mp3d_synth_pair()

static void mp3d_synth_pair ( mp3d_sample_t * pcm, int nch, const int32_t * z )

static

Definition at line 616 of file minimp3_synth_fixed.h.

{
    int64_t a;
    /* The sums and differences are computed with defined wraparound, then
       widened (FastLED#4133).
 
       `(int64_t)(x - y)` evaluates `x - y` in int32 first and only then widens,
       which overflows once the polyphase inputs get large -- undefined
       behaviour, not merely a wrong number. Ordinary audio never gets there; a
       malformed intensity-stereo stream does, and UBSan caught it on
       l3-nonstandard-big-iscf.
 
       Widening both operands first was the obvious fix and it is the wrong one
       here: an int64 add or subtract costs two instructions plus carry on a
       32-bit target, and the codegen ledger measured it at +34% on the Xtensa
       polyphase inner loop -- 251 instructions against 187 -- which is the
       hottest kernel on the most constrained platform FastLED targets.
 
       Unsigned arithmetic wraps by definition in C, so MP3D_WRAP_SUB and
       MP3D_WRAP_ADD emit exactly the same single 32-bit instruction the
       undefined version did, and produce exactly the same bits. No well-formed
       stream reaches the wrap; a malformed one now gets a defined, reproducible
       answer instead of whatever the optimiser felt entitled to assume. */
    a  = (int64_t)MP3D_WRAP_SUB(z[14*64], z[    0]) * 29;
    a += (int64_t)MP3D_WRAP_ADD(z[ 1*64], z[13*64]) * 213;
    a += (int64_t)MP3D_WRAP_SUB(z[12*64], z[ 2*64]) * 459;
    a += (int64_t)MP3D_WRAP_ADD(z[ 3*64], z[11*64]) * 2037;
    a += (int64_t)MP3D_WRAP_SUB(z[10*64], z[ 4*64]) * 5153;
    a += (int64_t)MP3D_WRAP_ADD(z[ 5*64], z[ 9*64]) * 6574;
    a += (int64_t)MP3D_WRAP_SUB(z[ 8*64], z[ 6*64]) * 37489;
    a += (int64_t) z[ 7*64]                         * 75038;
    pcm[0] = mp3d_scale_pcm(a);
 
    z += 2;
    a  = (int64_t)z[14*64] * 104;
    a += (int64_t)z[12*64] * 1567;
    a += (int64_t)z[10*64] * 9727;
    a += (int64_t)z[ 8*64] * 64019;
    a += (int64_t)z[ 6*64] * -9975;
    a += (int64_t)z[ 4*64] * -45;
    a += (int64_t)z[ 2*64] * 146;
    a += (int64_t)z[ 0*64] * -5;
    pcm[16*nch] = mp3d_scale_pcm(a);
}

References FL_NO_EXCEPT, mp3d_scale_pcm(), and z.

Referenced by mp3d_synth().

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