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

[Alexandre Isoard via llvm-dev]

Note that there is the same issue in ipconstprop (on the same example) for
the same reason. (thanks @Lin-Ya Yu <yu810226 at gmail.com>)

On Tue, Jul 14, 2020 at 5:02 PM Johannes Doerfert via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> On 7/14/20 4:34 PM, Hal Finkel via llvm-dev wrote:
>
>
> 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>
> <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.
>
> I don't think that was ever questioned ;)
>
> I think the comment about the Attributor was more to show that this
> behavior
>
> is not the same there which is yet another good indicator this is a bug.
>
>
> ~ Johannes
>
>
>  -Hal
>
>
>
> Thanks,
> Fangqing
>
> On Sat, Jul 11, 2020 at 5:12 AM Hal Finkel <hfinkel at anl.gov
> <mailto:hfinkel at anl.gov> <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/>
> <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
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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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-- 
*Alexandre Isoard*
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