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2020 Feb 07
3
Why does FPBinOp(X, undef) -> NaN?
I came across this comment in SelectionDAG.cpp:
case ISD::FADD:
case ISD::FSUB:
case ISD::FMUL:
case ISD::FDIV:
case ISD::FREM:
// If both operands are undef, the result is undef. If 1 operand is undef,
// the result is NaN. This should match the behavior of the IR optimizer.
That isn't intuitive to me. I would have expected a binary FP
operation with one undef operand to
2020 Feb 07
2
Why does FPBinOp(X, undef) -> NaN?
...correct (output of Alive2):
>
> define half @fn(half %a) {
> %b = fadd half %a, undef
> ret half %b
> }
> =>
> define half @fn(half %a) {
> ret half undef
> }
> Transformation doesn't verify!
> ERROR: Value mismatch
>
> Example:
> half %a = #x0e02 (0.000366687774?)
>
> Source:
> half %b = NaN [based on undef value]
>
> Target:
> Source value: NaN
> Target value: #x8000 (-0.0)
>
>
> Essentially, for some inputs, doing an operation with any value can't
> produce some value, as the example above shows.
Ok...