[llvm-bugs] [Bug 48317] New: __finite math calls generated prevents optimizations.

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https://bugs.llvm.org/show_bug.cgi?id=48317

            Bug ID: 48317
           Summary: __finite math calls generated prevents optimizations.
           Product: tools
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: opt
          Assignee: unassignedbugs at nondot.org
          Reporter: venkataramanan.kumar.llvm at gmail.com
                CC: llvm-bugs at lists.llvm.org

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);
}


Recently clang started supporting asm labels. 
Ref: https://reviews.llvm.org/D88712 


Before this patch clang generates the below LLVM IR for -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 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 as it generates finite calls now. 

Note the LLVM "opt" does optimizations like vectorization only on the
intrinsic, but fails to do so in the presence of calls to @__log_finite.

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

Older glibc versions are still supported OS distros. We are seeing performance
issues due to this finite calls.

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