[Patch][LoopVectorize] Late evaluate of runtime pointer check's threshold

[Tyler Nowicki via cfe-commits]

Hi Hal,

Thanks, the patches are committed in r244523 and r244526.

There are two possibilities to give the user control over pointer checking. We could add a loop hint option like vectorize(check_safety) that would be a safe version of ‘assume_safety’ or we could simply make it always emit the necessary pointer checks, no matter how many of them there are, when the loop hint vectorize(enable) is specified?

What do you think?

Tyler

> On Aug 9, 2015, at 9:41 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> Hi Tyler,
> 
> This looks very useful.
> 
> Please don't mention '-mllvm -runtime-memory-check-threshold=' in the error message. We should add some clause to #pragma clang loop to control this feature with some associated metadata. '-mllvm' things should not be part of the advertised interface to end users (but that should be a separate set of patches regardless).
> 
> Otherwise, LGTM.
> 
> Thanks again,
> Hal
> 
> ----- Original Message -----
>> From: "Tyler Nowicki" <tnowicki at apple.com <mailto:tnowicki at apple.com>>
>> To: "Hal J. Finkel" <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>, "Commit Messages and Patches for LLVM" <llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>>, "llvm
>> cfe" <cfe-commits at lists.llvm.org <mailto:cfe-commits at lists.llvm.org>>
>> Cc: "Gerolf Hoflehner" <ghoflehner at apple.com <mailto:ghoflehner at apple.com>>
>> Sent: Thursday, August 6, 2015 3:25:26 PM
>> Subject: [Patch][LoopVectorize] Late evaluate of runtime pointer check's threshold
>> 
>> 
>> Hi,
>> 
>> 
>> This patch extends the patches in ' Late evaluation of vectorization
>> requirements’ (still in review). This makes the num runtime pointer
>> checks test into a late diagnostic so we can give the user a better
>> diagnostic message. Clang appends the diagnostic with options for
>> getting the loop vectorized. Here is an example of the diagnostic
>> the user will see:
>> 
>> 
>> "remark: loop not vectorized: cannot prove pointers refer to
>> independent arrays in memory. The loop requires 9 runtime
>> independence checks to safely vectorize the loop, but that would
>> exceed the limit of 8 checks; increase the number of runtime pointer
>> checks with the compiler option ‘-mllvm
>> -runtime-memory-check-threshold=’. To avoid runtime pointer checking
>> when you know the arrays will always be independent specify ‘#pragma
>> clang loop vectorize(assume_safety)’ before the loop or specify
>> ‘restrict’ on the array arguments. Erroneous results will occur if
>> these options are incorrectly applied."
>> 
>> 
>> The diagnostic is produced when the command line option
>> -Rpass-analysis=loop-vectorize is provided.
>> 
>> 
>> Reviews/comments are much appreciated!
>> 
>> 
>> Tyler
>> 
>> 
>> 
>> 
>> 
>> 
>> 
> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory

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