Hi folks,
I found an issue while fixing a test using the wrong VMUL.f32, and I'd like to know what should be our choice on this topic that is slightly controversial.
Basically, LLVM chooses to lower single-precision FMUL to NEON's VMUL.f32 instead of VFP's version because, on some cores (A8, A5 and Apple's Swift), the VFP variant is really slow.
This is all cool and dandy, but NEON is not IEEE 754 compliant, so the result is slightly different. So slightly that only one test, that was really pushing the boundaries (ie. going below FLT_MIN) did catch it.
There are two ways we can go here:
1. Strict IEEE compatibility and *only* lower NEON's VMUL if unsafe-math is on. This will make generic single-prec. code slower but you can always turn unsafe-math on if you want more speed.
2. Continue using NEON for f32 by default and put a note somewhere that people should turn this option (FeatureNEONForFP) off on A5/A8 if they *really* care about maximum IEEE compliance.
Apple already said that for Darwin, 2 is still the option of choice. Do we agree and ignore this issue? Or for GNU/EABI we want strict conformance by default?
GCC uses fmuls...
cheers, --renato