Add a README entry noticed while investigating PR3216.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62558 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 22 insertions, 0 deletions

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 4e4668698a..756f38a649 100644
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1651,3 +1651,25 @@ globalopt to remove the "stored only" global.
 
 //===---------------------------------------------------------------------===//
 
+This code:
+
+define inreg i32 @foo(i8* inreg %p) nounwind {
+  %tmp0 = load i8* %p
+  %tmp1 = ashr i8 %tmp0, 5
+  %tmp2 = sext i8 %tmp1 to i32
+  ret i32 %tmp2
+}
+
+could be dagcombine'd to a sign-extending load with a shift.
+For example, on x86 this currently gets this:
+
+	movb	(%eax), %al
+	sarb	$5, %al
+	movsbl	%al, %eax
+
+while it could get this:
+
+	movsbl	(%eax), %eax
+	sarl	$5, %eax
+
+//===---------------------------------------------------------------------===//