Commit a5e3c8a6 authored by Jean-Marc Valin's avatar Jean-Marc Valin
Browse files

Inverse MDCT no longer requires any scratch space

parent e2bcb3fe
......@@ -241,13 +241,10 @@ void clt_mdct_backward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scala
int i;
int N, N2, N4;
kiss_twiddle_scalar sine;
VARDECL(kiss_fft_cpx, f2);
SAVE_STACK;
N = l->n;
N >>= shift;
N2 = N>>1;
N4 = N>>2;
ALLOC(f2, N4, kiss_fft_cpx);
/* sin(x) ~= x here */
#ifdef FIXED_POINT
sine = TRIG_UPSCALE*(QCONST16(0.7853981f, 15)+N2)/N;
......@@ -260,27 +257,27 @@ void clt_mdct_backward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scala
/* Temp pointers to make it really clear to the compiler what we're doing */
const kiss_fft_scalar * OPUS_RESTRICT xp1 = in;
const kiss_fft_scalar * OPUS_RESTRICT xp2 = in+stride*(N2-1);
kiss_fft_cpx * OPUS_RESTRICT yp = f2;
kiss_fft_scalar * OPUS_RESTRICT yp = out+(overlap>>1);
const kiss_twiddle_scalar * OPUS_RESTRICT t = &l->trig[0];
const opus_int16 * OPUS_RESTRICT bitrev = l->kfft[shift]->bitrev;
for(i=0;i<N4;i++)
{
int rev;
kiss_fft_scalar yr, yi;
kiss_fft_cpx yc;
rev = *bitrev++;
yr = -S_MUL(*xp2, t[i<<shift]) + S_MUL(*xp1,t[(N4-i)<<shift]);
yi = -S_MUL(*xp2, t[(N4-i)<<shift]) - S_MUL(*xp1,t[i<<shift]);
/* Works because the cos is nearly one. We swap real and imag because we
use an FFT instead of an IFFT. */
yc.i = yr - S_MUL(yi,sine);
yc.r = yi + S_MUL(yr,sine);
yp[2*rev+1] = yr - S_MUL(yi,sine);
yp[2*rev] = yi + S_MUL(yr,sine);
/* Storing the pre-rotation directly in the bitrev order. */
yp[*bitrev++] = yc;
xp1+=2*stride;
xp2-=2*stride;
}
}
opus_fft_impl(l->kfft[shift], f2);
opus_fft_impl(l->kfft[shift], (kiss_fft_cpx*)(out+(overlap>>1)));
/* Post-rotate and de-shuffle from both ends of the buffer at once to make
it in-place. */
......@@ -295,16 +292,16 @@ void clt_mdct_backward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scala
kiss_fft_scalar re, im, yr, yi;
kiss_twiddle_scalar t0, t1;
/* We swap real and imag because we're using an FFT instead of an IFFT. */
re = f2[i].i;
im = f2[i].r;
re = yp0[1];
im = yp0[0];
t0 = t[i<<shift];
t1 = t[(N4-i)<<shift];
/* We'd scale up by 2 here, but instead it's done when mixing the windows */
yr = S_MUL(re,t0) - S_MUL(im,t1);
yi = S_MUL(im,t0) + S_MUL(re,t1);
/* We swap real and imag because we're using an FFT instead of an IFFT. */
re = f2[N4-i-1].i;
im = f2[N4-i-1].r;
re = yp1[1];
im = yp1[0];
/* works because the cos is nearly one */
yp0[0] = -(yr - S_MUL(yi,sine));
yp1[1] = yi + S_MUL(yr,sine);
......@@ -340,5 +337,4 @@ void clt_mdct_backward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scala
wp2--;
}
}
RESTORE_STACK;
}
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