| commit | 7085065c02da6091dca91be201160912e43a63ec | [log] [tgz] |
|---|---|---|
| author | Amir Ayupov <aaupov@fb.com> | Fri Jun 20 03:17:08 2025 -0700 |
| committer | GitHub <noreply@github.com> | Fri Jun 20 03:17:08 2025 -0700 |
| tree | 673658822b190396a1033d9b9f60702a1edab592 | |
| parent | a5fa5bd2a890c588b99b07fc0f3fcef236888609 [diff] |
[BOLT] Support pre-aggregated returns (#143296) Intel's Architectural LBR supports capturing branch type information as part of LBR stack (SDM Vol 3B, part 2, October 2024): ``` 20.1.3.2 Branch Types The IA32_LBR_x_INFO.BR_TYPE and IA32_LER_INFO.BR_TYPE fields encode the branch types as shown in Table 20-3. Table 20-3. IA32_LBR_x_INFO and IA32_LER_INFO Branch Type Encodings Encoding | Branch Type 0000B | COND 0001B | NEAR_IND_JMP 0010B | NEAR_REL_JMP 0011B | NEAR_IND_CALL 0100B | NEAR_REL_CALL 0101B | NEAR_RET 011xB | Reserved 1xxxB | OTHER_BRANCH For a list of branch operations that fall into the categories above, see Table 20-2. Table 20-2. Branch Type Filtering Details Branch Type | Operations Recorded COND | Jcc, J*CXZ, and LOOP* NEAR_IND_JMP | JMP r/m* NEAR_REL_JMP | JMP rel* NEAR_IND_CALL | CALL r/m* NEAR_REL_CALL | CALL rel* (excluding CALLs to the next sequential IP) NEAR_RET | RET (0C3H) OTHER_BRANCH | JMP/CALL ptr*, JMP/CALL m*, RET (0C8H), SYS*, interrupts, exceptions (other than debug exceptions), IRET, INT3, INTn, INTO, TSX Abort, EENTER, ERESUME, EEXIT, AEX, INIT, SIPI, RSM ``` Linux kernel can preserve branch type when `save_type` is enabled, even if CPU does not support Architectural LBR: https://github.com/torvalds/linux/blob/f09079bd04a924c72d555cd97942d5f8d7eca98c/tools/perf/Documentation/perf-record.txt#L457-L460 > - save_type: save branch type during sampling in case binary is not available later. For the platforms with Intel Arch LBR support (12th-Gen+ client or 4th-Gen Xeon+ server), the save branch type is unconditionally enabled when the taken branch stack sampling is enabled. Kernel-reported branch type values: https://github.com/torvalds/linux/blob/8c6bc74c7f8910ed4c969ccec52e98716f98700a/include/uapi/linux/perf_event.h#L251-L269 This information is needed to disambiguate external returns (from DSO/JIT) to an entry point or a landing pad, when BOLT can't disassemble the branch source. This patch adds new pre-aggregated types: - return trace (R), - external return fall-through (r). For such types, the checks for fall-through start (not an entry or a landing pad) are relaxed. Depends on #143295. Test Plan: updated callcont-fallthru.s
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