https://bugs.freedesktop.org/show_bug.cgi?id=31837
Summary: Valgrind error due to alpha channel being left
uninitialized when sampling 24bit RGB textures
Product: Mesa
Version: 7.8
Platform: Other
OS/Version: All
Status: NEW
Severity: normal
Priority: medium
Component: Mesa core
AssignedTo: mesa-dev@lists.freedesktop.org
ReportedBy: bja...@mozilla.com
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.
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