c - Is branch predication used in gcc for ARM and how can we disable it? -


i compiling binaries armv8 , armv7 devices , want disable branch predication. know question has been asked once (here) target x86 machine. i'm using linaro gcc arm.

i'm doing experiment sake , don't recommend practice.

i have used perf measure branch instructions / branch misses count.

so have 2 questions:

  1. is branch predication utilised gcc/g++ arm?
  2. how can disable it?

i have tried following options:

  • fno-branch-target-load-optimize(2) - no effect on binary size generated different assembly. branch statistics identical without using option.
  • -fno-if-conversion(2) - binary size identical, assembly different. increased number of branches being executed , branch miss rate. appears option dose something.
  • -fno-guess-branch-probability - same above had lower impact.

so can sure using -fno-if-conversion , -fno-guess-branch-probability disabling branch predication?

after doing more research , tests have answer.

in armv7 isa there predicated instructions. compiler uses them when thinks beneficial. can disabled -fno-if-conversion -fno-if-conversion2 compilation flags. in cases branch predictor job, can potentially increase performance.

armv8 aarch64 isa has limited support branch predication there-s not disable. number of predicated instructions reduced @ minimum. when using -fno-if-conversion flags don-t see impact on number of executed instructions (used perf this). means branch predictor more important.


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