[llvm-exegesis] Allow benchmarking arbitrary code snippets.

[courbet]

Summary

This is a step towards fixing PR38048.

Note that right now the measurements are given per instruction. We'll
need to give measurements a per code snippet and update the analysis (PR38731).

[courbet]

Change diff title.

[gchatelet]

tools/llvm-exegesis/lib/BenchmarkRunner.cpp

39 ↗

(On Diff #165304)

while(Code.size() < (BC.size() + MinInstruction))

tools/llvm-exegesis/llvm-exegesis.cpp

108 ↗

(On Diff #165304)

Can we make this more explicit?

const size_t CountSetFlags =
 (OpcodeName.empty() ? 0 : 1) +
 (OpcodeIndex == 0 ? 0 : 1) +
 (SnippetsFile.empty() ? 0 : 1);
if(CountSetFlags != 1)
   ...

159 ↗

(On Diff #165304)

newline before

161 ↗

(On Diff #165304)

A streamer -> An MCStreamer

162 ↗

(On Diff #165304)

which -> with

213 ↗

(On Diff #165304)

AFAIK llvm does not provide any tool to do it. Not sure if it's worth the addition though.

231 ↗

(On Diff #165304)

std::string Filename -> llvm::StringRef Filename ?

236 ↗

(On Diff #165304)

Why pass the filename/message in the BenchmarkFailure and remove the llvm::errs line?

254 ↗

(On Diff #165304)

Add a comment explaining what 0 is in this context.

[courbet]

Address comments.

[courbet]

rebase

[courbet]

Rebase.

[courbet]

update doc

[courbet]

rebase

[gchatelet]

tools/llvm-exegesis/llvm-exegesis.cpp

192 ↗

(On Diff #166291)

Add comments (or variables) to explain the arguments

194 ↗

(On Diff #166291)

Maybe we want to crash here?

198 ↗

(On Diff #166291)

explain Value.size() * 4

254 ↗

(On Diff #165304)

Not done.

[courbet]

Address comments.

[courbet]

Add more comments.

[courbet]

rely on implicit default asm dialect

[courbet]

add missing stack setup for EFLAGS.

llvm/trunk/docs/CommandGuide/llvm-exegesis.rst

 instruction.
 The code snippet is jitted and executed on the host subtarget. The time taken
 (resp. resource usage) is measured using hardware performance counters. The
 result is printed out as YAML to the standard output.
 
 The main goal of this tool is to automatically (in)validate the LLVM's TableDef
 scheduling models. To that end, we also provide analysis of the results.
 
-EXAMPLES: benchmarking
-----------------------
+:program:`llvm-exegesis` can also benchmark arbitrary user-provided code
+snippets.
+
+EXAMPLE 1: benchmarking instructions
+------------------------------------
 
 Assume you have an X86-64 machine. To measure the latency of a single
 instruction, run:
 
 .. code-block:: bash
 
     $ llvm-exegesis -mode=latency -opcode-name=ADD64rr
 
.. code-block:: bash
   readonly INSTRUCTIONS=$(($(grep INSTRUCTION_LIST_END build/lib/Target/X86/X86GenInstrInfo.inc | cut -f2 -d=) - 1))
   for INSTRUCTION in $(seq 1 ${INSTRUCTIONS});
   do
     ./build/bin/llvm-exegesis -mode=latency -opcode-index=${INSTRUCTION} | sed -n '/---/,$p'
   done
 
 FIXME: Provide an :program:`llvm-exegesis` option to test all instructions.
 
