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Some SIMD implementations are not IEEE-754 compliant, for example ARM's NEON. This patch teaches the loop vectorizer to only allow transformations of loops that either contain no floating-point operations or have enough allowance flags supporting lack of precision (ex. -ffast-math, Darwin). For that, the target description now has a method which tells us if the vectorizer is allowed to handle FP math without falling into unsafe representations, plus a check on every FP instruction in the candidate loop to check for the safety flags. This commit makes LLVM behave like GCC with respect to ARM NEON support, but it stops short of fixing the underlying problem: sub-normals. Neither GCC nor LLVM have a flag for allowing sub-normal operations. Before this patch, GCC only allows it using unsafe-math flags and LLVM allows it by default with no way to turn it off (short of not using NEON at all). As a first step, we push this change to make it safe and in sync with GCC. The second step is to discuss a new sub-normal's flag on both communitues and come up with a common solution. The third step is to improve the FastMath flags in LLVM to encode sub-normals and use those flags to restrict NEON FP. Fixes PR16275. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266363 91177308-0d34-0410-b5e6-96231b3b80d8
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