Re: [boost] [beast] Formal review

Subject: Re: [boost] [beast] Formal review
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-07-11 14:24:25

On 11/07/2017 14:49, Peter Dimov via Boost wrote:
> Phil Endecott wrote:
>> > That hurts 32-bit ARM.
>>
>> I think that's an issue with whatever compiler you're using, not the
>> architecture; I've just done a quick test with
>> arm-linux-gnueabihf-g++-6 6.3.0 and I get about a 5% speedup by using
>> memcpy.
> 
> No, it's an issue with ARM32 not allowing unaligned loads. The memcpy
> code, at best, uses four byte loads, instead of one 32 bit one.
> __builtin_assume_aligned doesn't help.

Be aware that some recent ARM CPUs no longer penalise unaligned loads.
Cortex A15 I vaguely remember mostly does not except sometimes, Cortex
A57 definitely does not.

This may explain confounding results: a Cortex A9 most definitely
punishes unaligned loads badly, as do any of the lower end even if very
new ARM CPUs.

Niall

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