[LLVM][MC][DecoderEmitter] Add support to specialize decoder per bitwidth (#154865)

This change adds an option to specialize decoders per bitwidth, which
can help reduce the (compiled) code size of the decoder code.

**Current state**:
Currently, the code generated by the decoder emitter consists of two key
functions: `decodeInstruction` which is the entry point into the
generated code and `decodeToMCInst` which is invoked when a decode op is
reached while traversing through the decoder table. Both functions are
templated on `InsnType` which is the raw instruction bits that are
supplied to `decodeInstruction`.

Several backends call `decodeInstruction` with different `InsnType`
types, leading to several template instantiations of these functions in
the final code. As an example, AMDGPU instantiates this function with
type `DecoderUInt128` type for decoding 96/128-bit instructions,
`uint64_t` for decoding 64-bit instructions, and `uint32_t` for decoding
32-bit instructions. Since there is just one `decodeToMCInst` in the
generated code, it has code that handles decoding for *all* instruction
sizes. However, the decoders emitted for different instructions sizes
rarely have any intersection with each other. That means, in the AMDGPU
case, the instantiation with InsnType == DecoderUInt128 has decoder code
for 32/64-bit instructions that is *never exercised*. Conversely, the
instantiation with InsnType == uint64_t has decoder code for
128/96/32-bit instructions that is never exercised. This leads to
unnecessary dead code in the generated disassembler binary (that the
compiler cannot eliminate by itself).

**New state**:
With this change, we introduce an option
`specialize-decoders-per-bitwidth`. Under this mode, the DecoderEmitter
will generate several versions of `decodeToMCInst` function, one for
each bitwidth. The code is still templated, but will require backends to
specify, for each `InsnType` used, the bitwidth of the instruction that
the type is used to represent using a type-trait `InsnBitWidth`. This
will enable the templated code to choose the right variant of
`decodeToMCInst`. Under this mode, a particular instantiation will only
end up instantiating a single variant of `decodeToMCInst` generated and
that will include only those decoders that are applicable to a single
bitwidth, resulting in elimination of the code duplication through
instantiation and a reduction in code size.

Additionally, under this mode, decoders are uniqued only within a given
bitwidth (as opposed to across all bitwidths without this option), so
the decoder index values assigned are smaller, and consume less bytes in
their ULEB128 encoding. As a result, the generated decoder tables can
also reduce in size.

Adopt this feature for the AMDGPU and RISCV backend. In a release build,
this results in a net 55% reduction in the .text size of
libLLVMAMDGPUDisassembler.so and a 5% reduction in the .rodata size. For
RISCV, which today uses a single `uint64_t` type, this results in a 3.7%
increase in code size (expected as we instantiate the code 3 times now).

Actual measured sizes are as follows:
```
Baseline commit: 72c04bb
Configuration: Ubuntu clang version 18.1.3, release build with asserts disabled.
 
AMDGPU        Before       After      Change
======================================================
.text         612327       275607     55% reduction
.rodata       369728       351336      5% reduction          

RISCV:
======================================================
.text          47407       49187      3.7% increase   
.rodata        35768       35839      0.1% increase
```

Author: jurahul

llvm/include/llvm/MC/MCDecoder.h

@@ -12,6 +12,7 @@
 
 #include "llvm/MC/MCDisassembler/MCDisassembler.h"
 #include "llvm/Support/MathExtras.h"
+#include <bitset>
 #include <cassert>
 
 namespace llvm::MCD {
@@ -48,6 +49,15 @@ fieldFromInstruction(const InsnType &Insn, unsigned StartBit,
   return Insn.extractBitsAsZExtValue(NumBits, StartBit);
 }
 
