llvm dev - Jul 2021 - [754] Fold FP "Op SNaN" to QNaN?

I'm looking at lib/Analysis/InstructionSimplify.cpp where the function
propagateNaN() has a comment asking if it should quiet a signaling NaN.

If I understand the IEEE 754-2019 standard correctly: an SNaN shall be converted
to a QNaN whenever an "operation" is done. The standard doesn't
say, or I couldn't find it, exactly _when_ that operation must be done.
Which implies that the floating-point operation could be done by the compiler.
In which case folding an instruction that has an SNaN operand should result in a
QNaN.

Is my reading of the standard correct? And should we make this change to
InstructionSimplify.cpp:propagateNaN() and perhaps more generally in LLVM?
--
Kevin P. Neal
SAS/C and SAS/C++ Compiler
Compute Services
SAS Institute, Inc.




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On Fri, Jul 9, 2021, 08:41 Kevin Neal via llvm-dev <llvm-dev at
lists.llvm.org>
wrote:
> I’m looking at lib/Analysis/InstructionSimplify.cpp where the function
> propagateNaN() has a comment asking if it should quiet a signaling NaN.
>
> If I understand the IEEE 754-2019 standard correctly: an SNaN shall be
> converted to a QNaN whenever an “operation” is done. The standard doesn’t
> say, or I couldn’t find it, exactly _*when*_ that operation must be done.
> Which implies that the floating-point operation could be done by the
> compiler. In which case folding an instruction that has an SNaN operand
> should result in a QNaN.
>
That should work just fine, but only when you know that FP exceptions flags
are ignored, since the signalling NaN causes the operation to generate the
Invalid exception, and a quiet NaN doesn't generally cause exceptions.

Jacob Lifshay
>
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