Mesa (master): linker: Eliminate more dead code after demoting shader inputs and outputs

Ian Romanick idr at
Tue Oct 25 17:52:58 PDT 2011

Module: Mesa
Branch: master
Commit: 960d722bf7db636863a05ddf9258236fccb58ecd

Author: Ian Romanick <ian.d.romanick at>
Date:   Fri Oct 21 11:21:02 2011 -0700

linker: Eliminate more dead code after demoting shader inputs and outputs

Consider the following vertex shader and fragment shader:

// vertex shader
varying vec4 v;
uniform vec4 u;

void main() { gl_Position = vec4(0.0); v = u; }

// fragment shader
void main() { gl_FragColor = vec4(0.0); }

Since the fragment shader does not use 'v', it is demoted from a
varying to a simple global variable.  Once that happens, the
assignment to 'v' is useless, and it should be removed.  In addition,
'u' is no longer active, and it should also be removed.

Performing extra dead code elimination after demoting shader inputs
and outputs takes care of this.  This elimination must occur before
assigning uniform locations, or the declaration of 'u' cannot be

This change *breaks* the piglit test getuniform-01, but that test is
already incorrect.  The test uses a vertex shader that assigns to a
user-defined varying, but it has no fragment shader.  Since Mesa does
not support ARB_separate_shader_objects (we only support the EXT
version), the linker correctly eliminates the user-defined varying.
The cascading effect is that the uniform queried by the C code of the
test is also (correctly) eliminated.

Signed-off-by: Ian Romanick <ian.d.romanick at>
Tested-by: Brian Paul <brianp at>
Cc: Bryan Cain <bryancain3 at>
Cc: Vinson Lee <vlee at>
Cc: José Fonseca <jfonseca at>
Reviewed-by: Kenneth Graunke <kenneth at>
Reviewed-by: Yuanhan Liu <yuanhan.liu at>


 src/glsl/linker.cpp |   26 ++++++++++++++++++++++----
 1 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index d4d2496..beadec6 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -1746,10 +1746,6 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
-   update_array_sizes(prog);
-   assign_uniform_locations(prog);
    /* FINISHME: The value of the max_attribute_index parameter is
     * FINISHME: implementation dependent based on the value of
@@ -1785,6 +1781,12 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
    if (prog->_LinkedShaders[MESA_SHADER_VERTEX] != NULL) {
+      /* Eliminate code that is now dead due to unused vertex outputs being
+       * demoted.
+       */
+      while (do_dead_code(prog->_LinkedShaders[MESA_SHADER_VERTEX]->ir, false))
+	 ;
    if (prog->_LinkedShaders[MESA_SHADER_GEOMETRY] != NULL) {
@@ -1793,14 +1795,30 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
       demote_shader_inputs_and_outputs(sh, ir_var_in);
       demote_shader_inputs_and_outputs(sh, ir_var_inout);
       demote_shader_inputs_and_outputs(sh, ir_var_out);
+      /* Eliminate code that is now dead due to unused geometry outputs being
+       * demoted.
+       */
+      while (do_dead_code(prog->_LinkedShaders[MESA_SHADER_GEOMETRY]->ir, false))
+	 ;
    if (prog->_LinkedShaders[MESA_SHADER_FRAGMENT] != NULL) {
       gl_shader *const sh = prog->_LinkedShaders[MESA_SHADER_FRAGMENT];
       demote_shader_inputs_and_outputs(sh, ir_var_in);
+      /* Eliminate code that is now dead due to unused fragment inputs being
+       * demoted.  This shouldn't actually do anything other than remove
+       * declarations of the (now unused) global variables.
+       */
+      while (do_dead_code(prog->_LinkedShaders[MESA_SHADER_FRAGMENT]->ir, false))
+	 ;
+   update_array_sizes(prog);
+   assign_uniform_locations(prog);
    /* OpenGL ES requires that a vertex shader and a fragment shader both be
     * present in a linked program.  By checking for use of shading language
     * version 1.00, we also catch the GL_ARB_ES2_compatibility case.

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