[llvm-exegesis] Make duplicate snippet repetitor produce whole snippe…

…ts (#77224)

Currently, the duplicate snippet repetitor will truncate snippets that
do not exactly divide the minimum number of instructions. This patch
corrects that behavior by making the duplicate snippet repetitor
duplicate the snippet in its entirety until the minimum number of
instructions has been reached.

This makes the behavior consistent with the loop snippet repetitor,
which will execute at least `--num-repetitions` (soon to be renamed
`--min-instructions`) instructions.

llvm/tools/llvm-exegesis/lib/SnippetRepetitor.cpp

@@ -30,10 +30,10 @@ class DuplicateSnippetRepetitor : public SnippetRepetitor {
             CleanupMemory](FunctionFiller &Filler) {
       auto Entry = Filler.getEntry();
       if (!Instructions.empty()) {
-        // Add the whole snippet at least once.
-        Entry.addInstructions(Instructions);
-        for (unsigned I = Instructions.size(); I < MinInstructions; ++I) {
-          Entry.addInstruction(Instructions[I % Instructions.size()]);
+        const unsigned NumRepetitions =
+            divideCeil(MinInstructions, Instructions.size());
+        for (unsigned I = 0; I < NumRepetitions; ++I) {
+          Entry.addInstructions(Instructions);
         }
       }
       Entry.addReturn(State.getExegesisTarget(), CleanupMemory);

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

@@ -38,9 +38,11 @@ class X86SnippetRepetitorTest : public X86TestBase {
     MF = &createVoidVoidPtrMachineFunction("TestFn", Mod.get(), MMI.get());
   }
 
-  void TestCommon(Benchmark::RepetitionModeE RepetitionMode) {
+  void TestCommon(Benchmark::RepetitionModeE RepetitionMode,
+                  unsigned SnippetInstructions = 1) {
     const auto Repetitor = SnippetRepetitor::Create(RepetitionMode, State);
-    const std::vector<MCInst> Instructions = {MCInstBuilder(X86::NOOP)};
+    const std::vector<MCInst> Instructions(SnippetInstructions,
+                                           MCInstBuilder(X86::NOOP));
     FunctionFiller Sink(*MF, {X86::EAX});
     const auto Fill =
         Repetitor->Repeat(Instructions, kMinInstructions, kLoopBodySize, false);
@@ -74,6 +76,18 @@ TEST_F(X86SnippetRepetitorTest, Duplicate) {
                           HasOpcode(X86::NOOP), HasOpcode(X86::RET64)));
 }
 
+TEST_F(X86SnippetRepetitorTest, DuplicateSnippetInstructionCount) {
+  TestCommon(Benchmark::Duplicate, 2);
+  // Duplicating a snippet of two instructions with the minimum number of
+  // instructions set to three duplicates the snippet twice for a total of
+  // four instructions.
+  ASSERT_EQ(MF->getNumBlockIDs(), 1u);
+  EXPECT_THAT(MF->getBlockNumbered(0)->instrs(),
+              ElementsAre(HasOpcode(X86::NOOP), HasOpcode(X86::NOOP),
+                          HasOpcode(X86::NOOP), HasOpcode(X86::NOOP),
+                          HasOpcode(X86::RET64)));
+}
+
 TEST_F(X86SnippetRepetitorTest, Loop) {
   TestCommon(Benchmark::Loop);
   // Duplicating creates an entry block, a loop body and a ret block.