-EXAMPLES: analysis
-----------------------
+
+EXAMPLE 2: benchmarking a custom code snippet
+---------------------------------------------
+
+To measure the latency/uops of a custom piece of code, you can specify the
+`snippets-file` option (`-` reads from standard input).
+
+.. code-block:: bash
+
+    $ echo "vzeroupper" | llvm-exegesis -mode=uops -snippets-file=-
+
+Real-life code snippets typically depend on registers or memory.
+:program:`llvm-exegesis` checks the liveliness of registers (i.e. any register
+use has a corresponding def or is a "live in"). If your code depends on the
+value of some registers, you have two options:
+  - Mark the register as requiring a definition. :program:`llvm-exegesis` will
+    automatically assign a value to the register. This can be done using the
+    directive `LLVM-EXEGESIS-DEFREG <reg name> <hex_value>`, where `<hex_value>`
+    is a bit pattern used to fill `<reg_name>`. If `<hex_value>` is smaller than
+    the register width, it will be sign-extended.
+  - Mark the register as a "live in". :program:`llvm-exegesis` will benchmark
+    using whatever value was in this registers on entry. This can be done using
+    the directive `LLVM-EXEGESIS-LIVEIN <reg name>`.
+
+For example, the following code snippet depends on the values of XMM1 (which
+will be set by the tool) and the memory buffer passed in RDI (live in).
+
+.. code-block:: none
+
+  # LLVM-EXEGESIS-LIVEIN RDI
+  # LLVM-EXEGESIS-DEFREG XMM1 42
+  vmulps  (%rdi), %xmm1, %xmm2
+  vhaddps %xmm2, %xmm2, %xmm3
+  addq $0x10, %rdi
+
+
+EXAMPLE 3: analysis
+-------------------
 
 Assuming you have a set of benchmarked instructions (either latency or uops) as
 YAML in file `/tmp/benchmarks.yaml`, you can analyze the results using the
 following command:
 
 .. code-block:: bash
 
     $ llvm-exegesis -mode=analysis \

 -------
 
 .. option:: -help
 
  Print a summary of command line options.
 
 .. option:: -opcode-index=<LLVM opcode index>
 
- Specify the opcode to measure, by index.
- Either `opcode-index` or `opcode-name` must be set.
+ Specify the opcode to measure, by index. See example 1 for details.
+ Either `opcode-index`, `opcode-name` or `snippets-file` must be set.
 
 .. option:: -opcode-name=<LLVM opcode name>
 
- Specify the opcode to measure, by name.
- Either `opcode-index` or `opcode-name` must be set.
+ Specify the opcode to measure, by name. See example 1 for details.
+ Either `opcode-index`, `opcode-name` or `snippets-file` must be set.
+
+ .. option:: -snippets-file=<filename>
+
+  Specify the custom code snippet to measure. See example 2 for details.
+  Either `opcode-index`, `opcode-name` or `snippets-file` must be set.
 
 .. option:: -mode=[latency|uops|analysis]
 
  Specify the run mode.
 
 .. option:: -num-repetitions=<Number of repetition>
 
  Specify the number of repetitions of the asm snippet.

llvm/trunk/tools/llvm-exegesis/lib/Assembler.h

 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_TOOLS_LLVM_EXEGESIS_ASSEMBLER_H
 #define LLVM_TOOLS_LLVM_EXEGESIS_ASSEMBLER_H
 
 #include <memory>
 
+#include "BenchmarkCode.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/ExecutionEngine/ExecutionEngine.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
 #include "llvm/MC/MCInst.h"
 #include "llvm/Object/Binary.h"
 #include "llvm/Object/ObjectFile.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetMachine.h"
 
 namespace exegesis {
 
 class ExegesisTarget;
 
 // Gather the set of reserved registers (depends on function's calling
 // convention and target machine).
 llvm::BitVector getFunctionReservedRegs(const llvm::TargetMachine &TM);
 
-// A simple object storing the value for a particular register.
-struct RegisterValue {
-  unsigned Register;
-  llvm::APInt Value;
-};
-
 // Creates a temporary `void foo(char*)` function containing the provided
 // Instructions. Runs a set of llvm Passes to provide correct prologue and
 // epilogue. Once the MachineFunction is ready, it is assembled for TM to
 // AsmStream, the temporary function is eventually discarded.
 void assembleToStream(const ExegesisTarget &ET,
                       std::unique_ptr<llvm::LLVMTargetMachine> TM,
                       llvm::ArrayRef<unsigned> LiveIns,
                       llvm::ArrayRef<RegisterValue> RegisterInitialValues,

llvm/trunk/tools/llvm-exegesis/lib/BenchmarkCode.h

 //===-- BenchmarkCode.h -----------------------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_TOOLS_LLVM_EXEGESIS_BENCHMARKCODE_H
 #define LLVM_TOOLS_LLVM_EXEGESIS_BENCHMARKCODE_H
 
