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

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Merged
merged 3 commits into from
Nov 6, 2025
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[BOLT] Adding a unittest that covers Arm SPE PBT aggregation #160095

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5baecf8
[BOLT] Add unit test for SPE PBT feature
kaadam Sep 16, 2025
666ff2f
Description is updated
kaadam Oct 20, 2025
9958168
Fix the format
kaadam Oct 20, 2025
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88 changes: 88 additions & 0 deletions bolt/unittests/Profile/PerfSpeEvents.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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@paschalis-mpeis paschalis-mpeis Oct 21, 2025

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  // and MispredsCount = 0 because prediction data is absent. There is no branch

A note to have in mind (not for this patch):
if we get users with PBT with branch prediction bits utilized in optimization, we have to keep in mind that this won't be accurate. For example, reporting 0 misspredictions for 2 branches is interpreted as predictions being correct, where in fact the data was absent.

Maybe this could be marked as -1 instread, and treated as a NAN where its relevant.

// 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
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