[Mesa-dev] [PATCH 3/3] glsl: Generate smaller values for uniform locations

Ian Romanick idr at freedesktop.org
Mon Jun 10 10:52:58 PDT 2013

From: Ian Romanick <ian.d.romanick at intel.com>

Previously we would generate uniform locations as (slot << 16) +
array_index.  We do this two handle applications that assume the
location of a[2] will be +1 from the location of a[1].  This resulted in
every uniform location being at least 0x10000.  We've now encountered
two applications that assume uniform values will be smaller.


Our workaround for one sort of buggy application breaks a different sort
of buggy application.  Irony anyone?

Other implementations happen to have both these behaviors (sequential
array elements and small values), so let's just match their behavior and
let buggy apps break on other implementations.  /Sigh/

NOTE: This is a candidate for stable release branches.

Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
Cc: Chad Versace <chad.versace at linux.intel.com>
 src/glsl/link_uniforms.cpp | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/glsl/link_uniforms.cpp b/src/glsl/link_uniforms.cpp
index 84680be..6f67c70 100644
--- a/src/glsl/link_uniforms.cpp
+++ b/src/glsl/link_uniforms.cpp
@@ -739,7 +739,19 @@ link_assign_uniform_locations(struct gl_shader_program *prog)
-   prog->UniformLocationBaseScale = (1U<<16);
+   /* Determine the size of the largest uniform array queryable via
+    * glGetUniformLocation.  Using this as the location scale guarantees that
+    * there is enough "room" for the array index to be stored in the low order
+    * part of the uniform location.  It also makes the locations be more
+    * tightly packed.
+    */
+   unsigned max_array_size = 1;
+   for (unsigned i = 0; i < num_user_uniforms; i++) {
+      if (uniforms[i].array_elements > max_array_size)
+	 max_array_size = uniforms[i].array_elements;
+   }
+   prog->UniformLocationBaseScale = max_array_size;
 #ifndef NDEBUG
    for (unsigned i = 0; i < num_user_uniforms; i++) {

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