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

Alexandre Isoard via llvm-dev
Tue, 14 Jul 2020 21:28:32 -0000

I think here, I would fix ipsccp.

It transform an access of a function argument based on that argument into
an access of a function argument not based on that argument.

While one could argue it is still argmemonly (the documentation is
ambiguous on that) it makes it significantly harder to trace that
correlation for deadargelim or any form of alias analysis.

In general I wonder if we would not benefit from an attribute that list the
globals that a function may access, and having argmemonly include those
globals in what it consider "arg".

On Tue, Jul 14, 2020, 09:29 Fangqing Du via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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
>>
>> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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