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

[Kevin Neal via llvm-dev]

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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