InstCombine: Canonicalize (2^n)-1 - x into (2^n)-1 ^ x iff x is known to be smaller than 2^n.

This has the obvious advantage of being commutable and is always a win on x86 because
const - x wastes a register there. On less weird architectures this may lead to
a regression because other arithmetic doesn't fuse with it anymore. I'll address that
problem in a followup.

llvm-svn: 147254

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/sub-xor.ll b/llvm/test/Transforms/InstCombine/sub-xor.ll
new file mode 100644
index 00000000000..41e01fbdae8
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/sub-xor.ll
@@ -0,0 +1,12 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+define i32 @test1(i32 %x) nounwind {
+  %and = and i32 %x, 31
+  %sub = sub i32 63, %and
+  ret i32 %sub
+
+; CHECK: @test1
+; CHECK-NEXT: and i32 %x, 31
+; CHECK-NEXT: xor i32 %and, 63
+; CHECK-NEXT: ret
+}