[opus] [RFC PATCHv1] cover: celt_pitch_xcorr: Introduce ARM neon intrinsics

Jonathan Lennox jonathan at vidyo.com
Tue Nov 25 08:18:02 PST 2014

On Nov 25, 2014, at 11:13 AM, Viswanath Puttagunta <viswanath.puttagunta at linaro.org<mailto:viswanath.puttagunta at linaro.org>> wrote:

On 25 November 2014 at 10:11, Viswanath Puttagunta
<viswanath.puttagunta at linaro.org<mailto:viswanath.puttagunta at linaro.org>> wrote:

On 25 November 2014 at 09:39, Jonathan Lennox <jonathan at vidyo.com<mailto:jonathan at vidyo.com>> wrote:

On Nov 25, 2014, at 10:07 AM, Viswanath Puttagunta <viswanath.puttagunta at linaro.org<mailto:viswanath.puttagunta at linaro.org>> wrote:

Also is there plans to make the NEON optimisations on ARMv7 run time
detectable like they have in cairo/pixman? For generic distributions
it would nice to be able to be able to enable them as they offer
decent performance improvements but have the code fall back on devices
that don't support NEON.
Yep, adding support for ARMv8 is the final objective. I did not want to introduce too many changes in the first shot... and hence only introduced for ARMv7. In theory, most of the code (neon intrinsic code) in this patch should remain unchanged for ARMv8. Only the mechanism by which neon/asimd presence is detected during runtime and the flags used during compile are the only ones that should change. I will work on this once this patch gets reviewed and accepted. I made sure these changes are fairly localized.

And yes, this patch also supports runtime detection of neon. Actually, most of code to do run time detection of neon was already there in the project before this patch. I just re-used the infrastructure.

ARMv8 shouldn’t need Neon detection at all — Neon is a mandatory part of the ARMv8 architecture, unlike ARMv7, where it’s optional.
As I understand, your statement for ARMv8 is true for AAarch64 mode. But for ARMv8 in AAarch32 mode, neon is still optional (although I haven't heard of an implementation that does not support NEON even in AAarch32 mode). So, for AArch64 mode, I think rtcd can be disabled. Also, even the neon detection procedure currently in opus will not work on ARMv8 AArch32 mode. Please refer
This is one of my todo lists after this patchset gets reviewed and accepted.

It looks like this is what the configure script is already doing — arm64 sets rtcd_support to no.
I can't find any evidence of this in configure.ac. Can you please
point me to where rtcd_support is set to no for armv8 (AAarch64)?

Tip of git, starting line 202:

    202     case $host_cpu in
    203           arm64*|aarch64*)
    204         dnl Currently we only have asm for fixed-point
    205         AS_IF([test "$enable_float" != "yes"],[
    206             cpu_arm64=yes
    207             AC_DEFINE([OPUS_ARM64_ASM], [],  [Make use of ARM64 asm optimization])
    208             AS_GCC_INLINE_ASSEMBLY(
    209                 [inline_optimization="ARM64"],
    210                 [inline_optimization="disabled"]
    211             )
    212             AS_IF([test x"$inline_optimization" = x"ARM64"],[
    213                 AM_CONDITIONAL([OPUS_ARM64_INLINE_ASM],[true])
    214                 AC_DEFINE([OPUS_ARM64_INLINE_ASM], 1,
    215                     [Use ARM64 inline asm optimizations])
    216             ])
    217                         dnl Don't yet have external asm for arm64
    218                         asm_optimization="disabled"
    219                         dnl Don't need RTCD for arm64
    220                         rtcd_support=no
    221         ])
    222           ;;

This actually should probably be modified to support compilers that have ARM intrinsics but not GCC assembly (e.g. Visual Studio), however.
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