[BOLT] Ignore returns in DataAggregator

Returns are ignored in perf/pre-aggregated/fdata profile reader (see
DataReader::convertBranchData). They are also omitted in
YAMLProfileWriter by virtue of not having the profile attached to them
in the reader, and YAMLProfileWriter converting the profile attached to
BinaryFunctions. Thus, return profile is universally ignored across all
profile types except BAT YAML.

To make returns ignored for YAML produced in BAT mode, we can:
1) ignore them in YAMLProfileReader,
2) omit them from YAML profile in profile conversion/writing.

The first option is prone to profile staleness issue, where the profiled
binary doesn't match the one to be optimized, and thus returns in the
profile can no longer be reliably detected (as we don't distinguish them
from calls in the profile).

The second option is robust to staleness but requires disassembling the
branch source instruction.

Test Plan: Updated bolt-address-translation-yaml.test

Reviewers: rafaelauler, dcci, ayermolo, maksfb

Reviewed By: maksfb

Pull Request: https://github.com/llvm/llvm-project/pull/90807

bolt/include/bolt/Core/BinaryFunction.h

@@ -930,6 +930,8 @@ public:
     return const_cast<BinaryFunction *>(this)->getInstructionAtOffset(Offset);
   }
 
+  std::optional<MCInst> disassembleInstructionAtOffset(uint64_t Offset) const;
+
   /// Return offset for the first instruction. If there is data at the
   /// beginning of a function then offset of the first instruction could
   /// be different from 0

bolt/lib/Core/BinaryFunction.cpp

@@ -1167,6 +1167,21 @@ void BinaryFunction::handleAArch64IndirectCall(MCInst &Instruction,
   }
 }
 
+std::optional<MCInst>
+BinaryFunction::disassembleInstructionAtOffset(uint64_t Offset) const {
+  assert(CurrentState == State::Empty && "Function should not be disassembled");
+  assert(Offset < MaxSize && "Invalid offset");
+  ErrorOr<ArrayRef<unsigned char>> FunctionData = getData();
+  assert(FunctionData && "Cannot get function as data");
+  MCInst Instr;
+  uint64_t InstrSize = 0;
+  const uint64_t InstrAddress = getAddress() + Offset;
+  if (BC.DisAsm->getInstruction(Instr, InstrSize, FunctionData->slice(Offset),
+                                InstrAddress, nulls()))
+    return Instr;
+  return std::nullopt;
+}
+
 Error BinaryFunction::disassemble() {
   NamedRegionTimer T("disassemble", "Disassemble function", "buildfuncs",
                      "Build Binary Functions", opts::TimeBuild);

bolt/lib/Profile/DataAggregator.cpp

@@ -773,9 +773,19 @@ bool DataAggregator::doInterBranch(BinaryFunction *FromFunc,
 
 bool DataAggregator::doBranch(uint64_t From, uint64_t To, uint64_t Count,
                               uint64_t Mispreds) {
+  bool IsReturn = false;
   auto handleAddress = [&](uint64_t &Addr, bool IsFrom) -> BinaryFunction * {
     if (BinaryFunction *Func = getBinaryFunctionContainingAddress(Addr)) {
       Addr -= Func->getAddress();
+      if (IsFrom) {
+        auto checkReturn = [&](auto MaybeInst) {
+          IsReturn = MaybeInst && BC->MIB->isReturn(*MaybeInst);
+        };
+        if (Func->hasInstructions())
+          checkReturn(Func->getInstructionAtOffset(Addr));
+        else
+          checkReturn(Func->disassembleInstructionAtOffset(Addr));
+      }
 
       if (BAT)
         Addr = BAT->translate(Func->getAddress(), Addr, IsFrom);
@@ -792,6 +802,9 @@ bool DataAggregator::doBranch(uint64_t From, uint64_t To, uint64_t Count,
   };
 
   BinaryFunction *FromFunc = handleAddress(From, /*IsFrom=*/true);
+  // Ignore returns.
+  if (IsReturn)
+    return true;
   BinaryFunction *ToFunc = handleAddress(To, /*IsFrom=*/false);
   if (!FromFunc && !ToFunc)
     return false;

bolt/test/X86/bolt-address-translation-yaml.test

@@ -13,7 +13,7 @@ RUN: llvm-bolt %t.exe -data %t.fdata -w %t.yaml-fdata -o /dev/null
 RUN: FileCheck --input-file %t.yaml-fdata --check-prefix YAML-BAT-CHECK %s
 
 # Test resulting YAML profile with the original binary (no-stale mode)
-RUN: llvm-bolt %t.exe -data %t.yaml -o %t.null -dyno-stats \
+RUN: llvm-bolt %t.exe -data %t.yaml -o %t.null -dyno-stats 2>&1 \
 RUN:   | FileCheck --check-prefix CHECK-BOLT-YAML %s
 
 WRITE-BAT-CHECK: BOLT-INFO: Wrote 5 BAT maps
@@ -63,7 +63,8 @@ YAML-BAT-CHECK-NEXT:   blocks:
 YAML-BAT-CHECK:        - bid:   1
 YAML-BAT-CHECK-NEXT:       insns: [[#]]
 YAML-BAT-CHECK-NEXT:       hash:  0xD70DC695320E0010
-YAML-BAT-CHECK-NEXT:       succ:  {{.*}} { bid: 2, cnt: [[#]] }
+YAML-BAT-CHECK-NEXT:       succ:  {{.*}} { bid: 2, cnt: [[#]]
 
 CHECK-BOLT-YAML:      pre-processing profile using YAML profile reader
 CHECK-BOLT-YAML-NEXT: 5 out of 16 functions in the binary (31.2%) have non-empty execution profile
+CHECK-BOLT-YAML-NOT: invalid (possibly stale) profile