[PATCH] D88712: [CGBuiltin] Respect asm labels and redefine_extname for builtins with specialized emitting

[Venkataramanan Kumar via Phabricator via cfe-commits]

venkataramanan.kumar.llvm added a comment.

Hi,

I am on Ubuntu 18 machine and it has finite math header <bits/math-finite.h>.
This header is included by the glibc 2.27.  This header has this following definition.
extern double log (double) __asm__ ("" "__log_finite") __attribute__ ((__nothrow__ ));

Consider the following test case

#include <math.h>
double mylog (double d) {

  return log(d);

}

Before this patch clang generates  -O2 -ffast-math
--Snip--
; Function Attrs: nounwind readnone uwtable
define dso_local double @mlog(double %d) local_unnamed_addr #0 {
entry:

  %0 = tail call fast double @llvm.log.f64(double %d)
  ret double %0

}
---Snip--

After this patch  on machines with lesser glibc versions,  for -O2 -ffast-math, clang generates

--Snip--
; Function Attrs: nounwind readnone uwtable
define dso_local double @mlog(double %d) local_unnamed_addr #0 {
entry:

  %call = tail call fast double @__log_finite(double %d) #2
  ret double %call

}
--Snip--

Note on latest Ubuntu 20.04.1 LTS with Glibc 2.31 <bits/math-finite.h> header is not present
It is removed by glibc.  so there I see the @llvm.log.f64 intrinsic calls.

So on machines with lesser glibc versions where the header file is present this will be a problem.
Note the LLVM "opt" does optimizations like vectorization only on the intrinsic, but ignores the calls to  @__log_finite.

Example https://llvm.godbolt.org/z/765We8

I am not sure if this known issue.

It is causing some performance issues with some benchmarks  on my ubuntu 18 machine.

Shall I file a PR for this ?


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D88712/new/

https://reviews.llvm.org/D88712