Hi Toolchain Group,

I am trying to study the effect of loop buffer size on loop unrolling & the way gcc (aarch64) handles this.

To my understanding, Loop Buffer is like i-cache which contains pre-decoded instruction that can be re-used if branch instruction loopbacks to an instruction
which is still present in the buffer. For example, in Intel’s Nehalem loop buffer size is 28 u-ops. In LLVM compiler, it seems LoopMicroOpBufferSize is for the same purpose.
However, I could not find any parameter/variable inside config/aarch64 representing loop buffer size. I am using Linaro gcc 5.2.1

1. Is there any example inside aarch64 (or in general) which uses the loop buffer size in loop unrolling decision? If yes, could you please mention the relevant files or code section?
2. Otherwise any guidance/input on adding this support in aarch64 backend assuming architecture has the loop buffer support.

[My Experiments/Code Browsing]
I have collected following information from code browsing. Please correct if I missed or misunderstood something.

TARGET_LOOP_UNROLL_ADJUST - This target hook return the number of times a loop can be unrolled.
This can be used to handle the architecture constraint such number of memory references inside a loop e.g. ix86_loop_unroll_adjust() & s390_loop_unroll_adjust().
On the same note, can this be used to handle loop buffer size too?

Without above hook, in loop-unroll.c parameters like PARAM_MAX_UNROLLED_INSNS (default 200), PARAM_MAX_AVERAGE_UNROLLED_INSNS (default 80) decides the unrolling factor. e.g. nunroll = PARAM_VALUE (PARAM_MAX_UNROLLED_INSNS) / loop->ninsns;

In config/aarch64.c, I found align_loops variable in aarch64_override_options_after_change() function.
I guess this an alignment done before starting the loop header in the executable. This should not play any role in loop unrolling. Right?

So any guidance on how we can instruct aarch64 backend to utilize loop buffer size in deciding the loop unrolling factor?

Thanks in advance for your time.

--
with regards,
Virendra Kumar Pathak