[BOLT] Adding a unittest that covers Arm SPE PBT aggregation (#160095)

When the SPE Previous Branch Target address (FEAT_SPE_PBT) feature is
available, an SPE sample by combining this PBT feature, has two entries.
Arm SPE records SRC/DEST addresses of the latest sampled branch
operation, and it stores into the first entry. PBT records the target
address of most recently taken branch in program order before the
sampled operation, it places into the second entry. They are formed a
chain of two consecutive branches.

Where:
- The previous branch operation (PBT) is always taken.
- In SPE entry, the current source branch (SRC) may be either
fall-through or taken, and the target address (DEST) of the recorded
branch operation is always what was architecturally executed.

However PBT doesn't provide as much information as SPE does. It lacks
those information such as the address of source branch, branch type, and
prediction bit. These information are always filled with zero in PBT
entry. Therefore Bolt cannot evaluate the prediction, and source branch
fields, it leaves them zero during the aggregation process.

Tests includes a fully expanded example.

Author: kaadam

bolt/unittests/Profile/PerfSpeEvents.cpp

@@ -161,4 +161,92 @@ TEST_F(PerfSpeEventsTestHelper, SpeBranchesWithBrstack) {
   parseAndCheckBrstackEvents(1234, ExpectedSamples);
 }
 
+TEST_F(PerfSpeEventsTestHelper, SpeBranchesWithBrstackAndPbt) {
+  // Check perf input with SPE branch events as brstack format by
+  // combining with the previous branch target address (named as PBT).
+  // Example collection command:
+  // ```
+  // perf record -e 'arm_spe_0/branch_filter=1/u' -- BINARY
+  // ```
+  // How Bolt extracts the branch events:
+  // ```
+  // perf script -F pid,brstack --itrace=bl
+  // ```
+
+  opts::ArmSPE = true;
+  opts::ReadPerfEvents =
+      // "<PID> <SRC>/<DEST>/PN/-/-/10/COND/- <NULL>/<PBT>/-/-/-/0//-\n"
+      "  4567  0xa002/0xa003/PN/-/-/10/COND/- 0x0/0xa001/-/-/-/0//-\n"
+      "  4567  0xb002/0xb003/P/-/-/4/RET/- 0x0/0xb001/-/-/-/0//-\n"
+      "  4567  0xc456/0xc789/P/-/-/13/-/- 0x0/0xc123/-/-/-/0//-\n"
+      "  4567  0xd456/0xd789/M/-/-/7/RET/- 0x0/0xd123/-/-/-/0//-\n"
+      "  4567  0xe005/0xe009/P/-/-/14/RET/- 0x0/0xe001/-/-/-/0//-\n"
+      "  4567  0xd456/0xd789/M/-/-/7/RET/- 0x0/0xd123/-/-/-/0//-\n"
+      "  4567  0xf002/0xf003/MN/-/-/8/COND/- 0x0/0xf001/-/-/-/0//-\n"
+      "  4567  0xc456/0xc789/P/-/-/13/-/- 0x0/0xc123/-/-/-/0//-\n";
+
+  // ExpectedSamples contains the aggregated information about
+  // a branch {{From, To, TraceTo}, {TakenCount, MispredCount}}.
+  // Where
+  // - From: is the source address of the sampled branch operation.
+  // - To: is the target address of the sampled branch operation.
+  // - TraceTo could be either
+  //    - A 'Type = Trace::BR_ONLY', which means the trace only contains branch
+  //    data.
+  //    - Or an address, when the trace contains information about the previous
+  //    branch.
+  //
+  // When FEAT_SPE_PBT is present, Arm SPE emits two records per sample:
+  // - the current branch (Spe.From/Spe.To), and
+  // - the previous taken branch target (PBT) (PBT.From, PBT.To).
+  //
+  // Together they behave like a depth-1 branch stack where:
+  //   - the PBT entry is always taken
+  //   - the current branch entry may represent a taken branch or a fall-through
+  //   - the destination (Spe.To) is the architecturally executed target
+  //
+  // There can be fall-throughs to be inferred between the PBT entry and
+  // the current branch (Spe.From), but there cannot be between current
+  // branch's (Spe.From/Spe.To).
+  //
+  // PBT records only the target address (PBT.To), meaning we have no
+  // information as the branch source (PBT.From=0x0), branch type, and the
+  // prediction bit.
+  //
+  // Consider the trace pair:
+  // {{Spe.From, Spe.To, Type}, {TK, MP}},
+  //   {{PBT.From, PBT.To, TraceTo}, {TK, MP}}
+  // {{0xd456, 0xd789, Trace::BR_ONLY}, {2, 2}}, {{0x0, 0xd123, 0xd456}, {2, 0}}
+  //
+  // The first entry is the Spe record, which represents a trace from 0xd456
+  // (Spe.From) to 0xd789 (Spe.To). Type = Trace::BR_ONLY, as Bolt processes the
+  // current branch event first. At this point we have no information about the
+  // previous trace (PBT). This entry has a TakenCount = 2, as we have two
+  // samples for (0xd456, 0xd789) in our input. It also has MispredsCount = 2,
+  // as 'M' misprediction flag appears in both cases.
+  //
+  // The second entry is the PBT record. TakenCount = 2 because the
+  // (PBT.From = 0x0, PBT.To = 0xd123) branch target appears twice in the input,
+  // and MispredsCount = 0 because prediction data is absent. There is no branch
+  // source information, so the PBT.From field is zero (0x0). TraceTo = 0xd456
+  // connect the flow from the previous taken branch at 0xd123 (PBT.To) to the
+  // current source branch at 0xd456 (Spe.From), which then continues to 0xd789
+  // (Spe.To).
+  std::vector<std::pair<Trace, TakenBranchInfo>> ExpectedSamples = {
+      {{0xa002, 0xa003, Trace::BR_ONLY}, {1, 0}},
+      {{0x0, 0xa001, 0xa002}, {1, 0}},
+      {{0xb002, 0xb003, Trace::BR_ONLY}, {1, 0}},
+      {{0x0, 0xb001, 0xb002}, {1, 0}},
+      {{0xc456, 0xc789, Trace::BR_ONLY}, {2, 0}},
+      {{0x0, 0xc123, 0xc456}, {2, 0}},
+      {{0xd456, 0xd789, Trace::BR_ONLY}, {2, 2}},
+      {{0x0, 0xd123, 0xd456}, {2, 0}},
+      {{0xe005, 0xe009, Trace::BR_ONLY}, {1, 0}},
+      {{0x0, 0xe001, 0xe005}, {1, 0}},
+      {{0xf002, 0xf003, Trace::BR_ONLY}, {1, 1}},
+      {{0x0, 0xf001, 0xf002}, {1, 0}}};
+
+  parseAndCheckBrstackEvents(4567, ExpectedSamples);
+}
+
 #endif