+#include "llvm/ADT/APInt.h"
 #include "llvm/MC/MCInst.h"
 #include <string>
 #include <vector>
 
 namespace exegesis {
 
+// A simple object storing the value for a particular register.
+struct RegisterValue {
+  unsigned Register;
+  llvm::APInt Value;
+};
+
 // A collection of instructions that are to be assembled, executed and measured.
 struct BenchmarkCode {
   // The sequence of instructions that are to be repeated.
   std::vector<llvm::MCInst> Instructions;
 
   // Before the code is executed some instructions are added to setup the
   // registers initial values.
   std::vector<RegisterValue> RegisterInitialValues;

llvm/trunk/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

BenchmarkFailure::BenchmarkFailure(const llvm::Twine &S)
     : llvm::StringError(S, llvm::inconvertibleErrorCode()) {}
 
 BenchmarkRunner::BenchmarkRunner(const LLVMState &State,
                                  InstructionBenchmark::ModeE Mode)
     : State(State), Mode(Mode), Scratch(llvm::make_unique<ScratchSpace>()) {}
 
 BenchmarkRunner::~BenchmarkRunner() = default;
 
-// Repeat the snippet until there are at least NumInstructions in the resulting
+// Repeat the snippet until there are at least MinInstructions in the resulting
 // code.
 static std::vector<llvm::MCInst>
-GenerateInstructions(const BenchmarkCode &BC, const int MinInstructions) {
+GenerateInstructions(const BenchmarkCode &BC, const size_t MinInstructions) {
   std::vector<llvm::MCInst> Code = BC.Instructions;
-  for (int I = 0; I < MinInstructions; ++I)
+  for (int I = 0; Code.size() < MinInstructions; ++I)
     Code.push_back(BC.Instructions[I % BC.Instructions.size()]);
   return Code;
 }
 
 InstructionBenchmark
 BenchmarkRunner::runConfiguration(const BenchmarkCode &BC,
                                   unsigned NumRepetitions) const {
   InstructionBenchmark InstrBenchmark;

llvm/trunk/tools/llvm-exegesis/lib/X86/Target.cpp

return llvm::MCInstBuilder(llvm::X86::ADD64ri8)
       .addReg(llvm::X86::RSP)
       .addReg(llvm::X86::RSP)
       .addImm(Bytes);
 }
 
 // Reserves some space on the stack, fills it with the content of the provided
 // constant and provide methods to load the stack value into a register.
 struct ConstantInliner {
-  explicit ConstantInliner(const llvm::APInt &Constant)
-      : StackSize(Constant.getBitWidth() / 8) {
-    assert(Constant.getBitWidth() % 8 == 0 && "Must be a multiple of 8");
-    add(allocateStackSpace(StackSize));
-    size_t ByteOffset = 0;
-    for (; StackSize - ByteOffset >= 4; ByteOffset += 4)
-      add(fillStackSpace(
-          llvm::X86::MOV32mi, ByteOffset,
-          Constant.extractBits(32, ByteOffset * 8).getZExtValue()));
-    if (StackSize - ByteOffset >= 2) {
-      add(fillStackSpace(
-          llvm::X86::MOV16mi, ByteOffset,
-          Constant.extractBits(16, ByteOffset * 8).getZExtValue()));
-      ByteOffset += 2;
-    }
-    if (StackSize - ByteOffset >= 1)
-      add(fillStackSpace(
-          llvm::X86::MOV8mi, ByteOffset,
-          Constant.extractBits(8, ByteOffset * 8).getZExtValue()));
-  }
+  explicit ConstantInliner(const llvm::APInt &Constant) : Constant_(Constant) {}
 
   std::vector<llvm::MCInst> loadAndFinalize(unsigned Reg, unsigned RegBitWidth,
                                             unsigned Opcode) {
-    assert(StackSize * 8 == RegBitWidth &&
-           "Value does not have the correct size");
+    assert((RegBitWidth & 7) == 0 &&
+           "RegBitWidth must be a multiple of 8 bits");
+    initStack(RegBitWidth / 8);
     add(loadToReg(Reg, Opcode));
-    add(releaseStackSpace(StackSize));
+    add(releaseStackSpace(RegBitWidth / 8));
     return std::move(Instructions);
   }
 
