[llvm-dev] TryToShrinkGlobalToBoolean in GlobalOpt.cpp issue

[Mehdi Amini via llvm-dev]

> On Aug 30, 2016, at 2:57 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> 
> Yes, the full test case is:
> 
> static int x = 100;
> int y = whatever;
> 
> int main() {
>    x = -101;
>    y = whatever that's not whatever above;
>    printf("%d\n", y);
>    printf("%d\n", x);
>   return 0;
> }
> 
> You are correct, in the above test case the globalopt does not make the transformation….

I’m more annoyed that it does not turn x into an alloca. With this code:

$ cat /tmp/test.c
static int x = 100;
int y = 2;
int main() {
   x = -101;
   y = 1; 
   printf("%d\n", y);
   printf("%d\n", x);
  return 0;
}
$ clang -O3 -S -emit-llvm -o - /tmp/test.c 
; ModuleID = '/tmp/test.c'
source_filename = "/tmp/test.c"
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.12.0"

@y = global i32 2, align 4
@x = internal unnamed_addr global i1 false, align 4
@.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1

; Function Attrs: nounwind ssp uwtable
define i32 @main() #0 {
  store i1 true, i1* @x, align 4
  store i32 1, i32* @y, align 4, !tbaa !2
  %1 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str, i64 0, i64 0), i32 1)
  %2 = load i1, i1* @x, align 4
  %3 = select i1 %2, i32 -101, i32 100
  %4 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str, i64 0, i64 0), i32 %3)
  ret i32 0
}


> however, I think the original issue still stands, it really shouldn't be doing it here either.

Yes.

— 
Mehdi



> 
> On Tue, Aug 30, 2016 at 5:52 PM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> 
> > On Aug 30, 2016, at 1:01 PM, Ryan Taylor via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> >
> > Given some code:
> >
> > static int x = 100;
> >
> > int main() {
> >    x = 101;
> >    printf("%d\n", x);
> > }
> >
> > results in:
> >
> > %0 = load i1
> > %1 = select %0, 101, 100
> 
> If x is only touched in one function, I’d expect globlaopt to turn it into an alloca instead of leaving it global and shrinking it.
> Did you oversimplify the real case where you see this for the sake of the example here?
> 
> —
> Mehdi
> 
> 
> > ...
> > ...
> >
> > 1) What architecture(s) does this benefit?
> > 2) What's the best way to circumvent this in the backend (currently I am just not allowing this opt function to run)? I have tried a few setOperationAction methods to no result. Rather not just comment out core code because it makes porting more difficult.
> >
> > -Ryan
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
> 

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