[llvm-exegesis] Add options to SnippetGenerator.

Summary:
This adds a `-max-configs-per-opcode` option to limit the number of
configs per opcode.

Reviewers: gchatelet

Subscribers: tschuett, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68642

llvm-svn: 374054

docs/CommandGuide/llvm-exegesis.rst

@@ -195,11 +195,23 @@ OPTIONS
  to specify at least one of the `-analysis-clusters-output-file=` and
  `-analysis-inconsistencies-output-file=`.
 
-.. option:: -num-repetitions=<Number of repetition>
+.. option:: -num-repetitions=<Number of repetitions>
 
  Specify the number of repetitions of the asm snippet.
  Higher values lead to more accurate measurements but lengthen the benchmark.
 
+.. option:: -max-configs-per-opcode=<value>
+
+ Specify the maximum configurations that can be generated for each opcode.
+ By default this is `1`, meaning that we assume that a single measurement is
+ enough to characterize an opcode. This might not be true of all instructions:
+ for example, the performance characteristics of the LEA instruction on X86
+ depends on the value of assigned registers and immediates. Setting a value of
+ `-max-configs-per-opcode` larger than `1` allows `llvm-exegesis` to explore
+ more configurations to discover if some register or immediate assignments
+ lead to different performance characteristics.
+
+
 .. option:: -benchmarks-file=</path/to/file>
 
  File to read (`analysis` mode) or write (`latency`/`uops`/`inverse_throughput`

test/tools/llvm-exegesis/X86/max-configs.test

@@ -0,0 +1,24 @@
+# RUN: llvm-exegesis -mode=latency -opcode-name=SBB8rr -max-configs-per-opcode=1 | FileCheck -check-prefixes=CHECK,CHECK1 %s
+# RUN: llvm-exegesis -mode=latency -opcode-name=SBB8rr -max-configs-per-opcode=2 | FileCheck -check-prefixes=CHECK,CHECK2 %s
+
+CHECK:      ---
+CHECK-NEXT: mode: latency
+CHECK-NEXT: key:
+CHECK-NEXT:   instructions:
+CHECK-NEXT:     SBB8rr
+CHECK-NEXT: config: ''
+CHECK-NEXT: register_initial_values:
+CHECK-DAG: - '[[REG1:[A-Z0-9]+]]=0x0'
+CHECK-LAST: ...
+
+CHECK1-NOT: SBB8rr
+
+CHECK2:      ---
+CHECK2-NEXT: mode: latency
+CHECK2-NEXT: key:
+CHECK2-NEXT:   instructions:
+CHECK2-NEXT:     SBB8rr
+CHECK2-NEXT: config: ''
+CHECK2-NEXT: register_initial_values:
+CHECK2-DAG: - '[[REG1:[A-Z0-9]+]]=0x0'
+CHECK2-LAST: ...

tools/llvm-exegesis/lib/Latency.h

@@ -24,7 +24,7 @@ namespace exegesis {
 
 class LatencySnippetGenerator : public SnippetGenerator {
 public:
-  LatencySnippetGenerator(const LLVMState &State) : SnippetGenerator(State) {}
+  using SnippetGenerator::SnippetGenerator;
   ~LatencySnippetGenerator() override;
 
   llvm::Expected<std::vector<CodeTemplate>>

tools/llvm-exegesis/lib/SnippetGenerator.cpp

@@ -33,7 +33,8 @@ std::vector<CodeTemplate> getSingleton(CodeTemplate &&CT) {
 SnippetGeneratorFailure::SnippetGeneratorFailure(const llvm::Twine &S)
     : llvm::StringError(S, llvm::inconvertibleErrorCode()) {}
 
-SnippetGenerator::SnippetGenerator(const LLVMState &State) : State(State) {}
+SnippetGenerator::SnippetGenerator(const LLVMState &State, const Options &Opts)
+    : State(State), Opts(Opts) {}
 
 SnippetGenerator::~SnippetGenerator() = default;
 