   std::vector<llvm::MCInst>
   loadX87AndFinalize(unsigned Reg, unsigned RegBitWidth, unsigned Opcode) {
-    assert(StackSize * 8 == RegBitWidth &&
-           "Value does not have the correct size");
+    assert((RegBitWidth & 7) == 0 &&
+           "RegBitWidth must be a multiple of 8 bits");
+    initStack(RegBitWidth / 8);
     add(llvm::MCInstBuilder(Opcode)
             .addReg(llvm::X86::RSP) // BaseReg
             .addImm(1)              // ScaleAmt
             .addReg(0)              // IndexReg
             .addImm(0)              // Disp
             .addReg(0));            // Segment
     if (Reg != llvm::X86::ST0)
       add(llvm::MCInstBuilder(llvm::X86::ST_Frr).addReg(Reg));
-    add(releaseStackSpace(StackSize));
+    add(releaseStackSpace(RegBitWidth / 8));
     return std::move(Instructions);
   }
 
   std::vector<llvm::MCInst> popFlagAndFinalize() {
-    assert(StackSize * 8 == 64 && "Value does not have the correct size");
+    initStack(8);
     add(llvm::MCInstBuilder(llvm::X86::POPF64));
     return std::move(Instructions);
   }
 
 private:
   ConstantInliner &add(const llvm::MCInst &Inst) {
     Instructions.push_back(Inst);
     return *this;
   }
 
-  const size_t StackSize;
+  void initStack(unsigned Bytes) {
+    assert(Constant_.getBitWidth() <= Bytes * 8 &&
+           "Value does not have the correct size");
+    const llvm::APInt WideConstant = Constant_.getBitWidth() < Bytes * 8
+                                         ? Constant_.sext(Bytes * 8)
+                                         : Constant_;
+    add(allocateStackSpace(Bytes));
+    size_t ByteOffset = 0;
+    for (; Bytes - ByteOffset >= 4; ByteOffset += 4)
+      add(fillStackSpace(
+          llvm::X86::MOV32mi, ByteOffset,
+          WideConstant.extractBits(32, ByteOffset * 8).getZExtValue()));
+    if (Bytes - ByteOffset >= 2) {
+      add(fillStackSpace(
+          llvm::X86::MOV16mi, ByteOffset,
+          WideConstant.extractBits(16, ByteOffset * 8).getZExtValue()));
+      ByteOffset += 2;
+    }
+    if (Bytes - ByteOffset >= 1)
+      add(fillStackSpace(
+          llvm::X86::MOV8mi, ByteOffset,
+          WideConstant.extractBits(8, ByteOffset * 8).getZExtValue()));
+  }
+
+  llvm::APInt Constant_;
   std::vector<llvm::MCInst> Instructions;
 };
 
 class ExegesisX86Target : public ExegesisTarget {
   void addTargetSpecificPasses(llvm::PassManagerBase &PM) const override {
     // Lowers FP pseudo-instructions, e.g. ABS_Fp32 -> ABS_F.
     PM.add(llvm::createX86FloatingPointStackifierPass());
   }

llvm/trunk/tools/llvm-exegesis/llvm-exegesis.cpp

 #include "lib/BenchmarkRunner.h"
 #include "lib/Clustering.h"
 #include "lib/LlvmState.h"
 #include "lib/PerfHelper.h"
 #include "lib/Target.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/MC/MCInstBuilder.h"
+#include "llvm/MC/MCObjectFileInfo.h"
+#include "llvm/MC/MCParser/MCAsmParser.h"
+#include "llvm/MC/MCParser/MCTargetAsmParser.h"
 #include "llvm/MC/MCRegisterInfo.h"
+#include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSubtargetInfo.h"
+#include "llvm/Object/ObjectFile.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/TargetSelect.h"
 #include <algorithm>
 #include <random>
 #include <string>
 #include <unordered_map>
 
 static llvm::cl::opt<unsigned>
     OpcodeIndex("opcode-index", llvm::cl::desc("opcode to measure, by index"),
                 llvm::cl::init(0));
 
 static llvm::cl::opt<std::string>
     OpcodeName("opcode-name", llvm::cl::desc("opcode to measure, by name"),
                llvm::cl::init(""));
 
