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

Fangqing Du via llvm-dev
Tue, 14 Jul 2020 16:28:47 -0000

Thank you Hal and  Stefan!

Bug report is filed: https://bugs.llvm.org/show_bug.cgi?id=46717

And Stefan,
I think 'attributor' is a really nice pass, and can infer more precise
and
useful attributes, which may give more opportunities for optimization.
But we shouldn't depend on 'attributor' to correct wrong function
attributes, because we cannot run 'attributor' after every pass, right?

Thanks,
Fangqing

On Sat, Jul 11, 2020 at 5:12 AM Hal Finkel <hfinkel at anl.gov> wrote:

> 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
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at
lists.llvm.orghttps://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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