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authorDan Gohman <dan433584@gmail.com>2013-02-01 00:49:06 +0000
committerDan Gohman <dan433584@gmail.com>2013-02-01 00:49:06 +0000
commit901261d558d0b41ba75d8aa2b38aac72aaa41bae (patch)
tree67fbdd4c1fc16f5b7d8821e83a162f6f1491a6af /lib/Analysis
parentf715dbd263149efeb9c684dfdb0637cf84f94399 (diff)
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Add a comment explaining an unavailable optimization.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174131 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/Analysis')
-rw-r--r--lib/Analysis/InstructionSimplify.cpp28
1 files changed, 28 insertions, 0 deletions
diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index 2ca37cc..e296215 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/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,