tree 7bade4ad954fa3a6f959cc375af2494b0c9ec230
parent 98bc3eeb3abce0424f7063ecce06d214256015a5
author Jonathan Wright <jonathan.wright@arm.com> 1630666360 +0100
committer DRC <information@libjpeg-turbo.org> 1645815185 -0600

Neon/AArch64: Explicitly unroll quant loop w/Clang

The loop in jsimd_quantize_neon() is only executed twice and should be
unrolled for AArch64 targets.  GCC does that by default, but Clang 11
and later versions available at the time of this writing do not.  This
patch adds an unroll pragma when targetting AArch64 with Clang.  We do
not use the unroll pragma for AArch32 targets, because it causes the
Clang-generated assembly code to exhaust the available Neon registers
(32 x 64-bit) and spill to the stack.  (DRC: Referring to the discussion
in #570, this is likely due to compiler confusion that results in poor
register allocation.  It is possible to eliminate the spillage and
reduce the instruction count by loading the data on a just-in-time
basis, thus explicitly interleaving compute and I/O, but the performance
implications of that are currently unknown.)

The effects of unrolling the quantization loop are:
1) elimination of the loop control flow overhead and
2) enabling the use of LDP/STP instructions that work from a single
   base pointer, instead of using double the number of LDR/STR
   instructions, each requiring an address calculation.

Closes #570