cs:0x0(%rax,%rax,1)
 50a:	00 00 00
 50d:	0f 1f 00             	nopl   (%rax)

This means that any thread which later loads the library will then set the flush subnormals to zero (FTZ) and subnormals are zero (DAZ) flags (even if the executable itself was not compiled with -ffast-math).

Related discussion

• "Someone’s Been Messing With My Subnormals!" by Brendan Dolan-Gavitt (@moyix)
• "Beware of fast-math" by me (@simonbyrne)
• GCC issue #55522

[jyknight]

I wish nobody would ever use -Ofast or -ffast-math (as you say, "friends don't let friends use fast-math"). That said, this issue does seem like even more of a misfeature even than the other issues caused by fast-math.

However, currently, Clang will can link against a crtfastmath.o if one is present, but it doesn't actually ship one itself. This behavior will only occur if you have a system GCC installation.

So, since this behavior is effectively just for GCC compatibility, I think Clang probably ought to follow GCC's lead here -- that is, if any changes are made for GCC's 55522, then implement a parallel change to Clang, otherwise leave it as is.

[moyix]

The gcc bug is almost 10 years old at this point with no sign of movement, and this particular behavior is continuing to cause problems in projects, many of which don't realize that they're enabling something with global effects. As a harm reduction measure, it would be really great to at least avoid linking in crtfastmath when building a shared library (even if present)?

[simonbyrne]

However, currently, Clang will can link against a crtfastmath.o if one is present, but it doesn't actually ship one itself. This behavior will only occur if you have a system GCC installation.

Somehow this seems even worse.

[jyknight]

I agree this is an unfortunate behavior.

Yet, I think it would also be poor to diverge Clang's behavior from GCC's here. It wouldn't be particularly helpful from a practical standpoint, since I expect these python packages are generally built with GCC when targeting linux anyhow.

Note that the implementation in clang, to match GCC's behavior, was explicitly requested by a user in #14396 a decade back.

[simonbyrne]

If you don't want to remove it, then would it be possible to:

• have an option to specifically disable crtfastmath
• make it a warning to use crtfastmath with -shared
• document it

[kiufta]

However, currently, Clang will can link against a crtfastmath.o if one is present, but it doesn't actually ship one itself. This behavior will only occur if you have a system GCC installation.

I think there are enough safeguards. We can't protect every schlub from incorrect use.

Only schlubs vote this post down ;)

[llvmbot]

@llvm/issue-subscribers-clang-driver

[jcranmer-intel]

I agree this is an unfortunate behavior.

Yet, I think it would also be poor to diverge Clang's behavior from GCC's here. It wouldn't be particularly helpful from a practical standpoint, since I expect these python packages are generally built with GCC when targeting linux anyhow.

Note that the implementation in clang, to match GCC's behavior, was explicitly requested by a user in #14396 a decade back.

This wouldn't be the first case where appealing to GCC's behavior leads to outcome that virtually everyone agrees is problematic. (oh hi -fp-contract and #pragma STDC FP_CONTRACT).

We definitely need to at least document this behavior so that people are aware what the consequences of using -ffast-math are. I strongly suspect that very few users are aware that modules compiled with -ffast-math can have effects on modules not compiled with -ffast-math.

Actually, the behavior with crtfastmath.o is even worse than it seems! We have -ffp-denormal[32]= behavior that would seem to suggest how we are handling subnormals, but setting -ffp-denormal=ieee doesn't disable linking with crtfastmath.o, since that logic only looks for -f[no-]fast-math and -f[no-]unsafe-math-optimizations (unless you compile with -Ofast, at which point crtfastmath.o is unconditionally linked because GCC!).

[jyknight]

GCC has now changed the behavior in https://gcc.gnu.org/PR55522 to no longer link crtfastmath.o to -ffast-math -shared builds by default, and has also added a new -mdaz-ftz -mno-daz-ftz option to explicitly override and enable or disable linking of crtfastmath.o explicitly.