[llvm-dev] -ffast-math optimizations impacted by symbol renaming in header files

[Hal Finkel via llvm-dev]

On 03/20/2017 11:27 AM, Chris Chrulski via llvm-dev wrote:
> Hi,
>
> I came across an issue where some optimizations that would normally be applied to standard math function calls are not getting applied when the –ffast-math option is enabled on the Clang command line on a Linux x86_64 target.
>
> I tracked down the issue to occurring because the –ffast-math option is triggering Clang to preprocess the math.h header file with the __FINITE_MATH_ONLY__ macro set to 1. In this case, the Linux header files are redirecting several math functions using the __asm__ extension to rename the symbol. An example of what the header files contains is:
>        extern double exp (double) __asm__ ("" "__exp_finite") __attribute__ ((__nothrow__ ));
>
> In effect, ‘exp’ gets converted to be ‘__exp_finite’ in the IR. Because some of the optimizations like constant folding or vectorization are looking for the original function name, those optimizations do not get triggered under –ffast-math due to these alternative function names.
>
> Other affected functions include: acos, asin, exp2, log, log2, log10, and a few others.
>
> Anybody know if this is intentional, or have input regarding whether these optimizations should be extended to also look for the renamed versions of these functions?

It is not intentional; the optimizations should be extended. I imagine 
that we would extend TargetLibraryInfo to have names for these 
functions, and then we could recognize them when available, and generate 
them when available and the inputs have the 'fast' tag.

  -Hal

>
> Thanks,
> Chris
>
>
>
> A couple of example cases:
>
> // File: test_folding.c
>
> // Constant fold will occur with:
> // clang -O2 -S -emit-llvm test_folding.c
>
> // Constant fold will not occur with:
> // clang -O2 -S -emit-llvm -ffast-math test_folding.c
>
> #include <math.h>
>
> double test_fold_exp() {
>    return exp(0.0);
> }
>
> double test_fold_acos() {
>    return acos(1.0);
> }
>
> ----------------------------------------------
>
> // File: test_vectorize.c
>
> // Vectorization will occur with:
> // clang -O2 -S -emit-llvm -fno-math-errno test_vectorize.c
>
> // Vectorization will not occur with:
> // clang -O2 -S -emit-llvm -fno-math-errno -ffast-math test_vectorize.c
>
> #include <math.h>
>
> void test_vectorize_exp(long n,  float* restrict y,  float* restrict x) {
>    long i;
>    for (i = 0; i < n; i++) {
>      x[i] = expf(y[i]);
>    }
> }
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory