mesa: extend GLSLZeroInit semantics

This commit introduces a new way to zero-init variables but keep the
old one to not break any existing behavior.

With this change GLSLZeroInit becomes an integer, with the following
possible values:
 - 0: no 0 init
 - 1: current behavior
 - 2: new behavior. Similar to 1, except ir_var_function_out type are
      0 initialized but ir_var_shader_out.

The rationale behind 2 is: zero initializing ir_var_shader_out can
prevent some optimization where out variables are completely eliminated
when not written to.

On the other hand, zero initializing "ir_var_function_out" has no
effect on correct shaders but typically helps shadertoy since the main
function is:

   void mainImage(out vec4 fragColor) { ... }

So with this change we're sure that fragColor will always get a value.

Reviewed-by: Marek Olšák <[email protected]>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4607>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
index a46eb72da9a..7046e8adfa3 100644
--- a/src/mesa/main/mtypes.h
+++ b/src/mesa/main/mtypes.h
@@ -3868,9 +3868,9 @@ struct gl_constants
    GLboolean ForceGLSLAbsSqrt;
 
    /**
-    * Force uninitialized variables to default to zero.
+    * Types of variable to default initialized to zero.
     */
-   GLboolean GLSLZeroInit;
+   GLuint GLSLZeroInit;
 
    /**
     * Treat integer textures using GL_LINEAR filters as GL_NEAREST.