From 63998ec3c86eb090ac1a3086ba22784392b15b94 Mon Sep 17 00:00:00 2001 From: Davide Italiano Date: Tue, 9 May 2017 18:29:37 +0000 Subject: [PATCH] [NewGVN] Explain why sorting by pointer values doesn't introduce non-determinism. Thanks to Eli for pointing out in a post-commit review comment. llvm-svn: 302566 --- llvm/lib/Transforms/Scalar/NewGVN.cpp | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/llvm/lib/Transforms/Scalar/NewGVN.cpp b/llvm/lib/Transforms/Scalar/NewGVN.cpp index c7afd1d..e976f0e 100644 --- a/llvm/lib/Transforms/Scalar/NewGVN.cpp +++ b/llvm/lib/Transforms/Scalar/NewGVN.cpp @@ -734,6 +734,10 @@ PHIExpression *NewGVN::createPHIExpression(Instruction *I, bool &HasBackedge, // PHIs. LLVM doesn't seem to always guarantee this. While we need to fix // this in LLVM at some point we don't want GVN to find wrong congruences. // Therefore, here we sort uses in predecessor order. + // We're sorting the values by pointer. In theory this might be cause of + // non-determinism, but here we don't rely on the ordering for anything + // significant, e.g. we don't create new instructions based on it so we're + // fine. SmallVector PHIOperands; for (const Use &U : PN->operands()) PHIOperands.push_back(&U); -- 2.7.4