Fold the PPCISD shifts when presented with 0 inputs. This occurs for code

like:
long long test(long long X, int Y) {
  return 1ULL << Y;
}
long long test2(long long X, int Y) {
  return -1LL << Y;
}

which we used to compile to:

_test:
        li r2, 1
        subfic r3, r5, 32
        li r4, 0
        addi r6, r5, -32
        srw r3, r2, r3
        slw r4, r4, r5
        slw r6, r2, r6
        or r3, r4, r3
        slw r4, r2, r5
        or r3, r3, r6
        blr
_test2:
        li r2, -1
        subfic r3, r5, 32
        addi r6, r5, -32
        srw r3, r2, r3
        slw r4, r2, r5
        slw r2, r2, r6
        or r3, r4, r3
        or r3, r3, r2
        blr

Now we produce:

_test:
        li r2, 1
        addi r3, r5, -32
        subfic r4, r5, 32
        slw r3, r2, r3
        srw r4, r2, r4
        or r3, r4, r3
        slw r4, r2, r5
        blr
_test2:
        li r2, -1
        subfic r3, r5, 32
        addi r6, r5, -32
        srw r3, r2, r3
        slw r4, r2, r5
        slw r2, r2, r6
        or r3, r4, r3
        or r3, r3, r2
        blr


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30479 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 20 insertions, 0 deletions

diff --git a/lib/Target/PowerPC/PPCISelLowering.cpp b/lib/Target/PowerPC/PPCISelLowering.cpp
index db33f5f..63ae9e7 100644
--- a/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -2309,6 +2309,26 @@ SDOperand PPCTargetLowering::PerformDAGCombine(SDNode *N,
   SelectionDAG &DAG = DCI.DAG;
   switch (N->getOpcode()) {
   default: break;
+  case PPCISD::SHL:
+    if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(N->getOperand(0))) {
+      if (C->getValue() == 0)   // 0 << V -> 0.
+        return N->getOperand(0);
+    }
+    break;
+  case PPCISD::SRL:
+    if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(N->getOperand(0))) {
+      if (C->getValue() == 0)   // 0 >>u V -> 0.
+        return N->getOperand(0);
+    }
+    break;
+  case PPCISD::SRA:
+    if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(N->getOperand(0))) {
+      if (C->getValue() == 0 ||   //  0 >>s V -> 0.
+          C->isAllOnesValue())    // -1 >>s V -> -1.
+        return N->getOperand(0);
+    }
+    break;
+    
   case ISD::SINT_TO_FP:
     if (TM.getSubtarget<PPCSubtarget>().has64BitSupport()) {
       if (N->getOperand(0).getOpcode() == ISD::FP_TO_SINT) {