tree 543eb51f9fd07283412ffb41a05aed5d0b87704d
parent bbd8089297862efb6c39a22b5623f04567ff6443
author DRC <information@libjpeg-turbo.org> 1605291167 -0600
committer DRC <information@libjpeg-turbo.org> 1605302194 -0600

Neon: Auto-detect compiler intrinsics completeness

This allows the Neon intrinsics code to be built successfully (albeit
likely with reduced run-time performance) with Xcode 5.0-6.2
(iOS/AArch64) and Android NDK < r19 (AArch32).  Note that Xcode 5.0-6.2
will not build the Armv8 GAS code without gas-preprocessor.pl, and no
version of Xcode will build the Armv7 GAS code without
gas-preprocessor.pl, so we always use the full Neon intrinsics
implementation by default with macOS and iOS builds.

Auto-detecting the completeness of the compiler's set of Neon intrinsics
also allows us to more intelligently set the default value of
NEON_INTRINSICS, based on the values of HAVE_VLD1*.  This is a
reasonable, albeit imperfect, proxy for whether a compiler has a full
and optimal set of Neon intrinsics.  Specific notes:

  - 64-bit RGB-to-YCbCr color conversion
    does not use any of the intrinsics in question, regresses with GCC
  - 64-bit accurate integer forward DCT
    uses vld1_s16_x3(), regresses with GCC
  - 64-bit Huffman encoding
    uses vld1q_u8_x4(), regresses with GCC
  - 64-bit YCbCr-to-RGB color conversion
    does not use any of the intrinsics in question, regresses with GCC
  - 64-bit accurate integer inverse DCT
    uses vld1_s16_x3(), regresses with GCC
  - 64-bit 4x4 inverse DCT
    uses vld1_s16_x3().  I did not test this algorithm in isolation, so
    it may in fact regress with GCC, but the regression may be hidden by
    the speedup from the new SIMD-accelerated upsampling algorithms.

  - 32-bit RGB-to-YCbCr color conversion:
    uses vld1_u16_x2(), regresses with GCC
  - 32-bit accurate integer forward DCT
    uses vld1_s16_x3(), regression irrelevant because there was no
    previous implementation
  - 32-bit accurate integer inverse DCT
    uses vld1_s16_x3(), regresses with GCC
  - 32-bit fast integer inverse DCT
    does not use any of the intrinsics in question, regresses with GCC
  - 32-bit 4x4 inverse DCT
    uses vld1_s16_x3().  I did not test this algorithm in isolation, so
    it may in fact regress with GCC, but the regression may be hidden by
    the speedup from the new SIMD-accelerated upsampling algorithms.

Presumably when GCC includes a full and optimal set of Neon intrinsics,
the HAVE_VLD1* tests will pass, and the full Neon intrinsics
implementation will be enabled automatically.