[PATCH] D93326: [MCInstrDesc] [TableGen] Reduce size of MCOperandInfo instances

[Paul C. Anagnostopoulos via Phabricator via llvm-commits]

Paul-C-Anagnostopoulos created this revision.
Paul-C-Anagnostopoulos added reviewers: lattner, nhaehnle, craig.topper, madhur13490.
Paul-C-Anagnostopoulos requested review of this revision.
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This patch reduces the size of the MCOperandInfo instances. The Constraints member is reduced to 16 bits, since it contains only two values of 5 bits each. This leaves room for one more constraint before the member needs to be enlarged to 32 bits.

The next step is to reduce the size of MCInstrDesc instances. But that means changing pointers to indexes, which might get some pushback.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D93326

Files:
  llvm/include/llvm/MC/MCInstrDesc.h
  llvm/utils/TableGen/InstrInfoEmitter.cpp


Index: llvm/utils/TableGen/InstrInfoEmitter.cpp
===================================================================
--- llvm/utils/TableGen/InstrInfoEmitter.cpp
+++ llvm/utils/TableGen/InstrInfoEmitter.cpp
@@ -182,11 +182,13 @@
       if (Constraint.isNone())
         Res += "0";
       else if (Constraint.isEarlyClobber())
-        Res += "(1 << MCOI::EARLY_CLOBBER)";
+        Res += "MCOI_EARLY_CLOBBER(1)";
+////        Res += "(1 << MCOI::EARLY_CLOBBER)";
       else {
         assert(Constraint.isTied());
-        Res += "((" + utostr(Constraint.getTiedOperand()) +
-                    " << 16) | (1 << MCOI::TIED_TO))";
+        Res += "MCOI_TIED_TO(" + utostr(Constraint.getTiedOperand()) + ")";
+////        Res += "((" + utostr(Constraint.getTiedOperand()) +
+////                    " << 16) | (1 << MCOI::TIED_TO))";
       }
 
       Result.push_back(Res);
Index: llvm/include/llvm/MC/MCInstrDesc.h
===================================================================
--- llvm/include/llvm/MC/MCInstrDesc.h
+++ llvm/include/llvm/MC/MCInstrDesc.h
@@ -27,12 +27,22 @@
 //===----------------------------------------------------------------------===//
 
 namespace MCOI {
-// Operand constraints
+/// Operand constraints. These are encoded in 16 bits with one of the
+/// low-order 3 bits specifying that a constraint is present and the
+/// corresponding high-order hex digit specifying the constraint value.
+/// This allows for a maximum of 3 constraints.
 enum OperandConstraint {
-  TIED_TO = 0,  // Must be allocated the same register as.
-  EARLY_CLOBBER // Operand is an early clobber register operand
+  TIED_TO = 0,  // Must be allocated the same register as specified value.
+  EARLY_CLOBBER // Operand is an early clobber register.
 };
 
+// Define a macro to produce each constraint value.
+#define MCOI_TIED_TO(op) \
+  ((1 << MCOI::TIED_TO) | ((op) << (4 + MCOI::TIED_TO * 4)))
+
+#define MCOI_EARLY_CLOBBER(bit) \
+  ((1 << MCOI::EARLY_CLOBBER) | ((bit) << (4 + MCOI::EARLY_CLOBBER * 4)))
+
 /// These are flags set on operands, but should be considered
 /// private, all access should go through the MCOperandInfo accessors.
 /// See the accessors for a description of what these are.
@@ -84,10 +94,9 @@
 
   /// Information about the type of the operand.
   uint8_t OperandType;
-  /// The lower 16 bits are used to specify which constraints are set.
-  /// The higher 16 bits are used to specify the value of constraints (4 bits
-  /// each).
-  uint32_t Constraints;
+
+  /// Operand constraints (see OperandConstraint enum).
+  uint16_t Constraints;
 
   /// Set if this operand is a pointer value and it requires a callback
   /// to look up its register class.
@@ -197,14 +206,14 @@
   const MCPhysReg *ImplicitDefs; // Registers implicitly defined by this instr
   const MCOperandInfo *OpInfo;   // 'NumOperands' entries about operands
 
-  /// Returns the value of the specific constraint if
-  /// it is set. Returns -1 if it is not set.
+  /// Returns the value of the specified operand constraint if
+  /// it is present. Returns -1 if it is not present.
   int getOperandConstraint(unsigned OpNum,
                            MCOI::OperandConstraint Constraint) const {
     if (OpNum < NumOperands &&
         (OpInfo[OpNum].Constraints & (1 << Constraint))) {
-      unsigned Pos = 16 + Constraint * 4;
-      return (int)(OpInfo[OpNum].Constraints >> Pos) & 0xf;
+      unsigned ValuePos = 4 + Constraint * 4;
+      return (int)(OpInfo[OpNum].Constraints >> ValuePos) & 0x0f;
     }
     return -1;
   }


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