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Hack that makes the SMALL_FOOTPRINT CELT decoder use only 4.25 kB of stack.

--- a/celt/celt_decoder.c

+++ b/celt/celt_decoder.c

@@ -708,7 +708,11 @@

    opus_int32 bits;

    ec_dec _dec;

    VARDECL(celt_sig, freq);

+#ifdef SMALL_FOOTPRINT

+   celt_norm *X;

+#else

    VARDECL(celt_norm, X);

+#endif

    VARDECL(int, fine_quant);

    VARDECL(int, pulses);

    VARDECL(int, cap);

@@ -944,9 +948,21 @@

    unquant_fine_energy(mode, start, end, oldBandE, fine_quant, dec, C);

+   c=0; do {

+      OPUS_MOVE(decode_mem[c], decode_mem[c]+N, DECODE_BUFFER_SIZE-N+overlap/2);

+      out_syn[c] = decode_mem[c]+DECODE_BUFFER_SIZE-N;

+   } while (++c<CC);

+

    /* Decode fixed codebook */

    ALLOC(collapse_masks, C*nbEBands, unsigned char);

+

+#ifdef SMALL_FOOTPRINT

+   /* This is an ugly hack that breaks aliasing rules and would be easily broken,

+      but it saves almost 4kB of stack. */

+   X = (celt_norm*)(out_syn[CC-1]+overlap/2);

+#else

    ALLOC(X, C*N, celt_norm);   /**< Interleaved normalised MDCTs */

+#endif

    quant_all_bands(0, mode, start, end, X, C==2 ? X+N : NULL, collapse_masks,

          NULL, pulses, shortBlocks, spread_decision, dual_stereo, intensity, tf_res,

@@ -969,14 +985,6 @@

       for (i=0;i<C*nbEBands;i++)

          oldBandE[i] = -QCONST16(28.f,DB_SHIFT);

    }

-

-   c=0; do {

-      OPUS_MOVE(decode_mem[c], decode_mem[c]+N, DECODE_BUFFER_SIZE-N+overlap/2);

-   } while (++c<CC);

-

-   c=0; do {

-      out_syn[c] = decode_mem[c]+DECODE_BUFFER_SIZE-N;

-   } while (++c<CC);

    celt_synthesis(mode, X, out_syn, oldBandE, start, effEnd, C, CC, isTransient, LM, st->downsample, silence);