[PATCH] D110322: [ConstantFolding] Fold constrained compare intrinsics

[Nikita Popov via Phabricator via llvm-commits]

nikic added a reviewer: lebedev.ri.
nikic added a subscriber: lebedev.ri.
nikic added inline comments.


================
Comment at: llvm/lib/Analysis/ConstantFolding.cpp:1854
     if (EB && *EB != fp::ExceptionBehavior::ebIgnore)
-      CI->addFnAttr(Attribute::ReadNone);
+      const_cast<ConstrainedFPIntrinsic *>(CI)->addFnAttr(Attribute::ReadNone);
     return true;
----------------
sepavloff wrote:
> nikic wrote:
> > kpn wrote:
> > > This still worries me. Are _analysis_ passes allowed to change the input IR? What if the caller decides to not do a fold after calling this function to see if a fold is possible? And why is the ebIgnore case different from the other two?
> > > 
> > > Constant folding won't add this ReadNone attribute when the constant folding code doesn't have an Instruction to alter. I don't know what to do about that.
> > Analysis passes are indeed not allowed to change the IR. And this isn't a harmless change either if it gets left behind -- e.g. it invalidates MemorySSA.
> > Analysis passes are indeed not allowed to change the IR.
> 
> Constant folding is not an analysis, it changes IR. I don't know why this file is in Analysis directory.
> 
> >What if the caller decides to not do a fold after calling this function to see if a fold is possible?
> 
> Nothing bad would happen. Only side effect would be removed, but we know for sure that it is absent, we just evaluated the operation. The function call would not be removed if its result is used.
> 
> > And why is the ebIgnore case different from the other two?
> 
> If exception behavior is `ebIgnore`, such calls will get attribute `SDNodeFlags::NoFPExcept` in DAG and such instructions do not have side effects. But setting `ReadNone` for instructions with `ebIgnore` allows removal of these instructions earlier, at IR level, which could have positive effect.
> 
> > And this isn't a harmless change either if it gets left behind -- e.g. it invalidates MemorySSA.
> 
> Comparison of two floating point numbers do not use memory access. But it can change bits in the floating point state register (only Invalid bit can be set). This change is emulated as memory access so that instruction be ordered correctly. This is why constrained intrinsics declared with attribute `IntrInaccessibleMemOnly`. As no actual memory access occurs, it is harmless to set `ReadNone` in this case.
> 
> Constant folding is not an analysis, it changes IR. I don't know why this file is in Analysis directory.

The constant folding analysis does not change IR. Users of the constant folding analysis change IR based on the analysis result.

> Comparison of two floating point numbers do not use memory access. But it can change bits in the floating point state register (only Invalid bit can be set). This change is emulated as memory access so that instruction be ordered correctly. This is why constrained intrinsics declared with attribute IntrInaccessibleMemOnly. As no actual memory access occurs, it is harmless to set ReadNone in this case.

My point here was that adding a readnone attribute invalidates MemorySSA, because it means that the instruction should no longer have a MemoryAccess -- it would result in a verification failure. Just calling ConstFold/InstSimplify should never have this kind of effect.

Unless @spatel or @lebedev.ri tell me I'm wrong here, I believe this should be considered a blocker for further work in this area.


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  https://reviews.llvm.org/D110322/new/

https://reviews.llvm.org/D110322