[llvm-dev] Bug in pass 'ipsccp' on function attribute 'argmemonly'?

[Hal Finkel via llvm-dev]

Hi, Fangqing,

Yes, this seems like a bug. Can you please file a bug report at 
https://bugs.llvm.org/ ? If you need someone else to do this on your 
behalf, please let us know.

  -Hal

On 7/10/20 9:04 PM, Fangqing Du via llvm-dev wrote:
> Hi all,
>
> Let's see an example (from Alexandre Isoard) first:
> ****************************************************************************************
> ; RUN: opt -ipsccp -deadargelim -licm -O2 -S < %s
>
> @g = internal global i32 0
>
> ; Function Attrs: argmemonly
> define internal void @foo(i32* nonnull dereferenceable(4) %arg, i32 
> %val) #0 {
> entry:
>   store i32 %val, i32* %arg
>   ret void
> }
>
> define i32 @bar(i32 %n) {
> entry:
>   store i32 1, i32* @g
>   br label %loop
>
> loop: ; preds = %bb1, %bb
> %i = phi i32 [ %i.inc, %loop ], [ 0, %entry ]
> %g.val = load i32, i32* @g
> %g.inc = add nuw i32 %g.val, 1
> tail call void @foo(i32* @g, i32 %g.inc)
> %i.inc = add nuw i32 %i, 1
> %cond = icmp eq i32 %i.inc, %n
>   br i1 %cond, label %exit, label %loop
>
> exit: ; preds = %bb1
>   ret i32 %g.val
> }
>
> declare void @llvm.assume(i1)
>
> attributes #0 = { argmemonly }
> **************************************************************************************** 
>
> With opt cmd '-ipsccp -deadargelim -licm -O2', function @bar will 
> return constant value 1, instead of correct value %n. This is crazy, 
> right?
> Let's see what happens here.
> Due to pass 'ipsccp' replaced the argument of function '@foo' with 
> global variable '@g', but keeps attr 'argmemonly' there, so pass 
> 'licm' will hoist the load of '@g' before the loop (as the value of @g 
> isn't changed, but it is changed), and will cause function return 
> wrong constant value '1', instead of '%n' (the correct value) , like 
> following:
> **************************************************************************************** 
>
> ; Function Attrs: nofree norecurse nounwind writeonly
> define i32 @bar(i32 %n) local_unnamed_addr #0 {
> entry:
> ret i32 1
> }
>
> attributes #0 = { nofree norecurse nounwind writeonly }
> **************************************************************************************** 
>
>
> And if remove the 'argmemonly' attribute, function @bar will return 
> the correct value:
> **************************************************************************************** 
>
> ; Function Attrs: nofree norecurse nounwind
> define i32 @bar(i32 %n) local_unnamed_addr #0 {
> entry:
> ret i32 %n
> }
> **************************************************************************************** 
>
>
> So if function attribute 'argmemonly' on function @foo is correct, 
> then there's a bug in pass 'ipsccp'. When it replaces the function 
> local value with global variable, then it shoud remember to remove 
> this function attribute.
>
> Thanks,
> Fangqing
>
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> llvm-dev at lists.llvm.org
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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