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, 0 deletions

diff --git a/src/compiler/glsl/glsl_parser_extras.cpp b/src/compiler/glsl/glsl_parser_extras.cpp
index 5b8e5b4c48f..e610fda002c 100644
--- a/src/compiler/glsl/glsl_parser_extras.cpp
+++ b/src/compiler/glsl/glsl_parser_extras.cpp
@@ -84,6 +84,8 @@ _mesa_glsl_parse_state::_mesa_glsl_parse_state(struct gl_context *_ctx,
    this->forced_language_version = ctx->Const.ForceGLSLVersion;
    if (ctx->Const.GLSLZeroInit == 1) {
       this->zero_init = (1u << ir_var_auto) | (1u << ir_var_temporary) | (1u << ir_var_shader_out);
+   } else if (ctx->Const.GLSLZeroInit == 2) {
+      this->zero_init = (1u << ir_var_auto) | (1u << ir_var_temporary) | (1u << ir_var_function_out);
    } else {
       this->zero_init = 0;
    }