SPE (Statistical Profiling Extension) is an Arm hardware profiling unit that collects statistical samples of program execution with very low runtime overhead. SPE periodically samples microarchitectural events such as instruction execution, memory accesses, and branches. The processor records information about the sampled event in a trace buffer, which profiling tools later decode.
For BOLT, SPE branch samples are the relevant input as they provide an edge-based control-flow profile. Unlike BRBE profiling , SPE does not record sequences of taken branches. Instead, each sample describes only a single branch transition between two program locations, representing a single edge in the control-flow graph. Because of this limited context, SPE typically produces less detailed control-flow profiles than BRBE.
Some processors also support the Previous Branch Target (PBT) feature. PBT records the target of the most recently taken branch in addition to the sampled edge. This provides a depth-1 branch history, which slightly improves the quality of the reconstructed control-flow profile.
Even with PBT, SPE provides less branch history than BRBE, but it remains a useful profiling option when BRBE is not available.
SPE provides less detailed control-flow information than BRBE because it samples individual branch events rather than recording full branch histories. Despite this limitation, SPE can still capture useful branch behavior and guide code layout decisions. Use SPE when BRBE is unavailable or when instrumentation overhead is too high for the workload. In these cases, SPE offers a practical compromise between profiling overhead and profile quality.
SPE is an optional processor feature called FEAT_SPE (Statistical Profiling Extension), introduced in the
Armv8.1 architecture
.
To check whether your system supports SPE, attempt to record an SPE trace using perf.
If SPE is available, the command records the trace successfully:
perf record -e arm_spe/branch_filter=1/u -o prof/spe.data -- ./out/bsort
Bubble sorting 10000 elements
454 ms (first=100669 last=2147469841)
[ perf record: Woken up 7 times to write data ]
[ perf record: Captured and wrote 13.458 MB prof/spe.data ]
If the processor or kernel does not support SPE, perf reports an error similar to the following:
perf record -e arm_spe/branch_filter=1/u -o prof/spe.data -- ./out/bsort
event syntax error: 'arm_spe/branch_filter=1/u'
\___ Bad event or PMU
Unable to find PMU or event on a PMU of 'arm_spe'
Run 'perf list' for a list of valid events
Usage: perf record [<options>] [<command>]
or: perf record [<options>] -- <command> [<options>]
-e, --event <event> event selector. use 'perf list' to list available events
This error indicates that the system does not expose the arm_spe PMU, which usually means that the processor or kernel does not support SPE profiling.
Recording SPE traces requires a Linux kernel version 6.14 or later. Check the kernel version with:
uname -r
Next, collect an SPE profile by running the workload under perf. Then convert the recorded trace into a format that BOLT can use and run the BOLT optimizer.
The process consists of three steps:
mkdir -p prof
perf record -e arm_spe/branch_filter=1/u -o prof/spe.data -- ./out/bsort
perf2bolt -p prof/spe.data -o prof/spe.fdata ./out/bsort --spe
llvm-bolt out/bsort -o out/bsort.opt.spe --data prof/spe.fdata \
-reorder-blocks=ext-tsp -reorder-functions=cdsort -split-functions \
--dyno-stats
The perf record command collects branch samples using the SPE hardware profiler.
The perf2bolt tool converts the SPE trace into BOLT’s .fdata profile format, using the –spe option to interpret the samples correctly.
Finally, llvm-bolt uses the generated profile to reorganize functions and basic blocks in the binary, producing an optimized binary named out/bsort.opt.spe.
You’ve collected an SPE profile and used it to optimize the binary with BOLT. SPE provides useful control-flow information when BRBE is unavailable, though it captures less detailed branch history.
You can explore other profiling methods or move on to verify the optimization results.