[PATCH] Generate warning when explicitly specified vectorization fails

[Arnold]

LLVM patch lgtm.

Thanks Tyler.

> On Jul 9, 2014, at 7:40 PM, Tyler Nowicki <tnowicki at apple.com> wrote:
> 
> If these pragmas are silently ignored when then they don’t have much value. A big part of their value come from the confirmation that they are working. The warning will only be generated on loops with #pragma clang loop vectorize/interleave/unroll specified. It can also be silenced using the command line option -Wno-optimization-warning.
> 
> I’m still looking into the clang in 32-bit mode question posed earlier. Are there any other comments on the patches?
> 
> Thanks,
> 
> Tyler
> 
>> On Jul 7, 2014, at 10:45 AM, Mark Heffernan <meheff at google.com> wrote:
>> 
>>> On Thu, Jul 3, 2014 at 9:23 AM, Diego Novillo <dnovillo at google.com> wrote:
>>> This change is raising the severity of this diagnostic. Do we really want to cause build failures in this situation when the code is built with -Werror?
>>> 
>>> I think I would be fine with it, if this diagnostic was only triggered when the user specified a #pragma, but if the failure is in the vectorizer, with no hints from the user, then I think a warning is too strong. Even when using #pragma, I'm not sure I'd like a warning here.
>> 
>> I'd put my vote in for having a warning if a #pragma optimization directive could not be followed.  This assumes the warning is instructive enough that the user can easily fix the issue.  I added support for loop unrolling pragmas (following Tyler's vectorization work) so the focus for us is loop unrolling.  For the target we care about (CUDA/GPU) a missed loop unroll can be a massive performance loss and the user should know about it.  Whatever is decided, I'll make the diagnostic severity of loop unrolling pragma diagnostics match that of the vectorization ones.
>> 
>> Mark
>>  
>>> 
>>>>        return false;
>>>>      }
>>>> 
>>>> @@ -1276,8 +1292,7 @@
>>>>        emitOptimizationRemarkAnalysis(
>>>>            F->getContext(), DEBUG_TYPE, *F, L->getStartLoc(),
>>>>            "loop not vectorized due to NoImplicitFloat attribute");
>>>> -      emitOptimizationRemarkMissed(F->getContext(), DEBUG_TYPE, *F,
>>>> -                                   L->getStartLoc(), Hints.emitRemark());
>>>> +      emitMissedWarning(F, L, Hints);
>>> 
>>> Likewise.
>>> 
>>> 
>>> Diego.
> 
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