[cfe-dev] CLang and ISO C math functions

Hal Finkel via cfe-dev cfe-dev at lists.llvm.org
Wed Aug 31 17:47:57 PDT 2016

----- Original Message -----

> From: "Martin J. O'Riordan via cfe-dev" <cfe-dev at lists.llvm.org>
> To: "Clang Dev" <cfe-dev at lists.llvm.org>
> Sent: Wednesday, August 31, 2016 5:10:28 AM
> Subject: [cfe-dev] CLang and ISO C math functions

> When I updated our out of tree compiler to v3.9 RC3 from v3.8, I
> noticed a number of large performance regressions, some tests using
> 5 times as many instructions. However, when I examined the test, I
> realised that something quite different was going on concerning the
> ISO C math functions, and it is not a true performance regression at
> all.

> I will use the following example for this message, and this is
> compiled with ‘-S -O3’. The option ‘ -ffast-math ’ is not used, and
> I have verified that ‘ -fmath-errno ’ is present in the ‘ -cc1 ’
> options:

> extern double exp(double);
> extern double foo(double);

> int useMathName() {
> if ((exp(1.0) < 2.71) || (exp(1.0) > 2.72))
> return -1;
> return 0;
> }

> int useOtherName() {
> if ((foo(1.0) < 2.71) || (foo(1.0) > 2.72))
> return -1;
> return 0;
> }

> With v3.8, the implementation of the function ‘ useMathName ’ was
> reduced to simply ‘ return 0 ’. The compiler elided the calls to ‘
> exp ’, assumed the value that would have been returned if it was
> called, decided that the two tests would be ‘ false ’ and reduced
> the code-generation to ‘ return 0 ’. This does not happen for the
> other function ‘ useOtherName ’, and the code generated is as
> expected.

> After updating to v3.9 RC3, the compiler is no longer eliding the
> calls to ‘ exp ’ - probably because a bug was fixed since ‘ errno ’
> could be changed - but it is still presuming the returned value and
> eliding the tests, so the function is now 2 consecutive calls to ‘
> exp ’ and a ‘ return 0 ’.

> I have verified that this is the case for the unaltered X86 v3.8
> distribution version too.

> I would expect this behaviour if ‘ -ffast-math -fno-math-errno ’ was
> selected, but it isn’t, and I think that this is an invalid
> optimisation. It also means that some of my math functional tests
> are not reporting honestly (this only happens when the argument(s)
> are constants). Also, on our architecture, ‘ double ’ is FP32,
Does Clang for your target emit C-language "double" types as "double" at the IR level? If so, that's wrong. "double" at the IR level is assumed to be an IEEE double-precision number. All of the constant folding will do the wrong thing on your target if this is what is happening. 

> and it is probable that the compiler is using the host platform’s
> implementation which is FP64 for evaluating the test expressions,
Yes, that's right. See ConstantFoldScalarCall in LLVM's lib/Analysis/ConstantFolding.cpp. We're obviously aware this can cause issues when cross compiling. If you'd like to discuss this behavior, you should do so on llvm-dev. We might want to make this more configurable than it currently is. 


> and this will introduce precision differences that the test will not
> detect - in my real tests, the test expression ranges are more
> fine-grained to allow for legitimate FP32 ranges.

> Thanks,

> MartinO

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Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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