 static llvm::cl::opt<std::string>
+    SnippetsFile("snippets-file", llvm::cl::desc("code snippets to measure"),
+                 llvm::cl::init(""));
+
+static llvm::cl::opt<std::string>
     BenchmarkFile("benchmarks-file", llvm::cl::desc(""), llvm::cl::init(""));
 
 static llvm::cl::opt<exegesis::InstructionBenchmark::ModeE> BenchmarkMode(
     "mode", llvm::cl::desc("the mode to run"),
     llvm::cl::values(clEnumValN(exegesis::InstructionBenchmark::Latency,
                                 "latency", "Instruction Latency"),
                      clEnumValN(exegesis::InstructionBenchmark::Uops, "uops",
                                 "Uop Decomposition"),

 namespace exegesis {
 
 static llvm::ExitOnError ExitOnErr;
 
 #ifdef LLVM_EXEGESIS_INITIALIZE_NATIVE_TARGET
 void LLVM_EXEGESIS_INITIALIZE_NATIVE_TARGET();
 #endif
 
-static unsigned GetOpcodeOrDie(const llvm::MCInstrInfo &MCInstrInfo) {
-  if (OpcodeName.empty() && (OpcodeIndex == 0))
+// Checks that only one of OpcodeName, OpcodeIndex or SnippetsFile is provided,
+// and returns the opcode index or 0 if snippets should be read from
+// `SnippetsFile`.
+static unsigned getOpcodeOrDie(const llvm::MCInstrInfo &MCInstrInfo) {
+  const size_t NumSetFlags = (OpcodeName.empty() ? 0 : 1) +
+                             (OpcodeIndex == 0 ? 0 : 1) +
+                             (SnippetsFile.empty() ? 0 : 1);
+  if (NumSetFlags != 1)
     llvm::report_fatal_error(
-        "please provide one and only one of 'opcode-index' or 'opcode-name'");
+        "please provide one and only one of 'opcode-index', 'opcode-name' or "
+        "'snippets-file'");
+  if (!SnippetsFile.empty())
+    return 0;
   if (OpcodeIndex > 0)
     return OpcodeIndex;
   // Resolve opcode name -> opcode.
   for (unsigned I = 0, E = MCInstrInfo.getNumOpcodes(); I < E; ++I)
     if (MCInstrInfo.getName(I) == OpcodeName)
       return I;
   llvm::report_fatal_error(llvm::Twine("unknown opcode ").concat(OpcodeName));
 }
getBenchmarkResultContext(const LLVMState &State) {
   const llvm::MCRegisterInfo &RegInfo = State.getRegInfo();
   for (unsigned E = RegInfo.getNumRegs(), I = 0; I < E; ++I)
     Ctx.addRegEntry(I, RegInfo.getName(I));
 
   return Ctx;
 }
 
 // Generates code snippets for opcode `Opcode`.
-llvm::Expected<std::vector<BenchmarkCode>>
+static llvm::Expected<std::vector<BenchmarkCode>>
 generateSnippets(const LLVMState &State, unsigned Opcode) {
   const std::unique_ptr<SnippetGenerator> Generator =
       State.getExegesisTarget().createSnippetGenerator(BenchmarkMode, State);
-  if (!Generator) {
+  if (!Generator)
     llvm::report_fatal_error("cannot create snippet generator");
-  }
 
   const llvm::MCInstrDesc &InstrDesc = State.getInstrInfo().get(Opcode);
   // Ignore instructions that we cannot run.
   if (InstrDesc.isPseudo())
     return llvm::make_error<BenchmarkFailure>("Unsupported opcode: isPseudo");
   if (InstrDesc.isBranch() || InstrDesc.isIndirectBranch())
     return llvm::make_error<BenchmarkFailure>(
         "Unsupported opcode: isBranch/isIndirectBranch");
   if (InstrDesc.isCall() || InstrDesc.isReturn())
     return llvm::make_error<BenchmarkFailure>(
         "Unsupported opcode: isCall/isReturn");
 
