Let M68kMCCodeEmitter set Scratch size. (llvm#69898)

The Scratch buffer passed to getBinaryCodeForInst needs to be able to
hold any value returned by getMachineOpValue or other custom encoders.
It's better to let the caller of getBinaryCodeForInst set the size of
Scratch as it's impossible for VarLenCodeEmitterGen to know what the
smallest needed size is.

VarLenCodeEmitterGen now calculates its smallest needed Scratch bit
width based on the slice operations and zero extends Scratch if it's too
small. This only guarantees that Scratch has enough bits for the
generated code not for getMachineOpValue or custom encoders.

The smallest internal APInt representation uses one uint64_t word so
there is no point in using a smaller size.

Author: erikfjon

llvm/lib/Target/M68k/MCTargetDesc/M68kMCCodeEmitter.cpp

@@ -209,7 +209,7 @@ void M68kMCCodeEmitter::encodeInstruction(const MCInst &MI,
 
   // Try using the new method first.
   APInt EncodedInst(16, 0U);
-  APInt Scratch(16, 0U);
+  APInt Scratch(64, 0U); // One APInt word is enough.
   getBinaryCodeForInstr(MI, Fixups, EncodedInst, Scratch, STI);
 
   ArrayRef<uint64_t> Data(EncodedInst.getRawData(), EncodedInst.getNumWords());

llvm/test/TableGen/VarLenEncoder.td

@@ -65,7 +65,7 @@ def FOO32 : MyVarInst<MemOp32<"src">>;
 // CHECK: UINT64_C(46848), // FOO32
 
 // CHECK-LABEL: case ::FOO16: {
-// CHECK: Scratch = Scratch.zext(41);
+// CHECK: Scratch.getBitWidth() < 16
 // src.reg
 // CHECK: getMachineOpValue(MI, MI.getOperand(1), /*Pos=*/0, Scratch, Fixups, STI);
 // CHECK: Inst.insertBits(Scratch.extractBits(8, 0), 0);
@@ -83,7 +83,7 @@ def FOO32 : MyVarInst<MemOp32<"src">>;
 // CHECK: Inst.insertBits(Scratch.extractBits(2, 0), 39);
 
 // CHECK-LABEL: case ::FOO32: {
-// CHECK: Scratch = Scratch.zext(57);
+// CHECK: Scratch.getBitWidth() < 32
 // src.reg
 // CHECK: getMachineOpValue(MI, MI.getOperand(1), /*Pos=*/0, Scratch, Fixups, STI);
 // CHECK: Inst.insertBits(Scratch.extractBits(8, 0), 0);

llvm/utils/TableGen/VarLenCodeEmitterGen.cpp

@@ -60,6 +60,8 @@
 #include "llvm/TableGen/Error.h"
 #include "llvm/TableGen/Record.h"
 
+#include <algorithm>
+
 using namespace llvm;
 
 namespace {
@@ -445,20 +447,17 @@ std::string VarLenCodeEmitterGen::getInstructionCases(Record *R,
 std::string VarLenCodeEmitterGen::getInstructionCaseForEncoding(
     Record *R, AltEncodingTy Mode, const VarLenInst &VLI, CodeGenTarget &Target,
     int I) {
-  size_t BitWidth = VLI.size();
 
   CodeGenInstruction &CGI = Target.getInstruction(R);
 
   std::string Case;
   raw_string_ostream SS(Case);
-  // Resize the scratch buffer.
-  if (BitWidth && !VLI.isFixedValueOnly())
-    SS.indent(I) << "Scratch = Scratch.zext(" << BitWidth << ");\n";
   // Populate based value.
   SS.indent(I) << "Inst = getInstBits" << Modes[Mode] << "(opcode);\n";
 
   // Process each segment in VLI.
   size_t Offset = 0U;
+  unsigned HighScratchAccess = 0U;
   for (const auto &ES : VLI) {
     unsigned NumBits = ES.BitWidth;
     const Init *Val = ES.Value;
@@ -497,6 +496,8 @@ std::string VarLenCodeEmitterGen::getInstructionCaseForEncoding(
                    << "Scratch.extractBits(" << utostr(NumBits) << ", "
                    << utostr(LoBit) << ")"
                    << ", " << Offset << ");\n";
+
+      HighScratchAccess = std::max(HighScratchAccess, NumBits + LoBit);
     }
     Offset += NumBits;
   }
@@ -505,7 +506,16 @@ std::string VarLenCodeEmitterGen::getInstructionCaseForEncoding(
   if (!PostEmitter.empty())
     SS.indent(I) << "Inst = " << PostEmitter << "(MI, Inst, STI);\n";
 
-  return Case;
+  // Resize the scratch buffer if it's to small.
+  std::string ScratchResizeStr;
+  if (VLI.size() && !VLI.isFixedValueOnly()) {
+    raw_string_ostream RS(ScratchResizeStr);
+    RS.indent(I) << "if (Scratch.getBitWidth() < " << HighScratchAccess
+                 << ") { Scratch = Scratch.zext(" << HighScratchAccess
+                 << "); }\n";
+  }
+
+  return ScratchResizeStr + Case;
 }
 
 namespace llvm {