Re: Infinite loop in dcraw with current GCC versions

[dcoffin@xxxxxxxxxxxxxxxxxx]

Hi Manuel,

     Here's the blend_highlights() function from dcraw.c
with just the includes, defines, and globals it needs to
compile.

	gcc -Wall -O4 -march=athlon64 foo.c

     gives two "array bounds" warnings,

	gcc -Wall -O4 -march=x86-64 foo.c
	gcc -Wall -O4 foo.c
	gcc -Wall -O3 foo.c

     give one warning, while "-O2" or less gives no warnings.
Unlike -Wunused-result and -Waggressive-loop-optimizations,
-Warray-bound is not enabled by default, only because I use
-Wall.
				Dave Coffin  3/2/2015

On Mon, Mar 02, 2015 at 12:10:37PM +0100, Manuel López-Ibáñez wrote:
> On 1 March 2015 at 21:03,  <dcoffin@xxxxxxxxxxxxxxxxxx> wrote:
> >      I fixed all the "undefined behavior" warnings with
> > typecasts, but I'm still getting array-bounds warnings
> > with manifestly correct code.  E.g. when blend_highlights()
> > uses the global variable "colors":
> >
> >   static const float trans[2][4][4] = { ... }
> > ...
> >   if ((unsigned) (colors-3) > 1) return;
> > ...
> >   lab[i][c] += trans[colors-3][c][j] * cam[i][j];
> >
> >      How could "colors-3" be out of bounds?  I just verified
> > that it's either 0 or 1, and if I hard-code 0 or 1 the warning
> > goes away.
> 
> Could you post a small complete example? It doesn't need to have a
> main() function and it is better if it doesn't use any header files.
> 
> Cheers,
> 
> Manuel.

#include <stdio.h>
#include <limits.h>
#include <math.h>

#if !defined(ushort)
#define ushort unsigned short
#endif

#define FORC(cnt) for (c=0; c < cnt; c++)
#define FORCC FORC(colors)
#define MIN(a,b) ((a) < (b) ? (a) : (b))
#define SQR(x) ((x)*(x))
#define _(String) (String)

ushort (*image)[4], height, width;
unsigned colors;
float pre_mul[4];
int verbose=0;

void blend_highlights()
{
  int clip=INT_MAX, row, col, c, i, j;
  static const float trans[2][4][4] =
  { { { 1,1,1 }, { 1.7320508,-1.7320508,0 }, { -1,-1,2 } },
    { { 1,1,1,1 }, { 1,-1,1,-1 }, { 1,1,-1,-1 }, { 1,-1,-1,1 } } };
  static const float itrans[2][4][4] =
  { { { 1,0.8660254,-0.5 }, { 1,-0.8660254,-0.5 }, { 1,0,1 } },
    { { 1,1,1,1 }, { 1,-1,1,-1 }, { 1,1,-1,-1 }, { 1,-1,-1,1 } } };
  float cam[2][4], lab[2][4], sum[2], chratio;

  if ((unsigned) (colors-3) > 1) return;
  if (verbose) fprintf (stderr,_("Blending highlights...\n"));
  FORCC if (clip > (i = 65535*pre_mul[c])) clip = i;
  for (row=0; row < height; row++)
    for (col=0; col < width; col++) {
      FORCC if (image[row*width+col][c] > clip) break;
      if (c == colors) continue;
      FORCC {
	cam[0][c] = image[row*width+col][c];
	cam[1][c] = MIN(cam[0][c],clip);
      }
      for (i=0; i < 2; i++) {
	FORCC for (lab[i][c]=j=0; j < colors; j++)
	  lab[i][c] += trans[colors-3][c][j] * cam[i][j];
	for (sum[i]=0,c=1; c < colors; c++)
	  sum[i] += SQR(lab[i][c]);
      }
      chratio = sqrt(sum[1]/sum[0]);
      for (c=1; c < colors; c++)
	lab[0][c] *= chratio;
      FORCC for (cam[0][c]=j=0; j < colors; j++)
	cam[0][c] += itrans[colors-3][c][j] * lab[0][j];
      FORCC image[row*width+col][c] = cam[0][c] / colors;
    }
}