[PATCH] D102673: [ConstantFolding] Fold constrained arithmetic intrinsics

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


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Comment at: llvm/lib/Analysis/ConstantFolding.cpp:1907
+    // Even if the rounding mode is unknown, try evaluating the operation.
+    // If it does not raise inexact exception, rounding was not applied
+    // so the result does not depend on rounding mode.
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This code comment does not look accurate.
When we speculatively evaluate the expression, we check that *any* exception was not raised, not just the inexact exception, right?

This raises a question: if we evaluate the expression using NearestTiesToEven, then are we guaranteed that all potential exceptions (even underflow) are identical to any other rounding mode?


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Comment at: llvm/lib/Analysis/ConstantFolding.cpp:1910-1911
+  RoundingMode RM = RoundingMode::Dynamic;
+  Optional<RoundingMode> ORM = CI->getRoundingMode();
+  if (ORM)
+    RM = *ORM;
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sepavloff wrote:
> spatel wrote:
> > sepavloff wrote:
> > > spatel wrote:
> > > > I think this would be easier to read if we made it more like the code above here:
> > > > 
> > > > ```
> > > >   Optional<RoundingMode> ORM = CI->getRoundingMode();
> > > >   // If no rounding mode is specified by the intrinsic or the mode is dynamic,
> > > >   // try to evaluate using the default mode. If it does not raise an inexact
> > > >   // exception, rounding was not applied so the result is independent of
> > > >   // rounding mode. 
> > > >   if (!ORM || *ORM == RoundingMode::Dynamic) 
> > > >     return RoundingMode::NearestTiesToEven;
> > > >   // Use the mode specified by the intrinsic.
> > > >   return *ORM;
> > > > ```
> > > > That's assuming I'm reading it correctly right now - are there tests with no RM specified on the intrinsic?
> > > > That's assuming I'm reading it correctly right now - are there tests with no RM specified on the intrinsic?
> > > 
> > > No, all affected intrinsics have such argument. Bu in other places such check is made, so I put it here also. 
> > I am not understanding something then.
> > If the metadata args for rounding mode and exception behavior are required by the LangRef:
> > https://llvm.org/docs/LangRef.html#llvm-experimental-constrained-fadd-intrinsic
> > ...then why is the result of `getRoundingMode()` or `getExceptionBehavior()` an `Optional` value? If it is not valid IR without those args, we shouldn't be adding compile-time checks for things that can't happen.
> > I am not understanding something then.
> > If the metadata args for rounding mode and exception behavior are required by the LangRef:
> > https://llvm.org/docs/LangRef.html#llvm-experimental-constrained-fadd-intrinsic
> > ...then why is the result of `getRoundingMode()` or `getExceptionBehavior()` an `Optional` value? If it is not valid IR without those args, we shouldn't be adding compile-time checks for things that can't happen.
> 
> This function processes any `ConstrainedFPIntrinsic`. Some of them do not have rounding mode arguments, like constrained variants of `floor`, `trunc`. `frem` also do not depend on rounding mode, although it has rounding mode argument, probably some day it would be removed.
> 
> Using `Optional` as return value of `getRoundingMode()` or `getExceptionBehavior()` is now a part of `ConstrainedFPIntrinsic` interface. It is safer to process results of these function in more general way.
Ok - thanks for explaining. I didn't think of those other intrinsics. Presumably, we'll add constant folding for those later, so this code will be shared.


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