[llvm-dev] Dependence Analysis bug or undefined behavior?

[De Azevedo Piovezan, Felipe via llvm-dev]

Thanks for the answers!

I'll try to dig into DA's code to see if I can find anything, but I'm not familiar with the implementation details of the analysis. If anyone has pointers, I'd appreciate it.

From: Finkel, Hal J. [mailto:hfinkel at anl.gov]
Sent: Sunday, November 18, 2018 11:32 AM
To: De Azevedo Piovezan, Felipe <felipe.de.azevedo.piovezan at intel.com>; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Dependence Analysis bug or undefined behavior?


On 11/18/2018 08:02 AM, De Azevedo Piovezan, Felipe via llvm-dev wrote:
Hi,

Does this kind of IR have "undefined behavior"  under LLVM semantics or is it acceptable?

(TLDR: a store of i64 at offset n, followed by a load of i32 at offset n+1.)

Yes, this is well-defined at the IR level. It might have been undefined at the source level, and that might have been translated into TBAA metadata at the LLVM level which would have rendered it undefined at the IR level, but I see no aliasing metadata here.

 -Hal



define void @foo(i32* %A, i64 %n) {
entry:
  %arrayidx = getelementptr inbounds i32, i32* %A, i64 %n
  %arrayidx_cast = bitcast i32* %arrayidx to i64*
  store i64 0, i64* %arrayidx_cast, align 4
  %add1 = add i64 %n, 1
  %arrayidx2 = getelementptr inbounds i32, i32* %A, i64 %add1
  %0 = load i32, i32* %arrayidx2, align 4
  ret void
}

The result of Dependence Analysis is:

opt -analyze -da BitCasts.ll
Printing analysis 'Dependence Analysis' for function 'z0':
da analyze - none!          <<< this is between the store and itself, ok to be "none".
da analyze - none!          <<< this is between the store and the load!!!
da analyze - none!          <<< this is between the load and itself, ok to be "none".

Is dependence analysis right or wrong? This IR likely comes from some C++ code doing funny things with unions...

Thanks!

--
Felipe de Azevedo Piovezan





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--

Hal Finkel

Lead, Compiler Technology and Programming Languages

Leadership Computing Facility

Argonne National Laboratory
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