@@ -81,6 +82,9 @@ SnippetGenerator::generateConfigurations(
             computeRegisterInitialValues(CT.Instructions);
         BC.Key.Config = CT.Config;
         Output.push_back(std::move(BC));
+        if (Output.size() >= Opts.MaxConfigsPerOpcode)
+          return Output; // Early exit if we exceeded the number of allowed
+                         // configs.
       }
     }
     return Output;

tools/llvm-exegesis/lib/SnippetGenerator.h

@@ -51,7 +51,11 @@ public:
 // Common code for all benchmark modes.
 class SnippetGenerator {
 public:
-  explicit SnippetGenerator(const LLVMState &State);
+  struct Options {
+    unsigned MaxConfigsPerOpcode = 1;
+  };
+
+  explicit SnippetGenerator(const LLVMState &State, const Options &Opts);
 
   virtual ~SnippetGenerator();
 
@@ -66,6 +70,7 @@ public:
 
 protected:
   const LLVMState &State;
+  const Options Opts;
 
 private:
   // API to be implemented by subclasses.

tools/llvm-exegesis/lib/Target.cpp

@@ -36,17 +36,17 @@ void ExegesisTarget::registerTarget(ExegesisTarget *Target) {
   FirstTarget = Target;
 }
 
-std::unique_ptr<SnippetGenerator>
-ExegesisTarget::createSnippetGenerator(InstructionBenchmark::ModeE Mode,
-                                       const LLVMState &State) const {
+std::unique_ptr<SnippetGenerator> ExegesisTarget::createSnippetGenerator(
+    InstructionBenchmark::ModeE Mode, const LLVMState &State,
+    const SnippetGenerator::Options &Opts) const {
   switch (Mode) {
   case InstructionBenchmark::Unknown:
     return nullptr;
   case InstructionBenchmark::Latency:
-    return createLatencySnippetGenerator(State);
+    return createLatencySnippetGenerator(State, Opts);
   case InstructionBenchmark::Uops:
   case InstructionBenchmark::InverseThroughput:
-    return createUopsSnippetGenerator(State);
+    return createUopsSnippetGenerator(State, Opts);
   }
   return nullptr;
 }
@@ -66,14 +66,14 @@ ExegesisTarget::createBenchmarkRunner(InstructionBenchmark::ModeE Mode,
   return nullptr;
 }
 
-std::unique_ptr<SnippetGenerator>
-ExegesisTarget::createLatencySnippetGenerator(const LLVMState &State) const {
-  return std::make_unique<LatencySnippetGenerator>(State);
+std::unique_ptr<SnippetGenerator> ExegesisTarget::createLatencySnippetGenerator(
+    const LLVMState &State, const SnippetGenerator::Options &Opts) const {
+  return std::make_unique<LatencySnippetGenerator>(State, Opts);
 }
 
-std::unique_ptr<SnippetGenerator>
-ExegesisTarget::createUopsSnippetGenerator(const LLVMState &State) const {
-  return std::make_unique<UopsSnippetGenerator>(State);
+std::unique_ptr<SnippetGenerator> ExegesisTarget::createUopsSnippetGenerator(
+    const LLVMState &State, const SnippetGenerator::Options &Opts) const {
+  return std::make_unique<UopsSnippetGenerator>(State, Opts);
 }
 
 std::unique_ptr<BenchmarkRunner> ExegesisTarget::createLatencyBenchmarkRunner(

tools/llvm-exegesis/lib/Target.h

@@ -125,7 +125,8 @@ public:
   // Creates a snippet generator for the given mode.
   std::unique_ptr<SnippetGenerator>
   createSnippetGenerator(InstructionBenchmark::ModeE Mode,
-                         const LLVMState &State) const;
+                         const LLVMState &State,
+                         const SnippetGenerator::Options &Opts) const;
   // Creates a benchmark runner for the given mode.
   std::unique_ptr<BenchmarkRunner>
   createBenchmarkRunner(InstructionBenchmark::ModeE Mode,
@@ -151,9 +152,9 @@ private:
   // Targets can implement their own snippet generators/benchmarks runners by
   // implementing these.
   std::unique_ptr<SnippetGenerator> virtual createLatencySnippetGenerator(
-      const LLVMState &State) const;
+      const LLVMState &State, const SnippetGenerator::Options &Opts) const;
   std::unique_ptr<SnippetGenerator> virtual createUopsSnippetGenerator(
-      const LLVMState &State) const;
+      const LLVMState &State, const SnippetGenerator::Options &Opts) const;
   std::unique_ptr<BenchmarkRunner> virtual createLatencyBenchmarkRunner(
       const LLVMState &State, InstructionBenchmark::ModeE Mode) const;
   std::unique_ptr<BenchmarkRunner> virtual createUopsBenchmarkRunner(

