Enhance MemDep: When alias analysis returns a partial alias result,

return it as a clobber.  This allows GVN to do smart things.

Enhance GVN to be smart about the case when a small load is clobbered
by a larger overlapping load.  In this case, forward the value.  This
allows us to compile stuff like this:

int test(void *P) {
  int tmp = *(unsigned int*)P;
  return tmp+*((unsigned char*)P+1);
}

into:

_test:                                  ## @test
	movl	(%rdi), %ecx
	movzbl	%ch, %eax
	addl	%ecx, %eax
	ret

which has one load.  We already handled the case where the smaller
load was from a must-aliased base pointer.

llvm-svn: 130180

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/include/llvm/Analysis/MemoryDependenceAnalysis.h b/llvm/include/llvm/Analysis/MemoryDependenceAnalysis.h
index 4d5dd1987f2..f2e33535ae2 100644
--- a/llvm/include/llvm/Analysis/MemoryDependenceAnalysis.h
+++ b/llvm/include/llvm/Analysis/MemoryDependenceAnalysis.h
@@ -48,6 +48,11 @@ namespace llvm {
       /// this occurs when we see a may-aliased store to the memory location we
       /// care about.
       ///
+      /// There are several cases that may be interesting here:
+      ///   1. Loads are clobbered by may-alias stores.
+      ///   2. Loads are considered clobbered by partially-aliased loads.  The
+      ///      client may choose to analyze deeper into these cases.
+      ///
       /// A dependence query on the first instruction of the entry block will
       /// return a clobber(self) result.
       Clobber,