De Azevedo Piovezan, Felipe via llvm-dev
2018-Nov-18 14:02 UTC
[llvm-dev] Dependence Analysis bug or undefined behavior?
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.) 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181118/636dea06/attachment.html>
Juneyoung Lee via llvm-dev
2018-Nov-18 15:13 UTC
[llvm-dev] Dependence Analysis bug or undefined behavior?
> Does this kind of IR have “undefined behavior” under LLVM semantics oris it acceptable? I think this should be well defined. C11 says 6.5.2.3 Structure and union members 95) If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called ‘‘type punning’’). This might be a trap representation. (I'm using this pdf file for the text - http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf ) I guess peephole optimizations also can lead to the load/store with different types (e.g. memcpy to/from an unsigned char array can be removed if redundant), so the code seems to make sense. Best Regards, Juneyoung Lee On Sun, Nov 18, 2018 at 11:03 PM De Azevedo Piovezan, Felipe via llvm-dev < llvm-dev at lists.llvm.org> 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.) > > > > 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 > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-- Juneyoung Lee Software Foundation Lab, Seoul National University -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181119/0d63fd87/attachment.html>
Finkel, Hal J. via llvm-dev
2018-Nov-18 16:32 UTC
[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 _______________________________________________ LLVM Developers mailing list llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev -- 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/20181118/88e10b87/attachment.html>
De Azevedo Piovezan, Felipe via llvm-dev
2018-Nov-19 13:58 UTC
[llvm-dev] Dependence Analysis bug or undefined behavior?
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 _______________________________________________ LLVM Developers mailing list llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev -- 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/20181119/77061ba6/attachment.html>
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