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.

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.

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.

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.

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.