+template <size_t N>
+uint64_t fieldFromInstruction(const std::bitset<N> &Insn, unsigned StartBit,
+                              unsigned NumBits) {
+  assert(StartBit + NumBits <= N && "Instruction field out of bounds!");
+  assert(NumBits <= 64 && "Cannot support >64-bit extractions!");
+  const std::bitset<N> Mask(maskTrailingOnes<uint64_t>(NumBits));
+  return ((Insn >> StartBit) & Mask).to_ullong();
+}
+
 // Helper function for inserting bits extracted from an encoded instruction into
 // an integer-typed field.
 template <typename IntType>
@@ -62,6 +72,13 @@ insertBits(IntType &field, IntType bits, unsigned startBit, unsigned numBits) {
   field |= bits << startBit;
 }
 
+// InsnBitWidth is essentially a type trait used by the decoder emitter to query
+// the supported bitwidth for a given type. But default, the value is 0, making
+// it an invalid type for use as `InsnType` when instantiating the decoder.
+// Individual targets are expected to provide specializations for these based
+// on their usage.
+template <typename T> static constexpr uint32_t InsnBitWidth = 0;
+
 } // namespace llvm::MCD
 
 #endif // LLVM_MC_MCDECODER_H

llvm/lib/Target/AMDGPU/CMakeLists.txt

@@ -6,7 +6,8 @@ tablegen(LLVM AMDGPUGenAsmMatcher.inc -gen-asm-matcher)
 tablegen(LLVM AMDGPUGenAsmWriter.inc -gen-asm-writer)
 tablegen(LLVM AMDGPUGenCallingConv.inc -gen-callingconv)
 tablegen(LLVM AMDGPUGenDAGISel.inc -gen-dag-isel)
-tablegen(LLVM AMDGPUGenDisassemblerTables.inc -gen-disassembler)
+tablegen(LLVM AMDGPUGenDisassemblerTables.inc -gen-disassembler
+              --specialize-decoders-per-bitwidth)
 tablegen(LLVM AMDGPUGenInstrInfo.inc -gen-instr-info)
 tablegen(LLVM AMDGPUGenMCCodeEmitter.inc -gen-emitter)
 tablegen(LLVM AMDGPUGenMCPseudoLowering.inc -gen-pseudo-lowering)

llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp

@@ -38,6 +38,7 @@
 #include "llvm/Support/Compiler.h"
 
 using namespace llvm;
+using namespace llvm::MCD;
 
 #define DEBUG_TYPE "amdgpu-disassembler"
 
@@ -446,6 +447,14 @@ static DecodeStatus decodeVersionImm(MCInst &Inst, unsigned Imm,
 
 #include "AMDGPUGenDisassemblerTables.inc"
 
+// Define bitwidths for various types used to instantiate the decoder.
+template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint32_t> = 32;
+template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint64_t> = 64;
+template <>
+static constexpr uint32_t llvm::MCD::InsnBitWidth<std::bitset<96>> = 96;
+template <>
+static constexpr uint32_t llvm::MCD::InsnBitWidth<std::bitset<128>> = 128;
+
 //===----------------------------------------------------------------------===//
 //
 //===----------------------------------------------------------------------===//
@@ -498,26 +507,24 @@ template <typename T> static inline T eatBytes(ArrayRef<uint8_t>& Bytes) {
   return Res;
 }
 
-static inline DecoderUInt128 eat12Bytes(ArrayRef<uint8_t> &Bytes) {
+static inline std::bitset<96> eat12Bytes(ArrayRef<uint8_t> &Bytes) {
+  using namespace llvm::support::endian;
   assert(Bytes.size() >= 12);
-  uint64_t Lo =
-      support::endian::read<uint64_t, llvm::endianness::little>(Bytes.data());
+  std::bitset<96> Lo(read<uint64_t, endianness::little>(Bytes.data()));
   Bytes = Bytes.slice(8);
-  uint64_t Hi =
-      support::endian::read<uint32_t, llvm::endianness::little>(Bytes.data());
+  std::bitset<96> Hi(read<uint32_t, endianness::little>(Bytes.data()));
   Bytes = Bytes.slice(4);
-  return DecoderUInt128(Lo, Hi);
+  return (Hi << 64) | Lo;
 }
 
