[Clang][inlineasm] Add special support for "rm" output constraints

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -4962,6 +4962,11 @@ class TargetLowering : public TargetLoweringBase {
     /// Memory, Other, Unknown.
     TargetLowering::ConstraintType ConstraintType = TargetLowering::C_Unknown;
 
+    /// The register may be folded. This is used if the constraint is "rm",
+    /// where we prefer using a register, but can fall back to a memory slot
+    /// under register pressure.
+    bool MayFoldRegister = false;
+
     /// If this is the result output operand or a clobber, this is null,
     /// otherwise it is the incoming operand to the CallInst.  This gets
     /// modified as the asm is processed.

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -1018,7 +1018,8 @@ void RegsForValue::getCopyToRegs(SDValue Val, SelectionDAG &DAG,
 }
 
 void RegsForValue::AddInlineAsmOperands(InlineAsm::Kind Code, bool HasMatching,
-                                        unsigned MatchingIdx, const SDLoc &dl,
+                                        unsigned MatchingIdx,
+                                        bool MayFoldRegister, const SDLoc &dl,
                                         SelectionDAG &DAG,
                                         std::vector<SDValue> &Ops) const {
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
@@ -1034,7 +1035,9 @@ void RegsForValue::AddInlineAsmOperands(InlineAsm::Kind Code, bool HasMatching,
     // from the def.
     const MachineRegisterInfo &MRI = DAG.getMachineFunction().getRegInfo();
     const TargetRegisterClass *RC = MRI.getRegClass(Regs.front());
+
     Flag.setRegClass(RC->getID());
+    Flag.setRegMayBeFolded(MayFoldRegister);
   }
 
   SDValue Res = DAG.getTargetConstant(Flag, dl, MVT::i32);
@@ -9985,8 +9988,8 @@ void SelectionDAGBuilder::visitInlineAsm(const CallBase &Call,
         AsmNodeOperands.push_back(OpInfo.CallOperand);
       } else {
         // Otherwise, this outputs to a register (directly for C_Register /
-        // C_RegisterClass, and a target-defined fashion for
-        // C_Immediate/C_Other). Find a register that we can use.
+        // C_RegisterClass, and a target-defined fashion for C_Immediate /
+        // C_Other). Find a register that we can use.
         if (OpInfo.AssignedRegs.Regs.empty()) {
           emitInlineAsmError(
               Call, "couldn't allocate output register for constraint '" +
@@ -10002,7 +10005,8 @@ void SelectionDAGBuilder::visitInlineAsm(const CallBase &Call,
         OpInfo.AssignedRegs.AddInlineAsmOperands(
             OpInfo.isEarlyClobber ? InlineAsm::Kind::RegDefEarlyClobber
                                   : InlineAsm::Kind::RegDef,
-            false, 0, getCurSDLoc(), DAG, AsmNodeOperands);
+            false, 0, OpInfo.MayFoldRegister, getCurSDLoc(), DAG,
+            AsmNodeOperands);
       }
       break;
 
@@ -10044,9 +10048,9 @@ void SelectionDAGBuilder::visitInlineAsm(const CallBase &Call,
           SDLoc dl = getCurSDLoc();
           // Use the produced MatchedRegs object to
           MatchedRegs.getCopyToRegs(InOperandVal, DAG, dl, Chain, &Glue, &Call);
-          MatchedRegs.AddInlineAsmOperands(InlineAsm::Kind::RegUse, true,
-                                           OpInfo.getMatchedOperand(), dl, DAG,
-                                           AsmNodeOperands);
+          MatchedRegs.AddInlineAsmOperands(
+              InlineAsm::Kind::RegUse, true, OpInfo.getMatchedOperand(),
+              OpInfo.MayFoldRegister, dl, DAG, AsmNodeOperands);
           break;
         }
 
@@ -10178,16 +10182,17 @@ void SelectionDAGBuilder::visitInlineAsm(const CallBase &Call,
                                         &Call);
 
       OpInfo.AssignedRegs.AddInlineAsmOperands(InlineAsm::Kind::RegUse, false,
-                                               0, dl, DAG, AsmNodeOperands);
+                                               0, OpInfo.MayFoldRegister, dl,
+                                               DAG, AsmNodeOperands);
       break;
     }
     case InlineAsm::isClobber:
       // Add the clobbered value to the operand list, so that the register
       // allocator is aware that the physreg got clobbered.
       if (!OpInfo.AssignedRegs.Regs.empty())
         OpInfo.AssignedRegs.AddInlineAsmOperands(InlineAsm::Kind::Clobber,
-                                                 false, 0, getCurSDLoc(), DAG,
-                                                 AsmNodeOperands);
+                                                 false, 0, false, getCurSDLoc(),
+                                                 DAG, AsmNodeOperands);
       break;
     }
   }

