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Own Constant Folder in C/C++
I was talking with someone today that really really wanted the sqrtps to be used in some code they were writing. And because of a quirk with clang (still there as of clang 18.1.0), if you happened to use -ffast-math clang would butcher the use of the intrinsic. So for the code: __m128 test(const __m128 vec) { return _mm_sqrt_ps(vec); } Clang would compile it correctly without fast-math: test: # @test sqrtps xmm0, xmm0 ret And create this monstrosity with -ffast-math:
But that has precision issues between Intel and AMD due to a high ULP tolerance for the rsqrtps instruction. So LLVM does two newton-raphson iterations anytime it calls rsqrtps to correct the precision between the CPU implementations. Turns out GCC is a bit picky with this builtin, and it looks like LLVM has inherited that funky behaviour.
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