mesa: Add a _mesa_active_fragment_shader_has_side_effects helper

Some drivers can disable the FS unit if there is nothing in the shader code
that writes to an output (i.e. color, depth, etc). Right now, mesa has
a function to check for atomic buffers and the i965 driver also checks for
images. Refactor this logic into a generic function that we can use for
any source of side effects in a fragment shader. Suggested by Jason.

v2:
- Use '_Shader', as suggested by Tapani, to fix the following CTS test:

ES31-CTS.shader_atomic_counters.advanced-usage-many-draw-calls2

Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/mesa/drivers/dri/i965/gen7_wm_state.c

@@ -77,13 +77,9 @@ upload_wm_state(struct brw_context *brw)
       dw1 |= GEN7_WM_KILL_ENABLE;
    }
 
-   if (_mesa_active_fragment_shader_has_atomic_ops(&brw->ctx)) {
-      dw1 |= GEN7_WM_DISPATCH_ENABLE;
-   }
-
    /* _NEW_BUFFERS | _NEW_COLOR */
    if (brw_color_buffer_write_enabled(brw) || writes_depth ||
-       prog_data->base.nr_image_params ||
+       _mesa_active_fragment_shader_has_side_effects(&brw->ctx) ||
        dw1 & GEN7_WM_KILL_ENABLE) {
       dw1 |= GEN7_WM_DISPATCH_ENABLE;
    }

src/mesa/drivers/dri/i965/gen8_ps_state.c

@@ -90,8 +90,7 @@ gen8_upload_ps_extra(struct brw_context *brw,
     *
     * BRW_NEW_FS_PROG_DATA | BRW_NEW_FRAGMENT_PROGRAM | _NEW_BUFFERS | _NEW_COLOR
     */
-   if ((_mesa_active_fragment_shader_has_atomic_ops(&brw->ctx) ||
-        prog_data->base.nr_image_params) &&
+   if (_mesa_active_fragment_shader_has_side_effects(&brw->ctx) &&
        !brw_color_buffer_write_enabled(brw))
       dw1 |= GEN8_PSX_SHADER_HAS_UAV;
 

src/mesa/main/mtypes.h

@@ -4535,11 +4535,20 @@ enum _debug
    DEBUG_INCOMPLETE_FBO         = (1 << 3)
 };
 
+/**
+ * Checks if the active fragment shader program can have side effects due
+ * to use of things like atomic buffers or images
+ */
 static inline bool
-_mesa_active_fragment_shader_has_atomic_ops(const struct gl_context *ctx)
+_mesa_active_fragment_shader_has_side_effects(const struct gl_context *ctx)
 {
-   return ctx->Shader._CurrentFragmentProgram != NULL &&
-      ctx->Shader._CurrentFragmentProgram->_LinkedShaders[MESA_SHADER_FRAGMENT]->NumAtomicBuffers > 0;
+   const struct gl_shader *sh;
+
+   if (!ctx->_Shader->_CurrentFragmentProgram)
+      return false;
+
+   sh = ctx->_Shader->_CurrentFragmentProgram->_LinkedShaders[MESA_SHADER_FRAGMENT];
+   return sh->NumAtomicBuffers > 0 || sh->NumImages > 0;
 }
 
 #ifdef __cplusplus