[llvm] [AArch64] Consider negated powers of 2 when calculating throughput cost (PR #143013)

[David Green via llvm-commits]

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@@ -4005,25 +4005,34 @@ InstructionCost AArch64TTIImpl::getArithmeticInstrCost(
       // have similar cost.
       auto VT = TLI->getValueType(DL, Ty);
       if (VT.isScalarInteger() && VT.getSizeInBits() <= 64) {
-        if (Op2Info.isPowerOf2()) {
+        if (Op2Info.isPowerOf2() || Op2Info.isNegatedPowerOf2()) {
+          // Neg can be folded into the asr instruction.
+          // FIXME: Is the throughput cost of asr + neg the same as just asr?
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davemgreen wrote:

You can remove this FIXME I think. It will just mean that the cost of scalar is a little lower than vector. Or make it part of the note above, that we are assuming that the neg and the neg+asr share the same cost.

https://github.com/llvm/llvm-project/pull/143013