[InstCombine] Eliminate stores to constant memory
If we have a store to a piece of memory which is known constant, then we know the store must be storing back the same value. As a result, the store (or memset, or memmove) must either be down a dead path, or a noop. In either case, it is valid to simply remove the store.
The motivating case for this involves a memmove to a buffer which is constant down a path which is dynamically dead.
Note that I'm choosing to implement the less aggressive of two possible semantics here. We could simply say that the store *is undefined*, and prune the path. Consensus in the review was that the more aggressive form might be a good follow on change at a later date.
Differential Revision: https://reviews.llvm.org/D60659
llvm-svn: 358919
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 6f83284..0fe52b1 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -120,6 +120,15 @@
return MI;
}
+ // If we have a store to a location which is known constant, we can conclude
+ // that the store must be storing the constant value (else the memory
+ // wouldn't be constant), and this must be a noop.
+ if (AA->pointsToConstantMemory(MI->getDest())) {
+ // Set the size of the copy to 0, it will be deleted on the next iteration.
+ MI->setLength(Constant::getNullValue(MI->getLength()->getType()));
+ return MI;
+ }
+
// If MemCpyInst length is 1/2/4/8 bytes then replace memcpy with
// load/store.
ConstantInt *MemOpLength = dyn_cast<ConstantInt>(MI->getLength());
@@ -218,6 +227,15 @@
return MI;
}
+ // If we have a store to a location which is known constant, we can conclude
+ // that the store must be storing the constant value (else the memory
+ // wouldn't be constant), and this must be a noop.
+ if (AA->pointsToConstantMemory(MI->getDest())) {
+ // Set the size of the copy to 0, it will be deleted on the next iteration.
+ MI->setLength(Constant::getNullValue(MI->getLength()->getType()));
+ return MI;
+ }
+
// Extract the length and alignment and fill if they are constant.
ConstantInt *LenC = dyn_cast<ConstantInt>(MI->getLength());
ConstantInt *FillC = dyn_cast<ConstantInt>(MI->getValue());