[llvm] [TargetLowering] Deduplicate choosing InlineAsm constraint between ISels (PR #67057)

[via llvm-commits]

================
@@ -5812,59 +5820,50 @@ TargetLowering::ConstraintWeight
 ///  2) Otherwise, pick the most general constraint present.  This prefers
 ///     'm' over 'r', for example.
 ///
-static void ChooseConstraint(TargetLowering::AsmOperandInfo &OpInfo,
-                             const TargetLowering &TLI,
-                             SDValue Op, SelectionDAG *DAG) {
-  assert(OpInfo.Codes.size() > 1 && "Doesn't have multiple constraint options");
-  unsigned BestIdx = 0;
-  TargetLowering::ConstraintType BestType = TargetLowering::C_Unknown;
-  int BestGenerality = -1;
+TargetLowering::ConstraintGroup TargetLowering::getConstraintPreferences(
+    TargetLowering::AsmOperandInfo &OpInfo) const {
+  ConstraintGroup Ret;
 
-  // Loop over the options, keeping track of the most general one.
-  for (unsigned i = 0, e = OpInfo.Codes.size(); i != e; ++i) {
-    TargetLowering::ConstraintType CType =
-      TLI.getConstraintType(OpInfo.Codes[i]);
+  Ret.reserve(OpInfo.Codes.size());
+  for (StringRef Code : OpInfo.Codes) {
+    TargetLowering::ConstraintType CType = getConstraintType(Code);
 
     // Indirect 'other' or 'immediate' constraints are not allowed.
     if (OpInfo.isIndirect && !(CType == TargetLowering::C_Memory ||
                                CType == TargetLowering::C_Register ||
                                CType == TargetLowering::C_RegisterClass))
       continue;
 
-    // If this is an 'other' or 'immediate' constraint, see if the operand is
-    // valid for it. For example, on X86 we might have an 'rI' constraint. If
-    // the operand is an integer in the range [0..31] we want to use I (saving a
-    // load of a register), otherwise we must use 'r'.
-    if ((CType == TargetLowering::C_Other ||
-         CType == TargetLowering::C_Immediate) && Op.getNode()) {
-      assert(OpInfo.Codes[i].size() == 1 &&
-             "Unhandled multi-letter 'other' constraint");
-      std::vector<SDValue> ResultOps;
-      TLI.LowerAsmOperandForConstraint(Op, OpInfo.Codes[i],
-                                       ResultOps, *DAG);
-      if (!ResultOps.empty()) {
-        BestType = CType;
-        BestIdx = i;
-        break;
-      }
-    }
-
     // Things with matching constraints can only be registers, per gcc
     // documentation.  This mainly affects "g" constraints.
     if (CType == TargetLowering::C_Memory && OpInfo.hasMatchingInput())
       continue;
 
-    // This constraint letter is more general than the previous one, use it.
-    int Generality = getConstraintGenerality(CType);
-    if (Generality > BestGenerality) {
-      BestType = CType;
-      BestIdx = i;
-      BestGenerality = Generality;
-    }
+    Ret.emplace_back(Code, CType);
   }
 
-  OpInfo.ConstraintCode = OpInfo.Codes[BestIdx];
-  OpInfo.ConstraintType = BestType;
+  std::sort(Ret.begin(), Ret.end(), [](ConstraintPair a, ConstraintPair b) {
+    return getConstraintPiority(a.second) > getConstraintPiority(b.second);
+  });
+
+  return Ret;
+}
+
+// If we have an immediate, see if we can lower it. Return true if we can, or
+// don't have an immediate, false otherwise.
+static bool lowerImmediateIfPossible(TargetLowering::ConstraintPair &P,
----------------
qcolombet wrote:

I find it strange to have `true` when the input is not an immediate.
I guess the rationale hinge on the `IfPossible` part of the function name.

How does the code look like if you were to return a `std::optional<bool>` and return `none` for no immediate?

https://github.com/llvm/llvm-project/pull/67057