-static inline DecoderUInt128 eat16Bytes(ArrayRef<uint8_t> &Bytes) {
+static inline std::bitset<128> eat16Bytes(ArrayRef<uint8_t> &Bytes) {
+  using namespace llvm::support::endian;
   assert(Bytes.size() >= 16);
-  uint64_t Lo =
-      support::endian::read<uint64_t, llvm::endianness::little>(Bytes.data());
+  std::bitset<128> Lo(read<uint64_t, endianness::little>(Bytes.data()));
   Bytes = Bytes.slice(8);
-  uint64_t Hi =
-      support::endian::read<uint64_t, llvm::endianness::little>(Bytes.data());
+  std::bitset<128> Hi(read<uint64_t, endianness::little>(Bytes.data()));
   Bytes = Bytes.slice(8);
-  return DecoderUInt128(Lo, Hi);
+  return (Hi << 64) | Lo;
 }
 
 void AMDGPUDisassembler::decodeImmOperands(MCInst &MI,
@@ -600,14 +607,14 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
     // Try to decode DPP and SDWA first to solve conflict with VOP1 and VOP2
     // encodings
     if (isGFX1250() && Bytes.size() >= 16) {
-      DecoderUInt128 DecW = eat16Bytes(Bytes);
+      std::bitset<128> DecW = eat16Bytes(Bytes);
       if (tryDecodeInst(DecoderTableGFX1250128, MI, DecW, Address, CS))
         break;
       Bytes = Bytes_.slice(0, MaxInstBytesNum);
     }
 
     if (isGFX11Plus() && Bytes.size() >= 12) {
-      DecoderUInt128 DecW = eat12Bytes(Bytes);
+      std::bitset<96> DecW = eat12Bytes(Bytes);
 
       if (isGFX11() &&
           tryDecodeInst(DecoderTableGFX1196, DecoderTableGFX11_FAKE1696, MI,
@@ -642,7 +649,7 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
 
     } else if (Bytes.size() >= 16 &&
                STI.hasFeature(AMDGPU::FeatureGFX950Insts)) {
-      DecoderUInt128 DecW = eat16Bytes(Bytes);
+      std::bitset<128> DecW = eat16Bytes(Bytes);
       if (tryDecodeInst(DecoderTableGFX940128, MI, DecW, Address, CS))
         break;
 

llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.h

@@ -32,44 +32,6 @@ class MCOperand;
 class MCSubtargetInfo;
 class Twine;
 
-// Exposes an interface expected by autogenerated code in
-// FixedLenDecoderEmitter
-class DecoderUInt128 {
-private:
-  uint64_t Lo = 0;
-  uint64_t Hi = 0;
-
-public:
-  DecoderUInt128() = default;
-  DecoderUInt128(uint64_t Lo, uint64_t Hi = 0) : Lo(Lo), Hi(Hi) {}
-  operator bool() const { return Lo || Hi; }
-  uint64_t extractBitsAsZExtValue(unsigned NumBits,
-                                  unsigned BitPosition) const {
-    assert(NumBits && NumBits <= 64);
-    assert(BitPosition < 128);
-    uint64_t Val;
-    if (BitPosition < 64)
-      Val = Lo >> BitPosition | Hi << 1 << (63 - BitPosition);
-    else
-      Val = Hi >> (BitPosition - 64);
-    return Val & ((uint64_t(2) << (NumBits - 1)) - 1);
-  }
-  DecoderUInt128 operator&(const DecoderUInt128 &RHS) const {
-    return DecoderUInt128(Lo & RHS.Lo, Hi & RHS.Hi);
-  }
-  DecoderUInt128 operator&(const uint64_t &RHS) const {
-    return *this & DecoderUInt128(RHS);
-  }
-  DecoderUInt128 operator~() const { return DecoderUInt128(~Lo, ~Hi); }
-  bool operator==(const DecoderUInt128 &RHS) {
-    return Lo == RHS.Lo && Hi == RHS.Hi;
-  }
-  bool operator!=(const DecoderUInt128 &RHS) {
-    return Lo != RHS.Lo || Hi != RHS.Hi;
-  }
-  bool operator!=(const int &RHS) { return *this != DecoderUInt128(RHS); }
-};
-
 //===----------------------------------------------------------------------===//
 // AMDGPUDisassembler
 //===----------------------------------------------------------------------===//

