mesa: Only update sampler uniforms that are used by the shader stage

Previously a vertex shader that used no samplers would get updated (by
calling the driver's ProgramStringNotify) when a sampler in the
fragment shader was updated.  This was discovered while investigating
some spurious code generation for shaders in Cogs.  The behavior in
Cogs is especially pessimal because it ping-pongs sampler uniform
settings:

    glUniform1i(sampler1, 0);
    glUniform1i(sampler2, 1);
    draw();
    glUniform1i(sampler1, 1);
    glUniform1i(sampler2, 0);
    draw();
    glUniform1i(sampler1, 0);
    glUniform1i(sampler2, 1);
    draw();
    // etc.

ProgramStringNotify is still too big of a hammer.  Applications like
Cogs will still defeat the shader cache.  A lighter-weight mechanism
that can work with the shader cache is needed.  However, this patch at
least restores the previous behavior.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/uniform_query.cpp

@@ -698,11 +698,27 @@ _mesa_uniform(struct gl_context *ctx, struct gl_shader_program *shProg,
 
 	 prog = shProg->_LinkedShaders[i]->Program;
 
+	 /* If the shader stage doesn't use any samplers, don't bother
+	  * checking if any samplers have changed.
+	  */
+	 if (prog->SamplersUsed == 0)
+	    continue;
+
 	 assert(sizeof(prog->SamplerUnits) == sizeof(shProg->SamplerUnits));
 
-	 if (memcmp(prog->SamplerUnits,
-		    shProg->SamplerUnits,
-		    sizeof(shProg->SamplerUnits)) != 0) {
+	 /* Determine if any of the samplers used by this shader stage have
+	  * been modified.
+	  */
+	 bool changed = false;
+	 for (unsigned j = 0; j < Elements(prog->SamplerUnits); j++) {
+	    if ((prog->SamplersUsed & (1U << j)) != 0
+		&& (prog->SamplerUnits[j] != shProg->SamplerUnits[j])) {
+	       changed = true;
+	       break;
+	    }
+	 }
+
+	 if (changed) {
 	    if (!flushed) {
 	       FLUSH_VERTICES(ctx, _NEW_TEXTURE | _NEW_PROGRAM);
 	       flushed = true;