[llvm-bugs] [Bug 34208] New: Regression: const function hoisting not happening

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34208

            Bug ID: 34208
           Summary: Regression: const function hoisting not happening
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: nat-llvmbugs at mulle-kybernetik.com
                CC: llvm-bugs at lists.llvm.org

Starting with 3.9.0 a const function is not hoisted out of the loop anymore.
Compiling this little C snippet

```
extern __attribute__(( const))  int  foo( int i);

extern int  bar( int i);


int  foobar( void)
{
   int   i;
   int   x;

   x = 0;
   for( i = 0; i < 100; i++)
   {
      x += foo( 0x2373);
      x  = bar( x);
   }
   return( x == 1848);
}
```

with `-O3 -S -emit-llvm` produces

```
; ModuleID = '/tmp/x.c'
source_filename = "/tmp/x.c"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-freebsd10.3"

; Function Attrs: nounwind uwtable
define i32 @foobar() local_unnamed_addr #0 {
  br label %1

; <label>:1:                                      ; preds = %1, %0
  %2 = phi i32 [ 0, %0 ], [ %6, %1 ]
  %3 = phi i32 [ 0, %0 ], [ %7, %1 ]
  %4 = tail call i32 @foo(i32 9075) #3
  %5 = add nsw i32 %4, %2
  %6 = tail call i32 @bar(i32 %5) #4
  %7 = add nuw nsw i32 %3, 1
  %8 = icmp eq i32 %7, 100
  br i1 %8, label %9, label %1

; <label>:9:                                      ; preds = %1
  %10 = icmp eq i32 %6, 1848
  %11 = zext i1 %10 to i32
  ret i32 %11
}

; Function Attrs: nounwind readnone
declare i32 @foo(i32) local_unnamed_addr #1

declare i32 @bar(i32) local_unnamed_addr #2

attributes #0 = { nounwind uwtable "disable-tail-calls"="false"
"less-precise-fpmad"="false" "no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false"
"no-jump-tables"="false" "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8"
"target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87"
"unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { nounwind readnone "disable-tail-calls"="false"
"less-precise-fpmad"="false" "no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false"
"no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false"
"stack-protector-buffer-size"="8" "target-cpu"="x86-64"
"target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #2 = { "disable-tail-calls"="false" "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="false" "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8"
"target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87"
"unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #3 = { nounwind readnone }
attributes #4 = { nounwind }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.9.0 (tags/RELEASE_390/final)"}
```


But with clang 3.8.1 it hoists the function `foo` correctly out of the loop:

```
; ModuleID = 'x.c'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-freebsd10.3"

; Function Attrs: nounwind uwtable
define i32 @foobar() #0 {
entry:
  %call = tail call i32 @foo(i32 9075) #3
  br label %for.body

for.body:                                         ; preds = %for.body, %entry
  %x.09 = phi i32 [ 0, %entry ], [ %call1, %for.body ]
  %i.08 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
  %add = add nsw i32 %call, %x.09
  %call1 = tail call i32 @bar(i32 %add) #4
  %inc = add nuw nsw i32 %i.08, 1
  %exitcond = icmp eq i32 %inc, 100
  br i1 %exitcond, label %for.end, label %for.body

for.end:                                          ; preds = %for.body
  %call1.lcssa = phi i32 [ %call1, %for.body ]
  %cmp2 = icmp eq i32 %call1.lcssa, 1848
  %conv = zext i1 %cmp2 to i32
  ret i32 %conv
}

; Function Attrs: nounwind readnone
declare i32 @foo(i32) #1

declare i32 @bar(i32) #2

attributes #0 = { nounwind uwtable "disable-tail-calls"="false"
"less-precise-fpmad"="false" "no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false"
"no-nans-fp-math"="false" "stack-protector-buffer-size"="8"
"target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2"
"unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { nounwind readnone "disable-tail-calls"="false"
"less-precise-fpmad"="false" "no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false"
"no-nans-fp-math"="false" "stack-protector-buffer-size"="8"
"target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2"
"unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #2 = { "disable-tail-calls"="false" "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="false" "no-nans-fp-math"="false"
"stack-protector-buffer-size"="8" "target-cpu"="x86-64"
"target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #3 = { nounwind readnone }
attributes #4 = { nounwind }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.8.1 (tags/RELEASE_381/final)"}
```

If you remove the line `x = bar( x);` from the loop, the optimizer doesn't have
the problem anymore.

P.S. Do not use https://godbolt.org to test this, for some reasons it gives
different results.

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