Lower the i8 extension in memset to a multiply instead of a potentially long series of shifts and ors.
We could implement a DAGCombine to turn x * 0x0101 back into logic operations
on targets that doesn't support the multiply or it is slow (p4) if someone cares
enough.
Example code:
void test(char *s, int a) {
__builtin_memset(s, a, 4);
}
before:
_test: ## @test
movzbl 8(%esp), %eax
movl %eax, %ecx
shll $8, %ecx
orl %eax, %ecx
movl %ecx, %eax
shll $16, %eax
orl %ecx, %eax
movl 4(%esp), %ecx
movl %eax, 4(%ecx)
movl %eax, (%ecx)
ret
after:
_test: ## @test
movzbl 8(%esp), %eax
imull $16843009, %eax, %eax ## imm = 0x1010101
movl 4(%esp), %ecx
movl %eax, 4(%ecx)
movl %eax, (%ecx)
ret
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122707 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 9b2a601..b639b7c 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3132,6 +3132,17 @@
&ArgChains[0], ArgChains.size());
}
+/// SplatByte - Distribute ByteVal over NumBits bits.
+static APInt SplatByte(unsigned NumBits, uint8_t ByteVal) {
+ APInt Val = APInt(NumBits, ByteVal);
+ unsigned Shift = 8;
+ for (unsigned i = NumBits; i > 8; i >>= 1) {
+ Val = (Val << Shift) | Val;
+ Shift <<= 1;
+ }
+ return Val;
+}
+
/// getMemsetValue - Vectorized representation of the memset value
/// operand.
static SDValue getMemsetValue(SDValue Value, EVT VT, SelectionDAG &DAG,
@@ -3140,27 +3151,18 @@
unsigned NumBits = VT.getScalarType().getSizeInBits();
if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(Value)) {
- APInt Val = APInt(NumBits, C->getZExtValue() & 255);
- unsigned Shift = 8;
- for (unsigned i = NumBits; i > 8; i >>= 1) {
- Val = (Val << Shift) | Val;
- Shift <<= 1;
- }
+ APInt Val = SplatByte(NumBits, C->getZExtValue() & 255);
if (VT.isInteger())
return DAG.getConstant(Val, VT);
return DAG.getConstantFP(APFloat(Val), VT);
}
- const TargetLowering &TLI = DAG.getTargetLoweringInfo();
Value = DAG.getNode(ISD::ZERO_EXTEND, dl, VT, Value);
- unsigned Shift = 8;
- for (unsigned i = NumBits; i > 8; i >>= 1) {
- Value = DAG.getNode(ISD::OR, dl, VT,
- DAG.getNode(ISD::SHL, dl, VT, Value,
- DAG.getConstant(Shift,
- TLI.getShiftAmountTy())),
- Value);
- Shift <<= 1;
+ if (NumBits > 8) {
+ // Use a multiplication with 0x010101... to extend the input to the
+ // required length.
+ APInt Magic = SplatByte(NumBits, 0x01);
+ Value = DAG.getNode(ISD::MUL, dl, VT, Value, DAG.getConstant(Magic, VT));
}
return Value;