| ===================================== |
| AArch64 Optimization and Flags Status |
| ===================================== |
| |
| Overview |
| -------- |
| |
| This page summarizes default-off BOLT optimization flags that users may |
| explicitly enable when optimizing AArch64 binaries. |
| |
| BOLT is to be used with binaries linked with |
| relocations (``--emit-relocs`` or ``-Wl,-q``) and representative profile data. |
| |
| Main Code-Layout Optimizations |
| ------------------------------ |
| The following code-layout optimizations are typically the first options to |
| consider when optimizing AArch64 binaries with representative profile data. |
| They typically provide the largest performance gains among BOLT optimizations. |
| |
| .. list-table:: |
| :header-rows: 1 |
| :widths: 34 42 |
| :align: left |
| |
| * - Flag |
| - Optimization |
| * - | ``--reorder-functions=exec-count|hfsort|cdsort|pettis-hansen|random|user`` |
| | ``--function-order=<file>`` |
| - Reorder functions |
| * - ``--reorder-blocks=normal|ext-tsp|cache|branch-predictor|reverse|cluster-shuffle`` |
| - Reorder basic blocks |
| * - | ``--split-functions`` |
| | ``--split-strategy=profile2|random2|randomN|all`` |
| | ``--split-all-cold`` |
| | ``--split-eh`` |
| - Split hot and cold code |
| |
| |
| Other Supported Optimizations |
| ----------------------------- |
| The following optimizations are also supported for AArch64. |
| |
| .. list-table:: |
| :header-rows: 1 |
| :widths: 34 42 |
| :align: left |
| |
| * - Flag |
| - Optimization |
| * - | ``--align-blocks`` |
| | ``--block-alignment=<uint>`` |
| - Align basic blocks |
| * - ``--tail-duplication=aggressive|moderate|cache`` |
| - Duplicate branch tails |
| * - ``--peepholes=double-jumps|tailcall-traps|useless-branches|all`` |
| - Run peephole optimizations |
| * - | ``--inline-all`` |
| | ``--inline-small-functions`` |
| | Related options: |
| | ``--inline-ap`` |
| | ``--inline-limit=<uint>`` |
| | ``--inline-small-functions-bytes=<uint>`` |
| - Inline functions |
| * - ``--icf=safe|all`` |
| - Fold identical functions |
| |
| Supported Flags With Limitations |
| -------------------------------- |
| The following flags are implemented for AArch64, but require specific runtime |
| or option conditions. Enabling them without the required conditions may report |
| an error or perform no transformation. |
| |
| .. list-table:: |
| :header-rows: 1 |
| :widths: 30 28 44 |
| :align: left |
| |
| * - Flag |
| - Optimization |
| - Notes |
| * - ``--inline-memcpy`` |
| - Inline fixed-size ``memcpy`` calls |
| - Only applies when the copy size is a known constant; AArch64 skips sizes over 64 bytes. |
| * - ``--plt=hot|all`` |
| - Optimize PLT calls |
| - Requires immediate binding. If BOLT cannot update the binary, relink with ``-znow``. |
| * - ``--hugify`` |
| - Place hot code on huge pages |
| - Applies to binaries with a recognized entry point; skipped when ``--instrument`` is used. |
| * - | ``--reorder-data=<section1,section2,...>`` |
| | ``--reorder-data-algo=count|funcs`` |
| - Reorder data sections |
| - ``move``, ``split`` and ``aggressive`` disable data reordering. |
| * - ``--split-strategy=cdsplit`` |
| - Split functions using cache-directed splitting |
| - Requires ``--compact-code-model`` on AArch64. |
| |
| Unsupported Flags |
| ----------------- |
| |
| The following flags are not available for AArch64. ``Not applicable to |
| AArch64`` means the optimization targets architectural features or mechanisms |
| that do not apply to AArch64. ``Not implemented for AArch64`` means the |
| optimization could be relevant, but is not currently implemented for this |
| target. |
| |
| .. list-table:: |
| :header-rows: 1 |
| :widths: 30 28 42 |
| :align: left |
| |
| * - Flag |
| - Optimization |
| - Notes |
| * - ``--jt-footprint-reduction`` |
| - Reduce jump-table footprint |
| - Not implemented for AArch64. |
| * - ``--three-way-branch`` |
| - Reorder three-way branches |
| - Not implemented for AArch64. |
| * - ``--simplify-rodata-loads`` |
| - Replace read-only data loads with constants |
| - Not implemented for AArch64. |
| * - ``--frame-opt=hot|all`` |
| - Optimize stack-frame accesses |
| - Not implemented for AArch64. |
| * - ``--indirect-call-promotion=calls|jump-tables|all`` |
| - Promote indirect calls |
| - Not implemented for AArch64. |
| * - ``--memcpy1-spec=<func1,func2:cs1:cs2,...>`` |
| - Specialize one-byte ``memcpy`` calls |
| - Not implemented for AArch64. |
| * - ``--reg-reassign`` |
| - Reassign registers to reduce encoding size |
| - Not applicable to AArch64. |
| * - ``--cmov-conversion`` |
| - Convert branches to conditional moves |
| - Not applicable to AArch64. |
| * - | ``--stoke`` |
| | ``--stoke-out`` |
| - Emit STOKE optimization data |
| - Not applicable to AArch64. |
| * - ``--insert-retpolines`` |
| - Insert retpolines |
| - Not applicable to AArch64. |
