[llvm] [AMDGPU] [GlobalIsel] Combine Fmul with Select into ldexp instruction. (PR #120104)

[Jay Foad via llvm-commits]

================
@@ -445,3 +452,69 @@ void AMDGPUCombinerHelper::applyExpandPromotedF16FMed3(MachineInstr &MI,
   Builder.buildFMinNumIEEE(MI.getOperand(0), B1, C1);
   MI.eraseFromParent();
 }
+
+bool AMDGPUCombinerHelper::matchCombineFmulWithSelectToFldexp(
+    MachineInstr &MI, MachineInstr &Sel,
+    std::function<void(MachineIRBuilder &)> &MatchInfo) {
+  assert(MI.getOpcode() == TargetOpcode::G_FMUL);
+  assert(Sel.getOpcode() == TargetOpcode::G_SELECT);
+  assert(MI.getOperand(2).getReg() == Sel.getOperand(0).getReg());
+
+  Register Dst = MI.getOperand(0).getReg();
+  LLT DestTy = MRI.getType(Dst);
+  LLT ScalarDestTy = DestTy.getScalarType();
+
+  if ((ScalarDestTy == LLT::float64() || ScalarDestTy == LLT::float32() ||
+       ScalarDestTy == LLT::float16()) &&
+      (MRI.hasOneNonDBGUse(Sel.getOperand(0).getReg()))) {
+    Register SelectCondReg = Sel.getOperand(1).getReg();
+    MachineInstr *SelectTrue = MRI.getVRegDef(Sel.getOperand(2).getReg());
+    MachineInstr *SelectFalse = MRI.getVRegDef(Sel.getOperand(3).getReg());
+
+    const auto SelectTrueVal =
+        isConstantOrConstantSplatVectorFP(*SelectTrue, MRI);
+    if (!SelectTrueVal)
+      return false;
+    const auto SelectFalseVal =
+        isConstantOrConstantSplatVectorFP(*SelectFalse, MRI);
+    if (!SelectFalseVal)
+      return false;
+
+    if (SelectTrueVal->isNegative() != SelectFalseVal->isNegative())
+      return false;
+
+    // For f32, only non-inline constants should be transformed.
+    if (ScalarDestTy == LLT::float32() &&
+        TII.isInlineConstant(*SelectTrueVal) &&
+        TII.isInlineConstant(*SelectFalseVal))
+      return false;
+
+    int SelectTrueLog2Val = SelectTrueVal->getExactLog2Abs();
+    if (SelectTrueLog2Val == INT_MIN)
+      return false;
+    int SelectFalseLog2Val = SelectFalseVal->getExactLog2Abs();
+    if (SelectFalseLog2Val == INT_MIN)
+      return false;
+
+    MatchInfo = [=, &MI](MachineIRBuilder &Builder) {
+      LLT IntDestTy = DestTy.changeElementType(LLT::scalar(32));
+      auto NewSel = Builder.buildSelect(
+          IntDestTy, SelectCondReg,
+          Builder.buildConstant(IntDestTy, SelectTrueLog2Val),
+          Builder.buildConstant(IntDestTy, SelectFalseLog2Val));
+
+      Register XReg = MI.getOperand(1).getReg();
+      if (SelectTrueVal->isNegative()) {
+        auto NegX =
+            Builder.buildFNeg(DestTy, XReg, MRI.getVRegDef(XReg)->getFlags());
+        Builder.buildFLdexp(Dst, NegX, NewSel, MI.getFlags());
+      } else {
+        Builder.buildFLdexp(Dst, XReg, NewSel, MI.getFlags());
+      }
+    };
+
----------------
jayfoad wrote:

OK, thanks.

https://github.com/llvm/llvm-project/pull/120104