   return Generator->generateConfigurations(Opcode);
 }
 
+namespace {
+
+// An MCStreamer that reads a BenchmarkCode definition from a file.
+// The BenchmarkCode definition is just an asm file, with additional comments to
+// specify which registers should be defined or are live on entry.
+class BenchmarkCodeStreamer : public llvm::MCStreamer,
+                              public llvm::AsmCommentConsumer {
+public:
+  explicit BenchmarkCodeStreamer(llvm::MCContext *Context,
+                                 const llvm::MCRegisterInfo *TheRegInfo,
+                                 BenchmarkCode *Result)
+      : llvm::MCStreamer(*Context), RegInfo(TheRegInfo), Result(Result) {}
+
+  // Implementation of the llvm::MCStreamer interface. We only care about
+  // instructions.
+  void EmitInstruction(const llvm::MCInst &instruction,
+                       const llvm::MCSubtargetInfo &mc_subtarget_info,
+                       bool PrintSchedInfo) override {
+    Result->Instructions.push_back(instruction);
+  }
+
+  // Implementation of the llvm::AsmCommentConsumer.
+  void HandleComment(llvm::SMLoc Loc, llvm::StringRef CommentText) override {
+    CommentText = CommentText.trim();
+    if (!CommentText.consume_front("LLVM-EXEGESIS-"))
+      return;
+    if (CommentText.consume_front("DEFREG")) {
+      // LLVM-EXEGESIS-DEFREF <reg> <hex_value>
+      RegisterValue RegVal;
+      llvm::SmallVector<llvm::StringRef, 2> Parts;
+      CommentText.split(Parts, ' ', /*unlimited splits*/ -1,
+                        /*do not keep empty strings*/ false);
+      if (Parts.size() != 2) {
+        llvm::errs() << "invalid comment 'LLVM-EXEGESIS-DEFREG " << CommentText
+                     << "\n";
+        ++InvalidComments;
+      }
+      if (!(RegVal.Register = findRegisterByName(Parts[0].trim()))) {
+        llvm::errs() << "unknown register in 'LLVM-EXEGESIS-DEFREG "
+                     << CommentText << "\n";
+        ++InvalidComments;
+        return;
+      }
+      const llvm::StringRef HexValue = Parts[1].trim();
+      RegVal.Value = llvm::APInt(
+          /* each hex digit is 4 bits */ HexValue.size() * 4, HexValue, 16);
+      Result->RegisterInitialValues.push_back(std::move(RegVal));
+      return;
+    }
+    if (CommentText.consume_front("LIVEIN")) {
+      // LLVM-EXEGESIS-LIVEIN <reg>
+      if (unsigned Reg = findRegisterByName(CommentText.ltrim()))
+        Result->LiveIns.push_back(Reg);
+      else {
+        llvm::errs() << "unknown register in 'LLVM-EXEGESIS-LIVEIN "
+                     << CommentText << "\n";
+        ++InvalidComments;
+      }
+      return;
+    }
+  }
+
+  unsigned numInvalidComments() const { return InvalidComments; }
+
+private:
+  // We only care about instructions, we don't implement this part of the API.
+  void EmitCommonSymbol(llvm::MCSymbol *symbol, uint64_t size,
+                        unsigned byte_alignment) override {}
+  bool EmitSymbolAttribute(llvm::MCSymbol *symbol,
+                           llvm::MCSymbolAttr attribute) override {
+    return false;
+  }
+  void EmitValueToAlignment(unsigned byte_alignment, int64_t value,
+                            unsigned value_size,
+                            unsigned max_bytes_to_emit) override {}
+  void EmitZerofill(llvm::MCSection *section, llvm::MCSymbol *symbol,
+                    uint64_t size, unsigned byte_alignment,
+                    llvm::SMLoc Loc) override {}
+
+  unsigned findRegisterByName(const llvm::StringRef RegName) const {
+    // FIXME: Can we do better than this ?
+    for (unsigned I = 0, E = RegInfo->getNumRegs(); I < E; ++I) {
+      if (RegName == RegInfo->getName(I))
+        return I;
+    }
+    llvm::errs() << "'" << RegName
+                 << "' is not a valid register name for the target\n";
+    return 0;
+  }
+
+  const llvm::MCRegisterInfo *const RegInfo;
+  BenchmarkCode *const Result;
+  unsigned InvalidComments = 0;
+};
+
+} // namespace
+
+// Reads code snippets from file `Filename`.
+static llvm::Expected<std::vector<BenchmarkCode>>
+readSnippets(const LLVMState &State, llvm::StringRef Filename) {
+  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> BufferPtr =
+      llvm::MemoryBuffer::getFileOrSTDIN(Filename);
+  if (std::error_code EC = BufferPtr.getError()) {
+    return llvm::make_error<BenchmarkFailure>(
+        "cannot read snippet: " + Filename + ": " + EC.message());
+  }
+  llvm::SourceMgr SM;
+  SM.AddNewSourceBuffer(std::move(BufferPtr.get()), llvm::SMLoc());
+
+  BenchmarkCode Result;
+
+  llvm::MCObjectFileInfo ObjectFileInfo;
+  const llvm::TargetMachine &TM = State.getTargetMachine();
+  llvm::MCContext Context(TM.getMCAsmInfo(), TM.getMCRegisterInfo(),
+                          &ObjectFileInfo);
+  ObjectFileInfo.InitMCObjectFileInfo(TM.getTargetTriple(), /*PIC*/ false,
+                                      Context);
+  BenchmarkCodeStreamer Streamer(&Context, TM.getMCRegisterInfo(), &Result);
+  const std::unique_ptr<llvm::MCAsmParser> AsmParser(
+      llvm::createMCAsmParser(SM, Context, Streamer, *TM.getMCAsmInfo()));
+  if (!AsmParser)
+    return llvm::make_error<BenchmarkFailure>("cannot create asm parser");
+  AsmParser->getLexer().setCommentConsumer(&Streamer);
+
+  const std::unique_ptr<llvm::MCTargetAsmParser> TargetAsmParser(
+      TM.getTarget().createMCAsmParser(*TM.getMCSubtargetInfo(), *AsmParser,
+                                       *TM.getMCInstrInfo(),
+                                       llvm::MCTargetOptions()));
+
+  if (!TargetAsmParser)
+    return llvm::make_error<BenchmarkFailure>(
+        "cannot create target asm parser");
+  AsmParser->setTargetParser(*TargetAsmParser);
+
+  if (AsmParser->Run(false))
+    return llvm::make_error<BenchmarkFailure>("cannot parse asm file");
+  if (Streamer.numInvalidComments())
+    return llvm::make_error<BenchmarkFailure>(
+        llvm::Twine("found ")
+            .concat(llvm::Twine(Streamer.numInvalidComments()))
+            .concat(" invalid LLVM-EXEGESIS comments"));
+  return std::vector<BenchmarkCode>{std::move(Result)};
+}
+
 void benchmarkMain() {
   if (exegesis::pfm::pfmInitialize())
     llvm::report_fatal_error("cannot initialize libpfm");
 
