[llvm] [GlobalISel] fdiv to fmul transform (PR #144305)

[Jay Foad via llvm-commits]

================
@@ -6389,6 +6389,76 @@ bool CombinerHelper::matchCombineFMinMaxNaN(MachineInstr &MI,
   return MatchNaN(1) || MatchNaN(2);
 }
 
+// Combine multiple FDIVs with the same divisor into multiple FMULs by the
+// reciprocal.
+// E.g., (a / D; b / D;) -> (recip = 1.0 / D; a * recip; b * recip)
+bool CombinerHelper::matchRepeatedFPDivisor(
+    MachineInstr &MI, SmallVector<MachineInstr *> &MatchInfo) const {
+  assert(MI.getOpcode() == TargetOpcode::G_FDIV);
+  auto *MF = MI.getMF();
+  const TargetOptions &Options = MF->getTarget().Options;
+
+  Register X = MI.getOperand(1).getReg();
+  Register Y = MI.getOperand(2).getReg();
+
+  bool UnsafeMath = Options.UnsafeFPMath;
+  if (!UnsafeMath && !MI.getFlag(MachineInstr::MIFlag::FmArcp))
+    return false;
+
+  // Skip if current node is a reciprocal/fneg-reciprocal.
+  auto N0CFP = isConstantOrConstantSplatVectorFP(*MRI.getVRegDef(X), MRI);
+  if (N0CFP && (N0CFP->isExactlyValue(1.0) || N0CFP->isExactlyValue(-1.0)))
+    return false;
+
+  // Exit early if the target does not want this transform or if there can't
+  // possibly be enough uses of the divisor to make the transform worthwhile.
+  unsigned MinUses = getTargetLowering().combineRepeatedFPDivisors();
+
+  if (!MinUses)
+    return false;
+
+  // Find all FDIV users of the same divisor. Use a set because duplicates may
+  // be present in the user list. For the moment we limit all instructions to a
+  // single BB and use the first Instr in MatchInfo as the dominating position.
+  MatchInfo.push_back(&MI);
+  for (auto &U : MRI.use_nodbg_instructions(Y)) {
+    if (&U == &MI || U.getParent() != MI.getParent())
+      continue;
+    if (U.getOpcode() == TargetOpcode::G_FDIV &&
+        U.getOperand(2).getReg() == Y) {
+      // This division is eligible for optimization only if global unsafe math
+      // is enabled or if this division allows reciprocal formation.
+      if (UnsafeMath || U.getFlag(MachineInstr::MIFlag::FmArcp)) {
+        MatchInfo.push_back(&U);
+        if (dominates(U, *MatchInfo[0]))
+          std::swap(MatchInfo[0], MatchInfo.back());
----------------
jayfoad wrote:

I don't think you're handling the case where neither FDIV dominates the other. In that case, where would you insert the reciprocal instruction?

https://github.com/llvm/llvm-project/pull/144305