tree 6087083c35131860fa72456a56e20c03ecd8f881
parent 55acd713d02f61c984e10ff2d4cef775a7fb597c
author Elliott Hughes <enh@google.com> 1468551542 -0700
committer Elliott Hughes <enh@google.com> 1468551542 -0700

Fix AArch64 ABI conformance issue in SIMD code.

In the AArch64 ABI, the high (unused) DWORD of a 32-bit argument's
register is undefined, so it was incorrect to use 64-bit instructions
to transfer a JDIMENSION argument in the 64-bit NEON SIMD functions.
The code worked thus far only because the existing compiler optimizers
weren't smart enough to do anything else with the register in question,
so the upper 32 bits happened to be all zeroes.

The latest builds of Clang/LLVM have a smarter optimizer, and under
certain circumstances, it will attempt to load-combine adjacent 32-bit
integers from one of the libjpeg structures into a single 64-bit integer
and pass that 64-bit integer as a 32-bit argument to one of the SIMD
functions (which is allowed by the ABI, since the upper 32 bits of the
32-bit argument's register are undefined.)  This caused the
libjpeg-turbo regression tests to crash.

This patch tries to use the Wn registers whenever possible.  Otherwise,
it uses a zero-extend instruction to avoid using the upper 32 bits of
the 64-bit registers, which are not guaranteed to be valid for 32-bit
arguments.

Based on sebpop@1fbae13

Closes #91.  Refer also to android-ndk/ndk#110 and
https://llvm.org/bugs/show_bug.cgi?id=28393

(cherrypick from external/libjpeg-turbo of c8282be605a139b269f2b8f4b4fb6f4118e740ab.)

Change-Id: I935a3696106e7ffcf9b1a6c12cb7f31d95b4ccc8