   llvm::InitializeNativeTarget();
   llvm::InitializeNativeTargetAsmPrinter();
+  llvm::InitializeNativeTargetAsmParser();
 #ifdef LLVM_EXEGESIS_INITIALIZE_NATIVE_TARGET
   LLVM_EXEGESIS_INITIALIZE_NATIVE_TARGET();
 #endif
 
   const LLVMState State;
-  const auto Opcode = GetOpcodeOrDie(State.getInstrInfo());
+  const auto Opcode = getOpcodeOrDie(State.getInstrInfo());
 
-  // Ignore instructions without a sched class if -ignore-invalid-sched-class is
-  // passed.
+  std::vector<BenchmarkCode> Configurations;
+  if (Opcode > 0) {
+    // Ignore instructions without a sched class if -ignore-invalid-sched-class
+    // is passed.
     if (IgnoreInvalidSchedClass &&
         State.getInstrInfo().get(Opcode).getSchedClass() == 0) {
       llvm::errs() << "ignoring instruction without sched class\n";
       return;
     }
-
-  // FIXME: Allow arbitrary code.
-  const std::vector<BenchmarkCode> Configurations =
-      ExitOnErr(generateSnippets(State, Opcode));
+    Configurations = ExitOnErr(generateSnippets(State, Opcode));
+  } else {
+    Configurations = ExitOnErr(readSnippets(State, SnippetsFile));
+  }
 
   const std::unique_ptr<BenchmarkRunner> Runner =
       State.getExegesisTarget().createBenchmarkRunner(BenchmarkMode, State);
   if (!Runner) {
     llvm::report_fatal_error("cannot create benchmark runner");
   }
 
   if (NumRepetitions == 0)