[opus] Make check failure on clone from 31 January

[Marcello Caramma (mcaramma)]

I tracked down the bug to an incorrect use of restrict.

I would not consider this a compiler bug: we are lying to the optimizer by
telling it that a pointer is restrict when in fact it isn't.

This can be fixed like so:


diff --git a/celt/mdct.c b/celt/mdct.c
index 1634e8e..fa5098c 100644
--- a/celt/mdct.c
+++ b/celt/mdct.c
@@ -276,8 +276,8 @@ void clt_mdct_backward(const mdct_lookup *l,
kiss_fft_scalar *in, kiss_fft_scala
    /* Post-rotate and de-shuffle from both ends of the buffer at once to
make
       it in-place. */
    {
-      kiss_fft_scalar * OPUS_RESTRICT yp0 = out+(overlap>>1);
-      kiss_fft_scalar * OPUS_RESTRICT yp1 = out+(overlap>>1)+N2-2;
+      kiss_fft_scalar * yp0 = out+(overlap>>1);
+      kiss_fft_scalar * yp1 = out+(overlap>>1)+N2-2;
       const kiss_twiddle_scalar *t = &trig[0];
       /* Loop to (N4+1)>>1 to handle odd N4. When N4 is odd, the
          middle pair will be computed twice. */


Or like so:


diff --git a/celt/mdct.c b/celt/mdct.c
index 1634e8e..cdd053f 100644
--- a/celt/mdct.c
+++ b/celt/mdct.c
@@ -279,9 +279,8 @@ void clt_mdct_backward(const mdct_lookup *l,
kiss_fft_scalar *in, kiss_fft_scala
       kiss_fft_scalar * OPUS_RESTRICT yp0 = out+(overlap>>1);
       kiss_fft_scalar * OPUS_RESTRICT yp1 = out+(overlap>>1)+N2-2;
       const kiss_twiddle_scalar *t = &trig[0];
-      /* Loop to (N4+1)>>1 to handle odd N4. When N4 is odd, the
-         middle pair will be computed twice. */
-      for(i=0;i<(N4+1)>>1;i++)
+      /* Loop to N4>>1 to make sure pointers never overlap */
+      for(i=0;i<N4>>1;i++)
       {
          kiss_fft_scalar re, im, yr, yi;
          kiss_twiddle_scalar t0, t1;
@@ -309,6 +308,20 @@ void clt_mdct_backward(const mdct_lookup *l,
kiss_fft_scalar *in, kiss_fft_scala
          yp0 += 2;
          yp1 -= 2;
       }
+      // Handle the odd case
+      if(N4 & 1)
+      {
+         kiss_fft_scalar re, im;
+         kiss_twiddle_scalar t0, t1;
+         /* We swap real and imag because we're using an FFT instead of
an IFFT. */
+         re = yp0[1];
+         im = yp0[0];
+         t0 = t[i];
+         t1 = t[N4+i];
+         /* We'd scale up by 2 here, but instead it's done when mixing
the windows */
+         yp0[0] = S_MUL(re,t0) + S_MUL(im,t1);
+         yp0[1] = S_MUL(re,t1) - S_MUL(im,t0);
+      }
    }
 
    /* Mirror on both sides for TDAC */


Regards,

Marcello



On 05/02/2014 18:46, "Gregory Maxwell" <gmaxwell at gmail.com> wrote:

>On Wed, Feb 5, 2014 at 8:05 AM, Marcello Caramma (mcaramma)
><mcaramma at cisco.com> wrote:
>> Hi,
>>
>> Apologies if this is a known issue, but running make on revision
>>e3187444692195957eb66989622c7b1ad8448b06 fails one of the tests when
>>using fixed point configuration (floating point is ok) on my linux x86.
>> Note that libopus1.1, as extracted from the tar ball, is OK.
>
>I can't reproduce with Fedora 19, gcc version 4.8.2 20131212 (Red Hat
>4.8.2-7) (GCC) compiled with
>CFLAGS='-m32 -O2 -g'  ./configure --enable-fixed-point ; make clean ;
>make celt/tests/test_unit_mdct
>
>or with Clang 3.5 on the same system.
>
>None of the fixed point builds on or ci system are throwing errors on
>this either.
>
>Can you try compiling without optimizations? Smells like a compiler bug.