forked from OSchip/llvm-project
467b1f1cd2
As a side-effect of the change to default HoistCommonInsts to false early in the pipeline, we fail to convert conditional branch & phis to selects early on, which prevents vectorization for loops that contain conditional branches that effectively are selects (or if the loop gets vectorized, it will get vectorized very inefficiently). This patch updates SimplifyCFG to perform hoisting if the only instruction in both BBs is an equal branch. In this case, the only additional instructions are selects for phis, which should be cheap. Even though we perform hoisting, the benefits of this kind of hoisting should by far outweigh the negatives. For example, the loop in the code below will not get vectorized on AArch64 with the current default, but will with the patch. This is a fundamental pattern we should definitely vectorize. Besides that, I think the select variants should be easier to use for reasoning across other passes as well. https://clang.godbolt.org/z/sbjd8Wshx ``` double clamp(double v) { if (v < 0.0) return 0.0; if (v > 6.0) return 6.0; return v; } void loop(double* X, double *Y) { for (unsigned i = 0; i < 20000; i++) { X[i] = clamp(Y[i]); } } ``` Reviewed By: lebedev.ri Differential Revision: https://reviews.llvm.org/D100329 |
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