[NewGVN] Explain why sorting by pointer values doesn't introduce non-determinism.

Thanks to Eli for pointing out in a post-commit review comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302566 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/NewGVN.cpp b/lib/Transforms/Scalar/NewGVN.cpp
index c7afd1d..e976f0e 100644 (file)
--- a/lib/Transforms/Scalar/NewGVN.cpp
+++ b/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<const Use *, 4> PHIOperands;
   for (const Use &U : PN->operands())
     PHIOperands.push_back(&U);