[Patch][LoopInfo] Use CGLoopInfo to generate loop hints

[Tyler Nowicki]

Hi,

Could I get a review of the latest patch?

Thanks,

Tyler

> On Jul 14, 2015, at 4:45 PM, Tyler Nowicki <tnowicki at apple.com> wrote:
> 
> Hi Hal,
> 
> Thanks for the review! I have committed the renaming patch.
> 
> Comments in-line and I’ve attached an updated patch for review
> 
>> On Jul 9, 2015, at 6:12 PM, Hal Finkel <hfinkel at anl.gov <mailto:hfinkel at anl.gov>> wrote:
>> 
>> Hi Tyler,
>> 
>> 0001-Rename-Vectorizer-to-Vectorize-and-VectorizeUnroll-t.patch:
>> 
>>  This makes sense; it makes the variable names match the name of the metadata they control. LGTM.
>> 
>> 0002-Moved-functionality-from-EmitCondBr-to-CGLoopInfo.patch:
>> 
>> +      ValueInt = static_cast<unsigned>(ValueAPS.getSExtValue());
> 
> Removed.
> 
> 
>> Do Clang or GCC warn here without the static cast? If not, remove it.
>> 
>> +      case LoopHintAttr::UnrollCount:
>> +      case LoopHintAttr::VectorizeWidth:
>> +      case LoopHintAttr::InterleaveCount:
>> +        break;
>> 
>> The fact that there is no code here for either LoopHintAttr::Disable or case LoopHintAttr::Enable seems counter-intuitive. If this is right, please add a comment explaining why.
> 
> Specifying something like vectorizea_width(enable/disable) is invalid and produces an error. Perhaps we could improve on the message: error: use of undeclared identifier ‘disable’. So the case statements should never be triggered. I’ll add unreachable() calls to make sure that is clear.
> 
> 
>> --- a/test/CodeGenCXX/pragma-loop.cpp
>> +++ b/test/CodeGenCXX/pragma-loop.cpp
>> @@ -10,7 +10,7 @@ void while_test(int *List, int Length) {
>> #pragma clang loop vectorize_width(4)
>> #pragma clang loop unroll(full)
>>   while (i < Length) {
>> -    // CHECK: br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[LOOP_1:.*]]
>> +    // CHECK: br label {{.*}}, !llvm.loop ![[LOOP_1:.*]]
>>     List[i] = i * 2;
>>     i++;
>>   }
>> 
>> Why are these things changing? Are we now attaching the metadata to a different branch?
> 
> Yes, that is part of the reason for this change. 
> 
> "I noticed a problem with my implementation of #pragma clang loop
> vectorize(assume_safety). When it was specified on a loop other
> loop hints were lost. The problem is that CGLoopInfo attaches
> metadata a little bit differently than EmitCondBrHints in CGStmt.
> For do-loops CGLoopInfo attaches metadata to the br in the body
> block and for while and for loops, the inc block. EmitCondBrHints
> on the other hand always attaches data to the br in the cond
> block. When specifying assume_safety CGLoopInfo emits an empty
> llvm.loop metadata shadowing the metadata in the cond block. Loop
> transformations like rotate and unswitch would then eliminate the
> cond block.”
> 
> So when vectorize(assume_safety) is specified EmitCondBrHints would add the loop hint vectorize.enable = true to the cond br. But that would be lost by loop transformations. There certainly are other methods of fixing this problem, but I think unifying how metadata is attached makes the most sense and is the best for llvm in the long term.
> 
> I will post a patch for documentation of assume_safety once this problem has been fixed.
> 
> Tyler
> 
> <0001-Moved-functionality-from-EmitCondBr-to-CGLoopInfo.patch>

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