[llvm-dev] Bug in pass 'ipsccp' on function attribute 'argmemonly'? (original) (raw)

Hal Finkel via llvm-dev llvm-dev at lists.llvm.org
Tue Jul 14 14:34:04 PDT 2020

On 7/14/20 11:28 AM, Fangqing Du wrote:

Thank you Hal and Stefan!

Bug report is filed: https://bugs.llvm.org/showbug.cgi?id=46717 <https://bugs.llvm.org/showbug.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) localunnamedaddr #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) localunnamedaddr #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

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