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Re: [PATCH, v3] ARM: Integrate Cortex-A15 optimized memcpy using NEON/VFP.

On 12/04/13 08:42, Schwarz, Konrad wrote:
-----Original Message-----
From: []
On Behalf Of Will Newton
Sent: Thursday, April 11, 2013 10:14 PM
To: Ramana Radhakrishnan
Subject: Re: [PATCH, v3] ARM: Integrate Cortex-A15 optimized memcpy
using NEON/VFP.

My apologies if this has all been answered before, but the floating-point
units I am familiar with usually allow lazy context switching.
A common operating system optimization is give threads a floating point
context only after they actually start executing floating point operations.
(The initial floating point instruction causes a trap; the OS can
initializes the thread's floating point context before restarting
the instruction.)
Threads start off as integer (register) context only.

Won't an implementation of a nominally non-floating point function
such as memcpy break this optimization, and perhaps be harmful from
a system standpoint?

In bare-metal systems, the OS/Executive/whatever may not even have
have floating point support and would be in for a rude surprise
when they call memcpy().

Konrad Schwarz

I would have thought these days, with hardware floating-point support required by the Linux HF ABI, that this wasn't likely to be a major issue. The compiler will use FP insns freely as well whenever they are available, even for data moves. If you're using memcpy enough for performance to be an issue, then you'd want to use the fastest sequence possible. If you're not, then why would you care?

I see build options in the code for three variants: With Neon (and VFP), with VFP only and without either. That means that a bare metal systems have the option of using an integer-only variant (as does anyone else if they are really worried about using FP registers within memcpy).


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