[pulseaudio-discuss] ARM NEON patches

[Peter Meerwald]

Hello,

> If __ARM_ENON__ is defined, the compiler assumes NEON is available and can
> emit NEON opcodes for plain C code. In particular, any code that parses
> /proc/cpuinfo for "Features:.*neon" under #ifdef __ARM_NEON__ is completely
> *non-sensical*. Yes, this include patch 1 of 4 in your patchset - I already
> pointed that out but nobody seemed to pay attention.

ah, I see your point

the assumption is that the cross-compilation environment passes
meaningful CC and/or CFLAGS and PulseAudio takes advantage if NEON is 
available

I agree that the runtime-check for NEON is a bit pointless and can be 
dropped

> "-mfpu=neon", then the compiler assumes the code will run ONLY on
> NEON-capable ARM devices. If you want to do run-time detection, you MUST
> NOT pass the corresponding compiler flag. (The same is true of MMX and SSE
> by the way.)

so the simple solution is to 
- drop the runtime check
- use NEON if the compiler provides NEON

if the code has to run on non-NEON platforms, NEON support cannot be 
enabled in the compiler


the more involved solution is to 
- have the runtime check in place
- compile code with different compiler flags
- make a decision at runtime and call different code path

in PulseAudio, the MMX/SSE code path use inline assembler; surprisingly 
(for me at least), gcc happily compiles inline assembler SSE/MMX code even with 
-march=i386, i.e. arch=i386 does not get passed to the assembler

this behaviour makes it all too easy... :)

PulseAudio simply assumes that the compiler is recent enough to know about 
MMX/SSE, there is no compile-time probing or checks such as #ifdef 
__SSE__ (fair enough)

to take this solution, some build infrastructure is needed; it might be 
required as well for the SSE3 resampler patches in discussion

this means:
- probe compiler flags (such as -msse2, -mfpu=neon)
- probably configure options to override
- passing different compiler flags to different compilation units

which route shall we go?

regards, p.

-- 

Peter Meerwald
+43-664-2444418 (mobile)