tools/llvm-exegesis/lib/Uops.h

@@ -22,7 +22,7 @@ namespace exegesis {
 
 class UopsSnippetGenerator : public SnippetGenerator {
 public:
-  UopsSnippetGenerator(const LLVMState &State) : SnippetGenerator(State) {}
+  using SnippetGenerator::SnippetGenerator;
   ~UopsSnippetGenerator() override;
 
   llvm::Expected<std::vector<CodeTemplate>>

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

@@ -462,14 +462,16 @@ private:
                             sizeof(kUnavailableRegisters[0]));
   }
 
-  std::unique_ptr<SnippetGenerator>
-  createLatencySnippetGenerator(const LLVMState &State) const override {
-    return std::make_unique<X86LatencySnippetGenerator>(State);
+  std::unique_ptr<SnippetGenerator> createLatencySnippetGenerator(
+      const LLVMState &State,
+      const SnippetGenerator::Options &Opts) const override {
+    return std::make_unique<X86LatencySnippetGenerator>(State, Opts);
   }
 
-  std::unique_ptr<SnippetGenerator>
-  createUopsSnippetGenerator(const LLVMState &State) const override {
-    return std::make_unique<X86UopsSnippetGenerator>(State);
+  std::unique_ptr<SnippetGenerator> createUopsSnippetGenerator(
+      const LLVMState &State,
+      const SnippetGenerator::Options &Opts) const override {
+    return std::make_unique<X86UopsSnippetGenerator>(State, Opts);
   }
 
   bool matchesArch(llvm::Triple::ArchType Arch) const override {

tools/llvm-exegesis/llvm-exegesis.cpp

@@ -95,6 +95,12 @@ static cl::opt<unsigned>
                    cl::desc("number of time to repeat the asm snippet"),
                    cl::cat(BenchmarkOptions), cl::init(10000));
 
+static cl::opt<unsigned> MaxConfigsPerOpcode(
+    "max-configs-per-opcode",
+    cl::desc(
+        "allow to snippet generator to generate at most that many configs"),
+    cl::cat(BenchmarkOptions), cl::init(1));
+
 static cl::opt<bool> IgnoreInvalidSchedClass(
     "ignore-invalid-sched-class",
     cl::desc("ignore instructions that do not define a sched class"),
@@ -214,8 +220,11 @@ generateSnippets(const LLVMState &State, unsigned Opcode,
   if (InstrDesc.isCall() || InstrDesc.isReturn())
     return make_error<Failure>("Unsupported opcode: isCall/isReturn");
 
+  SnippetGenerator::Options Options;
+  Options.MaxConfigsPerOpcode = MaxConfigsPerOpcode;
   const std::unique_ptr<SnippetGenerator> Generator =
-      State.getExegesisTarget().createSnippetGenerator(BenchmarkMode, State);
+      State.getExegesisTarget().createSnippetGenerator(BenchmarkMode, State,
+                                                       Options);
   if (!Generator)
     llvm::report_fatal_error("cannot create snippet generator");
   return Generator->generateConfigurations(Instr, ForbiddenRegs);

unittests/tools/llvm-exegesis/X86/SnippetGeneratorTest.cpp

@@ -45,7 +45,7 @@ protected:
 template <typename SnippetGeneratorT>
 class SnippetGeneratorTest : public X86SnippetGeneratorTest {
 protected:
-  SnippetGeneratorTest() : Generator(State) {}
+  SnippetGeneratorTest() : Generator(State, SnippetGenerator::Options()) {}
 
   std::vector<CodeTemplate> checkAndGetCodeTemplates(unsigned Opcode) {
     randomGenerator().seed(0); // Initialize seed.
@@ -335,7 +335,8 @@ TEST_F(UopsSnippetGeneratorTest, MemoryUse) {
 
 class FakeSnippetGenerator : public SnippetGenerator {
 public:
-  FakeSnippetGenerator(const LLVMState &State) : SnippetGenerator(State) {}
+  FakeSnippetGenerator(const LLVMState &State, const Options &Opts)
+      : SnippetGenerator(State, Opts) {}
 
   Instruction createInstruction(unsigned Opcode) {
     return State.getIC().getInstr(Opcode);