From 9631d908b01f65d1ffffed82c2d873cc280c4b2f Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Fri, 1 Feb 2013 00:49:06 +0000 Subject: [PATCH] Add a comment explaining an unavailable optimization. llvm-svn: 174131 --- llvm/lib/Analysis/InstructionSimplify.cpp | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp index 2ca37cc..e296215 100644 --- a/llvm/lib/Analysis/InstructionSimplify.cpp +++ b/llvm/lib/Analysis/InstructionSimplify.cpp @@ -1688,6 +1688,34 @@ static Value *ExtractEquivalentCondition(Value *V, CmpInst::Predicate Pred, return 0; } +// A significant optimization not implemented here is assuming that alloca +// addresses are not equal to incoming argument values. They don't *alias*, +// as we say, but that doesn't mean they aren't equal, so we take a +// conservative approach. +// +// This is inspired in part by C++11 5.10p1: +// "Two pointers of the same type compare equal if and only if they are both +// null, both point to the same function, or both represent the same +// address." +// +// This is pretty permissive. +// +// It's also partly due to C11 6.5.9p6: +// "Two pointers compare equal if and only if both are null pointers, both are +// pointers to the same object (including a pointer to an object and a +// subobject at its beginning) or function, both are pointers to one past the +// last element of the same array object, or one is a pointer to one past the +// end of one array object and the other is a pointer to the start of a +// different array object that happens to immediately follow the first array +// object in the address space.) +// +// C11's version is more restrictive, however there's no reason why an argument +// couldn't be a one-past-the-end value for a stack object in the caller and be +// equal to the beginning of a stack object in the callee. +// +// If the C and C++ standards are ever made sufficiently restrictive in this +// area, it may be possible to update LLVM's semantics accordingly and reinstate +// this optimization. static Constant *computePointerICmp(const DataLayout *TD, const TargetLibraryInfo *TLI, CmpInst::Predicate Pred, -- 2.7.4