[llvm] [AArch64][GISel] Always fold G_SHL into addressing mode where possible, unless the subtarget has addr-lsl-slow-14 (PR #96603)

[David Green via llvm-commits]

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@@ -6717,19 +6722,70 @@ AArch64InstructionSelector::selectNegArithImmed(MachineOperand &Root) const {
   return select12BitValueWithLeftShift(Immed);
 }
 
+/// Checks if we are sure that folding MI into load/store addressing mode is
+/// beneficial or not.
+///
+/// Returns:
+/// - true if folding MI would be beneficial.
+/// - false if folding MI would be bad.
+/// - std::nullopt if it is not sure whether folding MI is beneficial.
+///
+/// \p MI can be the offset operand of G_PTR_ADD, e.g. G_SHL in the example:
+///
+/// %13:gpr(s64) = G_CONSTANT i64 1
+/// %8:gpr(s64) = G_SHL %6, %13(s64)
+/// %9:gpr(p0) = G_PTR_ADD %0, %8(s64)
+/// %12:gpr(s32) = G_LOAD %9(p0) :: (load (s16))
+std::optional<bool> AArch64InstructionSelector::isWorthFoldingIntoAddrMode(
+    MachineInstr &MI, const MachineRegisterInfo &MRI) const {
+  if (MI.getOpcode() == AArch64::G_SHL) {
+    // Address operands with shifts are free, except for running on sub targets
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davemgreen wrote:

"sub targets" -> subtargets

https://github.com/llvm/llvm-project/pull/96603