Make IMDCT scaling a bit more conservative

jmvalin · jmvalin · commit 08f5ff06bb4c · 2025-06-19T13:39:23.000-04:00
Fixes 2941f08
diff --git a/celt/mdct.c b/celt/mdct.c
@@ -294,7 +294,7 @@ void clt_mdct_backward_c(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_sca
          maxval = MAX32(maxval, ABS32(in[i*stride]));
          sumval = ADD32_ovflw(sumval, ABS32(SHR32(in[i*stride],11)));
       }
-      pre_shift = IMAX(0, 29-celt_ilog2(1+SHR32(maxval,2)*3));
+      pre_shift = IMAX(0, 29-celt_zlog2(1+maxval));
       /* Worst-case where all the energy goes to a single sample. */
       post_shift = IMAX(0, 19-celt_ilog2(ABS32(sumval)));
       post_shift = IMIN(post_shift, pre_shift);

Original file line number	Diff line number	Diff line change
`@@ -294,7 +294,7 @@ void clt_mdct_backward_c(const mdct_lookup l, kiss_fft_scalar in, kiss_fft_sca`
`294`	`294`	`maxval = MAX32(maxval, ABS32(in[i*stride]));`
`295`	`295`	`sumval = ADD32_ovflw(sumval, ABS32(SHR32(in[i*stride],11)));`
`296`	`296`	`}`
`297`		`- pre_shift = IMAX(0, 29-celt_ilog2(1+SHR32(maxval,2)*3));`
	`297`	`+ pre_shift = IMAX(0, 29-celt_zlog2(1+maxval));`
`298`	`298`	`/* Worst-case where all the energy goes to a single sample. */`
`299`	`299`	`post_shift = IMAX(0, 19-celt_ilog2(ABS32(sumval)));`
`300`	`300`	`post_shift = IMIN(post_shift, pre_shift);`