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Hash : c5f269eb
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Date : 2021-09-03T11:52:40
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
