driconf, glsl: Add a vs_position_always_invariant option

Many applications use multi-pass rendering and require their vertex
shader position to be computed the same way each time.  Optimizations
may consider, say, fusing a multiply-add based on global usage of an
expression in a shader.  But a second shader with the same expression
may have different code, causing that optimization to make the other
choice the second time around.

The correct solution is for applications to mark their VS outputs
'invariant', indicating they need multiple shaders to compute that
output in the same manner.  However, most applications fail to do so.

So, we add a new driconf option - vs_position_always_invariant - which
forces the gl_Position output in vertex shaders to be marked invariant.

Fixes: 7025dbe794b ("nir: Skip emitting no-op movs from the builder.")
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 9b577f2a887968483b88b629673d3f9904a179ff)

8 files changed, 23 insertions, 0 deletions

diff --git a/src/compiler/glsl/builtin_variables.cpp b/src/compiler/glsl/builtin_variables.cpp
index c10ea9e61a5..ef8aca76e0c 100644
--- a/src/compiler/glsl/builtin_variables.cpp
+++ b/src/compiler/glsl/builtin_variables.cpp
@@ -1435,6 +1435,9 @@ builtin_variable_generator::add_varying(int slot, const glsl_type *type,
 void
 builtin_variable_generator::generate_varyings()
 {
+   struct gl_shader_compiler_options *options =
+      &state->ctx->Const.ShaderCompilerOptions[state->stage];
+
    /* gl_Position and gl_PointSize are not visible from fragment shaders. */
    if (state->stage != MESA_SHADER_FRAGMENT) {
       add_varying(VARYING_SLOT_POS, vec4_t, GLSL_PRECISION_HIGH, "gl_Position");
@@ -1526,6 +1529,9 @@ builtin_variable_generator::generate_varyings()
          var->data.sample = fields[i].sample;
          var->data.patch = fields[i].patch;
          var->init_interface_type(per_vertex_out_type);
+
+         var->data.invariant = fields[i].location == VARYING_SLOT_POS &&
+                               options->PositionAlwaysInvariant;
       }
    }
 }
diff --git a/src/gallium/auxiliary/pipe-loader/driinfo_gallium.h b/src/gallium/auxiliary/pipe-loader/driinfo_gallium.h
index 69967d916f2..bac60215d15 100644
--- a/src/gallium/auxiliary/pipe-loader/driinfo_gallium.h
+++ b/src/gallium/auxiliary/pipe-loader/driinfo_gallium.h
@@ -38,6 +38,7 @@ DRI_CONF_SECTION_END
 DRI_CONF_SECTION_MISCELLANEOUS
    DRI_CONF_ALWAYS_HAVE_DEPTH_BUFFER("false")
    DRI_CONF_GLSL_ZERO_INIT("false")
+   DRI_CONF_VS_POSITION_ALWAYS_INVARIANT("false")
    DRI_CONF_ALLOW_RGB10_CONFIGS("true")
    DRI_CONF_ALLOW_FP16_CONFIGS("false")
 DRI_CONF_SECTION_END
diff --git a/src/gallium/include/state_tracker/st_api.h b/src/gallium/include/state_tracker/st_api.h
index 297954d70bf..9fd36447c7b 100644
--- a/src/gallium/include/state_tracker/st_api.h
+++ b/src/gallium/include/state_tracker/st_api.h
@@ -228,6 +228,7 @@ struct st_config_options
    bool allow_glsl_builtin_variable_redeclaration;
    bool allow_higher_compat_version;
    bool glsl_zero_init;
+   bool vs_position_always_invariant;
    bool force_glsl_abs_sqrt;
    bool allow_glsl_cross_stage_interpolation_mismatch;
    bool allow_glsl_layout_qualifier_on_function_parameters;
diff --git a/src/gallium/state_trackers/dri/dri_screen.c b/src/gallium/state_trackers/dri/dri_screen.c
index 8e4d5ca52df..9f31b09dfb5 100644
--- a/src/gallium/state_trackers/dri/dri_screen.c
+++ b/src/gallium/state_trackers/dri/dri_screen.c
@@ -84,6 +84,8 @@ dri_fill_st_options(struct dri_screen *screen)
    options->allow_higher_compat_version =
       driQueryOptionb(optionCache, "allow_higher_compat_version");
    options->glsl_zero_init = driQueryOptionb(optionCache, "glsl_zero_init");
+   options->vs_position_always_invariant =
+      driQueryOptionb(optionCache, "vs_position_always_invariant");
    options->force_glsl_abs_sqrt =
       driQueryOptionb(optionCache, "force_glsl_abs_sqrt");
    options->allow_glsl_cross_stage_interpolation_mismatch =
diff --git a/src/mesa/drivers/dri/i965/intel_screen.c b/src/mesa/drivers/dri/i965/intel_screen.c
index 6040161d8f9..523609afc6f 100644
--- a/src/mesa/drivers/dri/i965/intel_screen.c
+++ b/src/mesa/drivers/dri/i965/intel_screen.c
@@ -98,6 +98,7 @@ DRI_CONF_BEGIN
 
