[SDAG] Introduce a new BITREVERSE node along with a corresponding LLVM intrinsic

Several backends have instructions to reverse the order of bits in an integer. Conceptually matching such patterns is similar to @llvm.bswap, and it was mentioned in http://reviews.llvm.org/D14234 that it would be best if these patterns were matched in InstCombine instead of reimplemented in every different target.

This patch introduces an intrinsic @llvm.bitreverse.i* that operates similarly to @llvm.bswap. For plumbing purposes there is also a new ISD node ISD::BITREVERSE, with simple expansion and promotion support.

The intention is that InstCombine's BSWAP detection logic will be extended to support BITREVERSE too, and @llvm.bitreverse intrinsics emitted (if the backend supports lowering it efficiently).

llvm-svn: 252878

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/PowerPC/bitreverse.ll b/llvm/test/CodeGen/PowerPC/bitreverse.ll
new file mode 100644
index 00000000000..1c3741a9a69
--- /dev/null
+++ b/llvm/test/CodeGen/PowerPC/bitreverse.ll
@@ -0,0 +1,23 @@
+; RUN: llc -march=ppc64 %s -o - | FileCheck %s
+
+; These tests just check that the plumbing is in place for @llvm.bitreverse. The
+; actual output is massive at the moment as llvm.bitreverse is not yet legal.
+
+declare <2 x i16> @llvm.bitreverse.v2i16(<2 x i16>) readnone
+
+define <2 x i16> @f(<2 x i16> %a) {
+; CHECK-LABEL: f:
+; CHECK: rlwinm
+  %b = call <2 x i16> @llvm.bitreverse.v2i16(<2 x i16> %a)
+  ret <2 x i16> %b
+}
+
+declare i8 @llvm.bitreverse.i8(i8) readnone
+
+define i8 @g(i8 %a) {
+; CHECK-LABEL: g:
+; CHECK: rlwinm
+; CHECK: rlwimi
+  %b = call i8 @llvm.bitreverse.i8(i8 %a)
+  ret i8 %b
+}