[PATCH] D57359: [GlobalISel] Introduce a G_FSQRT generic instruction

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added inline comments.


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Comment at: include/llvm/Target/GenericOpcodes.td:572-577
+// Floating point square root of a value.
+def G_FSQRT : GenericInstruction {
+  let OutOperandList = (outs type0:$dst);
+  let InOperandList = (ins type0:$src1);
+  let hasSideEffects = 0;
+}
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paquette wrote:
> arsenm wrote:
> > arsenm wrote:
> > > Needs a comment about the < 0 behavior. One particularly awful detail of SelectionDAG is ISD::FSQRT 's behavior differs from the llvm.sqrt intrinsic.
> > To be more specific, the IR claims this is undefined, which is the wrong answer. This should return a NAN
> Hmm, actually, I'm thinking maybe this should be G_INTRINSIC_SQRT, so that we can be clear which cases in which this ought to be used.
> 
> Reading up on this a bit more, I found this (12-year-old) thread: http://lists.llvm.org/pipermail/llvm-dev/2007-August/010253.html
> 
> "Also, this makes llvm.sqrt side-effect free, where sqrt potentially has side-effects (making it harder to CSE, hoist out of loops etc)."
> 
> If that's true today, then I guess the llvm.sqrt case should be handled separately.
I think having both would be helpful. Maybe say G_FSQRT is side effect free and defined for NAN, but G_INTRINSIC_SQRT is undefined for NAN?


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  https://reviews.llvm.org/D57359/new/

https://reviews.llvm.org/D57359