clang can't optimize calls to nexttoward()

[nico]

% cat test.cc
#include <math.h>
int main() {
    float f = nexttoward(1.0, 0.0);
    return f;
}
% out/gn/bin/clang -c test.cc -O2 -S -o - | grep nexttow
	bl	_nexttoward

Expected: No call to nexttoward
Actual: Call

gcc can optimize it: https://godbolt.org/z/1jPbcfbxx

[d-musique]

It should be noted too that the similar nextafter functions are also affected by the problem.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Nico Weber (nico)

``` % cat test.cc #include <math.h> int main() { float f = nexttoward(1.0, 0.0); return f; } % out/gn/bin/clang -c test.cc -O2 -S -o - | grep nexttow bl _nexttoward ```

Expected: No call to nexttoward
Actual: Call

gcc can optimize it: https://godbolt.org/z/1jPbcfbxx

[dtcxzyw]

Two points:

1. Recognize nexttoward/nextafter in TargetLibraryInfo.
2. Add constant fold support for LibFunc_nexttoward/LibFunc_nextafter in ConstantFoldLibCall2.

[sivakusayan]

I'd like to take this if possible.