Add a neat little two's complement hack for x86.

On x86 we can't encode an immediate LHS of a sub directly. If the RHS comes from a XOR with a constant we can
fold the negation into the xor and add one to the immediate of the sub. Then we can turn the sub into an add,
which can be commuted and encoded efficiently.

This code is generated for __builtin_clz and friends.

llvm-svn: 136167

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/X86/sub.ll b/llvm/test/CodeGen/X86/sub.ll
new file mode 100644
index 00000000000..2a4d2d6e3fc
--- /dev/null
+++ b/llvm/test/CodeGen/X86/sub.ll
@@ -0,0 +1,11 @@
+; RUN: llc -march=x86 < sub.ll | FileCheck %s
+
+define i32 @test1(i32 %x) {
+  %xor = xor i32 %x, 31
+  %sub = sub i32 32, %xor
+  ret i32 %sub
+; CHECK: test1:
+; CHECK:      xorl $-32
+; CHECK-NEXT: addl $33
+; CHECK-NEXT: ret
+}