[llvm-dev] Is there any real downside to constructing the new SimplifyQuery once

Philip Reames via llvm-dev llvm-dev at lists.llvm.org
Wed Apr 26 16:55:26 PDT 2017

Agreed.  This seems like overall a huge win for readability.

I am a bit worried about all the miscompiles you're going to expose by 
using all that extra information, but we should definitely just fix 
those.  :)


On 04/26/2017 08:53 AM, Hal Finkel via llvm-dev wrote:
> On 04/25/2017 11:32 PM, Daniel Berlin via llvm-dev wrote:
>> For those not following along, startingin r301379, Simplify* in 
>> InstructionSimplify now can just take a query struct  instead of 8000 
>> optional arguments.  Nothing is really new since it used the same 
>> thing under the covers.
>> I'm slowly converting the old uses away (deletion of the old APIs is 
>> a different question).
>> Staring at most of them, i could just directly convert them using 
>> braced list initialization, and construct the objects again and 
>> again, but the vast majority of uses actually:
>> A. don't change the relevant query arguments over the lifetime of the 
>> pass
>> B. actually have more information sitting around than they are 
>> passing along.
>> For example, instcombine has a DomTree and AssumptionCache that are 
>> required, but it doesn't pass them to SimplifyBinOp in a bunch of cases.
>> JumpThreading has TLI but doesn't pass it to SimplifyCmpInst.
>> CVP at least admits it has a problem:
>> "CorrelatedValuePropagation.cpp:  // FIXME: Provide TLI, DT, AT to 
>> SimplifyInstruction.
>> CorrelatedValuePropagation.cpp:  if (Value *V = 
>> SimplifyInstruction(P, DL)) {
>> "
>> (This is because it uses LVI, which requires those things, but it 
>> doesn't ask for them itself)
>> Assuming this is not deliberate, it would seem to me to just be 
>> easiest to set up the query structure once in the pass and use it 
>> everywhere, which would hopefully avoid these issues in the future, 
>> besides making most of the call strings dramatically shorter :)
>> Is there any real downside (compile time performance, etc) to this vs 
>> what we do now (which is basically constructing the query objects 
>> again and again under the covers) that anyone can think of?
>> (again, assuming we are not deliberately avoiding passing info to 
>> Simplify* in most of these cases)
> I don't think it is deliberate; it is just that no one has done the 
> work to make it uniform. It is possible that doing this will expose 
> some compile-time issues, but if so, we should deal with them for real 
> (not just hide them by a failure to provide optional-but-available 
> analyses).
> I'm certainly in favor of more uniformity here.
>  -Hal
>> I figured i'd ask before i went writing the scripts/etc to help 
>> convert a bunch of this stuff :)
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> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
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