CRC32 affects performance for both image decoding (PNG)
as also in general browsing while accessing websites that serve
content using compression (i.e. Content-Encoding: gzip).

This patch implements an optimized CRC32 function using the
dedicated instruction available in ARMv8a. We only support
ARM Little-Endian (LE).

This instruction is available in new Android devices featuring an
ARMv8 SoC, like Nexus 5x and Google Pixel. It should be between
3x (A72) to 7x faster (A53) than the C implementation currently used
by zlib for 8KB vectors.

This is performance critical code and can be called with both large (8KB)
or small vectors, therefore we must avoid extraneous function calls or
branching (otherwise the performance benefits are negated). So the use
of 'public' variables to read the CPU features status flags
(i.e. arm_cpu_enable_crc32 | pmull).

Finally it also introduces code to perform run-time ARM CPU feature
detection on the supported platforms: Android and Linux/CrOS. We build
and link the CRC32 instruction dependent code, but will decide to use it
at run-time if the ARM CPU supports the CRC32 instruction. Otherwise,
we fallback to using zlib's default C implementation.

This approach allows to use the instruction in both 32bits and 64bits and
works fine either in ARMv7 or ARMv8 processor. I tested the generated
Chrome apk in both a Nexus 6 (ARMv7) and a Google Pixel (ARMv8).

The crc32 function benefited from input from Yang Zang and Mike Klein,
while the arm_features benefited from input from Noel Gordon.

Bug: 709716
Change-Id: I315c1216f8b3a8d88607630a28737c41f52a2f5d
Reviewed-on: https://chromium-review.googlesource.com/801108


Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#537179}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 28c9623083688b3a354c33bf77746f4c51f58826