llvm dev - May 2014 - [LLVMdev] clang -O2 versus opt -O2 | llc | clang

I'm investigating a miscompilation bug (
http://llvm.org/bugs/show_bug.cgi?id=19823 ), but I've run into a problem:
the output of the program is different when I compile the IR with clang
compared to opt | llc | clang. Any clues on how to resolve this difference?

$ ./opt -O1 19823.ll | ./llc | ./clang -x assembler - -o a.out ; ./a.out ;
echo $?
1
$ ./opt -O2 19823.ll | ./llc | ./clang -x assembler - -o a.out ; ./a.out ;
echo $?
1
$ ./clang -O1 19823.ll ; ./a.out ; echo $?
1
$ ./clang -O2 19823.ll ; ./a.out ; echo $?
0   <--- that's bad!


The IR is pretty simple:
@a = common global i16 0, align 2
@c = global i16* @a, align 8
@d = global i8 0, align 1
@b = common global i16 0, align 2

; Function Attrs: nounwind ssp uwtable
define i32 @main() #0 {
entry:
  %0 = load i16** @c, align 8, !tbaa !1
  %d.promoted = load i8* @d, align 1, !tbaa !5
  br label %lbl

lbl:                                              ; preds = %lbl, %entry
  %1 = phi i8 [ %dec, %lbl ], [ %d.promoted, %entry ]
  %dec = add i8 %1, -1
  %conv = sext i8 %1 to i16
  store i16 %conv, i16* @b, align 2, !tbaa !6
  store i16 1, i16* %0, align 2, !tbaa !6
  %tobool = icmp eq i8 %dec, 0
  br i1 %tobool, label %if.end, label %lbl

if.end:                                           ; preds = %lbl
  store i8 0, i8* @d, align 1, !tbaa !5
  %2 = load i16* @a, align 2, !tbaa !6
  %conv1 = sext i16 %2 to i32
  ret i32 %conv1
}
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Hi Sanjay,
> I'm investigating a miscompilation bug (
> http://llvm.org/bugs/show_bug.cgi?id=19823 ), but I've run into a
problem:
> the output of the program is different when I compile the IR with clang
> compared to opt | llc | clang. Any clues on how to resolve this difference?
>From the bug, it looks like there might be a problem with loop
unrolling. Running "opt -loop-unroll -unroll-runtime" on its own
performs the dodgy transformation and changes the output.

I've not tracked down quite how Clang sets that extra
"-unroll-runtime" option (I'd be interested to know myself,
actually,
having failed).

Cheers.

Tim.

----- Original Message -----
> From: "Tim Northover" <t.p.northover at gmail.com>
> To: "Sanjay Patel" <spatel at rotateright.com>
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Friday, May 23, 2014 2:22:55 PM
> Subject: Re: [LLVMdev] clang -O2 versus opt -O2 | llc | clang
> 
> Hi Sanjay,
> 
> > I'm investigating a miscompilation bug (
> > http://llvm.org/bugs/show_bug.cgi?id=19823 ), but I've run into a
> > problem:
> > the output of the program is different when I compile the IR with
> > clang
> > compared to opt | llc | clang. Any clues on how to resolve this
> > difference?
> 
> From the bug, it looks like there might be a problem with loop
> unrolling. Running "opt -loop-unroll -unroll-runtime" on its own
> performs the dodgy transformation and changes the output.
> 
> I've not tracked down quite how Clang sets that extra
> "-unroll-runtime" option (I'd be interested to know myself,
actually,
> having failed).
It doesn't, but the backend can by overriding TTI::getUnrollingPreferences
or setting LoopMicroOpBufferSize in the processor scheduling model.

 -Hal
> 
> Cheers.
> 
> Tim.
> _______________________________________________
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> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory