[llvm] [GlobaISel] Allow expanding of sdiv -> mul by constant (PR #146504)

[David Green via llvm-commits]

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@@ -5587,31 +5608,104 @@ MachineInstr *CombinerHelper::buildSDivUsingMul(MachineInstr &MI) const {
     // Calculate the multiplicative inverse modulo BW.
     // 2^W requires W + 1 bits, so we have to extend and then truncate.
     APInt Factor = Divisor.multiplicativeInverse();
-    Shifts.push_back(MIB.buildConstant(ScalarShiftAmtTy, Shift).getReg(0));
-    Factors.push_back(MIB.buildConstant(ScalarTy, Factor).getReg(0));
+    ExactShifts.push_back(MIB.buildConstant(ScalarShiftAmtTy, Shift).getReg(0));
+    ExactFactors.push_back(MIB.buildConstant(ScalarTy, Factor).getReg(0));
     return true;
   };
 
-  // Collect all magic values from the build vector.
+  if (MI.getFlag(MachineInstr::MIFlag::IsExact)) {
+    // Collect all magic values from the build vector.
+    bool Matched = matchUnaryPredicate(MRI, RHS, BuildExactSDIVPattern);
+    (void)Matched;
+    assert(Matched && "Expected unary predicate match to succeed");
+
+    Register Shift, Factor;
+    if (Ty.isVector()) {
+      Shift = MIB.buildBuildVector(ShiftAmtTy, ExactShifts).getReg(0);
+      Factor = MIB.buildBuildVector(Ty, ExactFactors).getReg(0);
+    } else {
+      Shift = ExactShifts[0];
+      Factor = ExactFactors[0];
+    }
+
+    Register Res = LHS;
+
+    if (UseSRA)
+      Res = MIB.buildAShr(Ty, Res, Shift, MachineInstr::IsExact).getReg(0);
+
+    return MIB.buildMul(Ty, Res, Factor);
+  }
+
+  SmallVector<Register, 16> MagicFactors, Factors, Shifts, ShiftMasks;
+
+  auto BuildSDIVPattern = [&](const Constant *C) {
+    auto *CI = cast<ConstantInt>(C);
+    const APInt &Divisor = CI->getValue();
+
+    SignedDivisionByConstantInfo magics =
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davemgreen wrote:

magics -> Magics

https://github.com/llvm/llvm-project/pull/146504