From: Nick Lewycky <nicholas@mxc.ca>
Date: Sun, 15 Nov 2009 17:51:23 +0000 (+0000)
Subject: Add a complex missed optimization opportunity I came across while investigating
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=93f9f7a44084543f637f15a2b38791cb3e5645f2;p=oota-llvm.git

Add a complex missed optimization opportunity I came across while investigating
bug 5438.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@88855 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index bcc55b41e1a..aad621f440a 100644
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1719,3 +1719,18 @@ static int foo(const char *X) { return strlen(X); }
 int bar() { return foo("abcd"); }
 
 //===---------------------------------------------------------------------===//
+
+InstCombine should use SimplifyDemandedBits to remove the or instruction:
+
+define i1 @test(i8 %x, i8 %y) {
+  %A = or i8 %x, 1
+  %B = icmp ugt i8 %A, 3
+  ret i1 %B
+}
+
+Currently instcombine calls SimplifyDemandedBits with either all bits or just
+the sign bit, if the comparison is obviously a sign test. In this case, we only
+need all but the bottom two bits from %A, and if we gave that mask to SDB it
+would delete the or instruction for us.
+
+//===---------------------------------------------------------------------===//