[llvm] [GlobalISel] Handle div-by-pow2 (PR #83155)

[Shilei Tian via llvm-commits]

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@@ -5286,6 +5286,106 @@ MachineInstr *CombinerHelper::buildSDivUsingMul(MachineInstr &MI) {
   return MIB.buildMul(Ty, Res, Factor);
 }
 
+bool CombinerHelper::matchSDivByPow2(MachineInstr &MI) {
+  assert(MI.getOpcode() == TargetOpcode::G_SDIV && "Expected SDIV");
+  if (MI.getFlag(MachineInstr::MIFlag::IsExact))
+    return false;
+  auto &SDiv = cast<GenericMachineInstr>(MI);
+  Register RHS = SDiv.getReg(2);
+  auto MatchPow2 = [&](const Constant *C) {
+    if (auto *CI = dyn_cast<ConstantInt>(C))
+      return CI->getValue().isPowerOf2() || CI->getValue().isNegatedPowerOf2();
+    return false;
+  };
+  return matchUnaryPredicate(MRI, RHS, MatchPow2, /* AllowUndefs */ false);
+}
+
+void CombinerHelper::applySDivByPow2(MachineInstr &MI) {
+  assert(MI.getOpcode() == TargetOpcode::G_SDIV && "Expected SDIV");
+  auto &SDiv = cast<GenericMachineInstr>(MI);
+  Register Dst = SDiv.getReg(0);
+  Register LHS = SDiv.getReg(1);
+  Register RHS = SDiv.getReg(2);
+  LLT Ty = MRI.getType(Dst);
+  LLT ShiftAmtTy = getTargetLowering().getPreferredShiftAmountTy(Ty);
+
+  Builder.setInstrAndDebugLoc(MI);
+
+  auto RHSC = getIConstantVRegValWithLookThrough(RHS, MRI);
----------------
shiltian wrote:

It looks like this will not work when `RHS` is a vector.
```
define <2 x i128> @v_sdiv_v2i128(<2 x i128> %num) {
  %result = sdiv <2 x i128> %num, <i128 8589934592, i128 4096>
  ret <2 x i128> %result
}
```
We might still need to build the instructions and then fold them accordingly.

https://github.com/llvm/llvm-project/pull/83155