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

[Stefan Stipanovic via llvm-dev]

Hi Fangqing,

It looks like that after -deadargelim `foo()` should not have an
`argmemonly` attribute. If you also run Attributor here, you get the
expected result: https://godbolt.org/z/8hcG9c

Unfortunately, Attributor doesn't yet run by default.

-Stefan

On Sat, Jul 11, 2020 at 4:05 AM Fangqing Du via llvm-dev <
llvm-dev at lists.llvm.org> 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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>
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