[MSP430] Optimize A >> (8 + N) case

Author: chbessonova

src/llvm/lib/Target/MSP430/MSP430ISelLowering.cpp

@@ -949,10 +949,17 @@ SDValue MSP430TargetLowering::LowerShifts(SDValue Op,
   uint64_t ShiftAmount = cast<ConstantSDNode>(N->getOperand(1))->getZExtValue();
 
   // Expand the stuff into sequence of shifts.
-  // FIXME: for some shift amounts this might be done better!
-  // E.g.: foo >> (8 + N) => sxt(swpb(foo)) >> N
   SDValue Victim = N->getOperand(0);
 
+  if ((Opc == ISD::SRA || Opc == ISD::SRL) && ShiftAmount > 8) {
+    // E.g.: foo >> (8 + N) => sxt(swpb(foo)) >> N
+    Victim = DAG.getNode(ISD::BSWAP, dl, VT, Victim);
+    unsigned ExtOpc =
+        Opc == ISD::SRA ? ISD::SIGN_EXTEND_INREG : ISD::ZERO_EXTEND;
+    Victim = DAG.getNode(ExtOpc, dl, VT, Victim, DAG.getValueType(MVT::i8));
+    ShiftAmount -= 8;
+  }
+
   if (Opc == ISD::SRL && ShiftAmount) {
     // Emit a special goodness here:
     // srl A, 1 => clrc; rrc A

src/llvm/test/CodeGen/MSP430/shifts.ll

@@ -5,6 +5,7 @@ target triple = "msp430-elf"
 define zeroext i8 @lshr8(i8 zeroext %a, i8 zeroext %cnt) nounwind readnone {
 entry:
 ; CHECK-LABEL: lshr8:
+; CHECK: clrc
 ; CHECK: rrc.b
   %shr = lshr i8 %a, %cnt
   ret i8 %shr
@@ -29,6 +30,7 @@ entry:
 define zeroext i16 @lshr16(i16 zeroext %a, i16 zeroext %cnt) nounwind readnone {
 entry:
 ; CHECK-LABEL: lshr16:
+; CHECK: clrc
 ; CHECK: rrc
   %shr = lshr i16 %a, %cnt
   ret i16 %shr
@@ -49,3 +51,26 @@ entry:
   %shl = shl i16 %a, %cnt
   ret i16 %shl
 }
+
+define i16 @ashr10_i16(i16 %a) #0 {
+entry:
+; CHECK-LABEL: ashr10_i16:
+; CHECK:      swpb	r12
+; CHECK-NEXT: sxt	r12
+; CHECK-NEXT: rra	r12
+; CHECK-NEXT: rra	r12
+  %shr = ashr i16 %a, 10
+  ret i16 %shr
+}
+
+define i16 @lshr10_i16(i16 %a) #0 {
+entry:
+; CHECK-LABEL: lshr10_i16:
+; CHECK:      swpb	r12
+; CHECK-NEXT: mov.b	r12, r12
+; CHECK-NEXT: clrc
+; CHECK-NEXT: rrc	r12
+; CHECK-NEXT: rra	r12
+  %shr = lshr i16 %a, 10
+  ret i16 %shr
+}