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h

@@ -787,8 +787,9 @@ struct RegsForValue {
   /// code marker, matching input operand index (if applicable), and includes
   /// the number of values added into it.
   void AddInlineAsmOperands(InlineAsm::Kind Code, bool HasMatching,
-                            unsigned MatchingIdx, const SDLoc &dl,
-                            SelectionDAG &DAG, std::vector<SDValue> &Ops) const;
+                            unsigned MatchingIdx, bool MayFoldRegister,
+                            const SDLoc &dl, SelectionDAG &DAG,
+                            std::vector<SDValue> &Ops) const;
 
   /// Check if the total RegCount is greater than one.
   bool occupiesMultipleRegs() const {

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -21,6 +21,7 @@
 #include "llvm/CodeGen/MachineModuleInfoImpls.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/SelectionDAG.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
@@ -33,6 +34,7 @@
 #include "llvm/Support/KnownBits.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Target/TargetMachine.h"
+#include "llvm/TargetParser/Triple.h"
 #include <cctype>
 using namespace llvm;
 
@@ -5695,6 +5697,7 @@ TargetLowering::ParseConstraints(const DataLayout &DL,
   unsigned ResNo = 0; // ResNo - The result number of the next output.
   unsigned LabelNo = 0; // LabelNo - CallBr indirect dest number.
 
+  const Triple &T = getTargetMachine().getTargetTriple();
   for (InlineAsm::ConstraintInfo &CI : IA->ParseConstraints()) {
     ConstraintOperands.emplace_back(std::move(CI));
     AsmOperandInfo &OpInfo = ConstraintOperands.back();
@@ -5705,6 +5708,16 @@ TargetLowering::ParseConstraints(const DataLayout &DL,
 
     OpInfo.ConstraintVT = MVT::Other;
 
+    // Special treatment for all platforms (currently only x86) that can fold a
+    // register into a spill. This is used for the "rm" constraint, where we
+    // would vastly prefer to use 'r' over 'm', but can't because of LLVM's
+    // architecture picks the most "conservative" constraint to ensure that (in
+    // the case of "rm") register pressure cause bad things to happen.
+    if (T.isX86() && !OpInfo.hasMatchingInput() && OpInfo.Codes.size() == 2 &&
+        llvm::is_contained(OpInfo.Codes, "r") &&
+        llvm::is_contained(OpInfo.Codes, "m"))
+      OpInfo.MayFoldRegister = true;
+
     // Compute the value type for each operand.
     switch (OpInfo.Type) {
     case InlineAsm::isOutput:
@@ -5981,14 +5994,29 @@ TargetLowering::ConstraintWeight
 ///  1) If there is an 'other' constraint, and if the operand is valid for
 ///     that constraint, use it.  This makes us take advantage of 'i'
 ///     constraints when available.
-///  2) Otherwise, pick the most general constraint present.  This prefers
+///  2) Special processing is done for the "rm" constraint. If specified, we
+///     opt for the 'r' constraint, but mark the operand as being "foldable."
+///     In the face of register exhaustion, the register allocator is free to
+///     choose to use a stack slot. This only applies to the greedy and default
+///     register allocators. FIXME: Support other allocators (fast?).
+///  3) Otherwise, pick the most general constraint present.  This prefers
 ///     'm' over 'r', for example.
 ///
 TargetLowering::ConstraintGroup TargetLowering::getConstraintPreferences(
     TargetLowering::AsmOperandInfo &OpInfo) const {
   ConstraintGroup Ret;
 
   Ret.reserve(OpInfo.Codes.size());
+
+  // If we can fold the register (i.e. it has an "rm" constraint), opt for the
+  // 'r' constraint, and allow the register allocator to spill if need be.
+  // Applies only to the greedy and default register allocators.
+  if (OpInfo.MayFoldRegister) {
+    Ret.emplace_back(ConstraintPair("r", getConstraintType("r")));
+    Ret.emplace_back(ConstraintPair("m", getConstraintType("m")));
+    return Ret;
+  }
+
   for (StringRef Code : OpInfo.Codes) {
     TargetLowering::ConstraintType CType = getConstraintType(Code);