This comes from https://bugzilla.mozilla.org/show_bug.cgi?id=588918, see especially comments 16 and 18 there.

I have only tried with Mesa 7.8. Source code references below are against Mesa 7.8.2.

*** Description of the problem ***

Running under valgrind a program that does a glDrawArrays call using a GL_RGB8 texture, I get this error:

==14218== Conditional jump or move depends on uninitialised value(s)
==14218==    at 0x2111134F: _mesa_convert_colors (image.c:5421)
==14218==    by 0x21176CBA: convert_color_type (s_span.c:941)
==14218==    by 0x211784B4: _swrast_write_rgba_span (s_span.c:1226)
==14218==    by 0x21187818: general_triangle (s_tritemp.h:819)
==14218==    by 0x21153E6C: _tnl_render_triangles_verts (t_vb_rendertmp.h:184)
==14218==    by 0x211554B1: run_render (t_vb_render.c:320)
==14218==    by 0x2114D5F6: _tnl_run_pipeline (t_pipeline.c:153)
==14218==    by 0x2114E262: _tnl_draw_prims (t_draw.c:478)
==14218==    by 0x2114E340: _tnl_vbo_draw_prims (t_draw.c:384)
==14218==    by 0x21146FB0: vbo_exec_DrawArrays (vbo_exec_array.c:524)
==14218==    by 0x5A81AC8: mozilla::gl::GLContext::fDrawArrays(unsigned int,
int, int) (GLContext.h:1185)
==14218==    by 0x5A670B0: mozilla::WebGLContext::DrawArrays(unsigned int, int,
int) (WebGLContextGL.cpp:1075)
==14218==  Uninitialised value was created by a stack allocation
==14218==    at 0x211699D8: fetch_texel (prog_execute.c:373)

Stepping in GDB through fetch_texel(), I arrive at this location:
   opt_sample_rgb_2d at swrast/s_texfilter.c:1366

The code there does:

   for (k=0; k<n; k++) {
      GLint i = IFLOOR(texcoords[k][0] * width) & colMask;
      GLint j = IFLOOR(texcoords[k][1] * height) & rowMask;
      GLint pos = (j << shift) | i;
      GLubyte *texel = ((GLubyte *) img->Data) + 3*pos;
      rgba[k][RCOMP] = UBYTE_TO_FLOAT(texel[2]);
      rgba[k][GCOMP] = UBYTE_TO_FLOAT(texel[1]);
      rgba[k][BCOMP] = UBYTE_TO_FLOAT(texel[0]);
   }

It seems that it's leaving the alpha channel uninitialized.

When we look at the location of the first reported valgrind error using that
uninitialized value, _mesa_convert_colors (image.c:5421), the source code is:


   case GL_FLOAT:
      if (dstType == GL_UNSIGNED_BYTE) {
         const GLfloat (*src4)[4] = (const GLfloat (*)[4]) src;
         GLubyte (*dst1)[4] = (GLubyte (*)[4]) (useTemp ? tempBuffer : dst);
         GLuint i;
         for (i = 0; i < count; i++) {
            if (!mask || mask[i]) {
               UNCLAMPED_FLOAT_TO_UBYTE(dst1[i][RCOMP], src4[i][RCOMP]);
               UNCLAMPED_FLOAT_TO_UBYTE(dst1[i][GCOMP], src4[i][GCOMP]);
               UNCLAMPED_FLOAT_TO_UBYTE(dst1[i][BCOMP], src4[i][BCOMP]);
               UNCLAMPED_FLOAT_TO_UBYTE(dst1[i][ACOMP], src4[i][ACOMP]);
            }
         }
         if (useTemp)
            memcpy(dst, tempBuffer, count * 4 * sizeof(GLubyte));
      }


The error line 5421 is the one converting the alpha channel:
               UNCLAMPED_FLOAT_TO_UBYTE(dst1[i][ACOMP], src4[i][ACOMP]);

This confirms the theory that the valgrind error is caused by the alpha value here being uninitialized.