[PATCH] DAGCombiner: fold (xor (shl 1, x), -1) -> (rotl -2, x)

[David Majnemer]

Hi chandlerc, nadav, resistor,

Targets which provide a rotate make it possible to replace a sequence of
(XOR (SHL 1, x), -1) with (ROTL -2, x).  This saves an instruction on
architectures like X86 and POWER(64).

http://reviews.llvm.org/D8350

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/xor.ll

Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3980,6 +3980,15 @@
   if (N0 == N1)
     return tryFoldToZero(SDLoc(N), TLI, VT, DAG, LegalOperations, LegalTypes);
 
+  // fold (xor (shl 1, x), -1) -> (rotl -2, x)
+  if (TLI.isOperationLegalOrCustom(ISD::ROTL, VT))
+    if (auto *N1C = dyn_cast<ConstantSDNode>(N1.getNode()))
+      if (N0.getOpcode() == ISD::SHL)
+        if (auto *ShlLHS = dyn_cast<ConstantSDNode>(N0.getOperand(0)))
+          if (N1C->isAllOnesValue() && ShlLHS->isOne())
+            return DAG.getNode(ISD::ROTL, SDLoc(N), VT, DAG.getConstant(-2, VT),
+                               N0.getOperand(1));
+
   // Simplify: xor (op x...), (op y...)  -> (op (xor x, y))
   if (N0.getOpcode() == N1.getOpcode()) {
     SDValue Tmp = SimplifyBinOpWithSameOpcodeHands(N);
Index: test/CodeGen/X86/xor.ll
===================================================================
--- test/CodeGen/X86/xor.ll
+++ test/CodeGen/X86/xor.ll
@@ -181,3 +181,15 @@
 ; X32-LABEL: PR17487:
 ; X32: andn
 }
+
+define i32 @test11(i32 %b) {
+  %shl = shl i32 1, %b
+  %neg = xor i32 %shl, -1
+  ret i32 %neg
+; X64-LABEL: test11:
+; X64: movl    $-2, %[[REG:.*]]
+; X64: roll    %{{.*}}, %[[REG]]
+; X32-LABEL: test11:
+; X32: movl    $-2, %[[REG:.*]]
+; X32: roll    %{{.*}}, %[[REG]]
+}

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