llvm/lib/Target/RISCV/CMakeLists.txt

@@ -7,7 +7,8 @@ tablegen(LLVM RISCVGenAsmWriter.inc -gen-asm-writer)
 tablegen(LLVM RISCVGenCompressInstEmitter.inc -gen-compress-inst-emitter)
 tablegen(LLVM RISCVGenMacroFusion.inc -gen-macro-fusion-pred)
 tablegen(LLVM RISCVGenDAGISel.inc -gen-dag-isel)
-tablegen(LLVM RISCVGenDisassemblerTables.inc -gen-disassembler)
+tablegen(LLVM RISCVGenDisassemblerTables.inc -gen-disassembler
+              --specialize-decoders-per-bitwidth)
 tablegen(LLVM RISCVGenInstrInfo.inc -gen-instr-info)
 tablegen(LLVM RISCVGenMCCodeEmitter.inc -gen-emitter)
 tablegen(LLVM RISCVGenMCPseudoLowering.inc -gen-pseudo-lowering)

llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp

@@ -558,7 +558,7 @@ static DecodeStatus decodeXqccmpRlistS0(MCInst &Inst, uint32_t Imm,
   return decodeZcmpRlist(Inst, Imm, Address, Decoder);
 }
 
-static DecodeStatus decodeCSSPushPopchk(MCInst &Inst, uint32_t Insn,
+static DecodeStatus decodeCSSPushPopchk(MCInst &Inst, uint16_t Insn,
                                         uint64_t Address,
                                         const MCDisassembler *Decoder) {
   uint32_t Rs1 = fieldFromInstruction(Insn, 7, 5);
@@ -701,6 +701,12 @@ static constexpr DecoderListEntry DecoderList32[]{
     {DecoderTableZdinxRV32Only32, {}, "RV32-only Zdinx (Double in Integer)"},
 };
 
+// Define bitwidths for various types used to instantiate the decoder.
+template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint16_t> = 16;
+template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint32_t> = 32;
+// Use uint64_t to represent 48 bit instructions.
+template <> static constexpr uint32_t llvm::MCD::InsnBitWidth<uint64_t> = 48;
+
 DecodeStatus RISCVDisassembler::getInstruction32(MCInst &MI, uint64_t &Size,
                                                  ArrayRef<uint8_t> Bytes,
                                                  uint64_t Address,
@@ -711,9 +717,7 @@ DecodeStatus RISCVDisassembler::getInstruction32(MCInst &MI, uint64_t &Size,
   }
   Size = 4;
 
-  // Use uint64_t to match getInstruction48. decodeInstruction is templated
-  // on the Insn type.
-  uint64_t Insn = support::endian::read32le(Bytes.data());
+  uint32_t Insn = support::endian::read32le(Bytes.data());
 
   for (const DecoderListEntry &Entry : DecoderList32) {
     if (!Entry.haveContainedFeatures(STI.getFeatureBits()))
@@ -759,9 +763,7 @@ DecodeStatus RISCVDisassembler::getInstruction16(MCInst &MI, uint64_t &Size,
   }
   Size = 2;
 
-  // Use uint64_t to match getInstruction48. decodeInstruction is templated
-  // on the Insn type.
-  uint64_t Insn = support::endian::read16le(Bytes.data());
+  uint16_t Insn = support::endian::read16le(Bytes.data());
 
   for (const DecoderListEntry &Entry : DecoderList16) {
     if (!Entry.haveContainedFeatures(STI.getFeatureBits()))