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

[Vikash Gupta via llvm-commits]

================
@@ -445,3 +445,75 @@ void AMDGPUCombinerHelper::applyExpandPromotedF16FMed3(MachineInstr &MI,
   Builder.buildFMinNumIEEE(MI.getOperand(0), B1, C1);
   MI.eraseFromParent();
 }
+
+bool AMDGPUCombinerHelper::matchCombineFmulWithSelectToLdexp(
+    MachineInstr &MI, MachineInstr &Sel,
+    std::function<void(MachineIRBuilder &)> &MatchInfo) {
+  assert(MI.getOpcode() == TargetOpcode::G_FMUL);
+  assert(Sel.getOpcode() == TargetOpcode::G_SELECT);
+
+  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 SelectCond = Sel.getOperand(1).getReg();
+    Register SelectTrue = Sel.getOperand(2).getReg();
+    Register SelectFalse = Sel.getOperand(3).getReg();
+
+    const auto SelectTrueCst =
+        DestTy.isVector()
+            ? getFConstantSplat(SelectTrue, MRI, /* allowUndef */ true)
+            : getFConstantVRegValWithLookThrough(SelectTrue, MRI);
+    if (!SelectTrueCst)
+      return false;
+    const auto SelectFalseCst =
+        DestTy.isVector()
+            ? getFConstantSplat(SelectFalse, MRI, /* allowUndef */ true)
+            : getFConstantVRegValWithLookThrough(SelectFalse, MRI);
----------------
vg0204 wrote:

> After looking into isConstOrConstSplatFP's counter part for integer in globalIsel i.e. **_isConstantOrConstantSplatVector_**, what I wrote can be considered the equivalent, so if it requires, I can turn it into a utility function as isConstOrConstSplatFP for globalIsel, if you can review that!!

I think would be this better as separate PR as well!




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