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, 27 insertions, 0 deletions

diff --git a/llvm/test/Transforms/GVN/rle.ll b/llvm/test/Transforms/GVN/rle.ll
index 2e433217507..42363d8bb2b 100644
--- a/llvm/test/Transforms/GVN/rle.ll
+++ b/llvm/test/Transforms/GVN/rle.ll
@@ -544,3 +544,30 @@ entry:
 ; CHECK: ret i32 0
 }
 
+
+;;===----------------------------------------------------------------------===;;
+;; Load -> Load forwarding in partial alias case.
+;;===----------------------------------------------------------------------===;;
+
+define i32 @load_load_partial_alias(i8* %P) nounwind ssp {
+entry:
+  %0 = bitcast i8* %P to i32*
+  %tmp2 = load i32* %0
+  %add.ptr = getelementptr inbounds i8* %P, i64 1
+  %tmp5 = load i8* %add.ptr
+  %conv = zext i8 %tmp5 to i32
+  %add = add nsw i32 %tmp2, %conv
+  ret i32 %add
+
+; CHECK: @load_load_partial_alias
+; CHECK: load i32*
+; CHECK-NOT: load
+; CHECK: lshr i32 {{.*}}, 8
+; CHECK-NOT: load
+; CHECK: trunc i32 {{.*}} to i8
+; CHECK-NOT: load
+; CHECK: ret i32
+}
+
+
+