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

Hal Finkel via llvm-dev
Tue, 14 Jul 2020 21:34:04 -0000

On 7/14/20 11:28 AM, Fangqing Du wrote:
> Thank you Hal and Stefan!
>
> Bug report is filed: https://bugs.llvm.org/show_bug.cgi?id=46717
> <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?

Correct. Each pass must be correct on its own. We need to fix ipsccp.

  -Hal


>
> Thanks,
> Fangqing
>
> On Sat, Jul 11, 2020 at 5:12 AM Hal Finkel <hfinkel at anl.gov
> <mailto: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/ <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 list
>>     llvm-dev at lists.llvm.org  <mailto:llvm-dev at
lists.llvm.org>
>>     https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>
>     --
>     Hal Finkel
>     Lead, Compiler Technology and Programming Languages
>     Leadership Computing Facility
>     Argonne National Laboratory
>
--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200714/2f453cc3/attachment.html>