    DRI_CONF_SECTION_MISCELLANEOUS
       DRI_CONF_GLSL_ZERO_INIT("false")
+      DRI_CONF_VS_POSITION_ALWAYS_INVARIANT("false")
       DRI_CONF_ALLOW_RGB10_CONFIGS("false")
       DRI_CONF_ALLOW_RGB565_CONFIGS("true")
       DRI_CONF_ALLOW_FP16_CONFIGS("false")
@@ -2798,6 +2799,8 @@ __DRIconfig **intelInitScreen2(__DRIscreen *dri_screen)
    screen->compiler->constant_buffer_0_is_relative = devinfo->gen < 8 ||
       !(screen->kernel_features & KERNEL_ALLOWS_CONTEXT_ISOLATION);
 
+   screen->compiler->glsl_compiler_options[MESA_SHADER_VERTEX].PositionAlwaysInvariant = driQueryOptionb(&screen->optionCache, "vs_position_always_invariant");
+
    screen->compiler->supports_pull_constants = true;
 
    screen->has_exec_fence =
diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
index ae93dff40da..05c684cf6b4 100644
--- a/src/mesa/main/mtypes.h
+++ b/src/mesa/main/mtypes.h
@@ -3193,6 +3193,9 @@ struct gl_shader_compiler_options
    /** Clamp UBO and SSBO block indices so they don't go out-of-bounds. */
    GLboolean ClampBlockIndicesToArrayBounds;
 
+   /** (driconf) Force gl_Position to be considered invariant */
+   GLboolean PositionAlwaysInvariant;
+
    const struct nir_shader_compiler_options *NirOptions;
 };
 
diff --git a/src/mesa/state_tracker/st_context.c b/src/mesa/state_tracker/st_context.c
index 9f72a8ef871..9798e0374cf 100644
--- a/src/mesa/state_tracker/st_context.c
+++ b/src/mesa/state_tracker/st_context.c
@@ -754,6 +754,8 @@ st_create_context_priv(struct gl_context *ctx, struct pipe_context *pipe,
    ctx->Const.ShaderCompilerOptions[MESA_SHADER_VERTEX].EmitNoSat =
       !screen->get_param(screen, PIPE_CAP_VERTEX_SHADER_SATURATE);
 
+   ctx->Const.ShaderCompilerOptions[MESA_SHADER_VERTEX].PositionAlwaysInvariant = options->vs_position_always_invariant;
+
    if (ctx->Const.GLSLVersion < 400) {
       for (i = 0; i < MESA_SHADER_STAGES; i++)
          ctx->Const.ShaderCompilerOptions[i].EmitNoIndirectSampler = true;
diff --git a/src/util/xmlpool/t_options.h b/src/util/xmlpool/t_options.h
index 7a665bfd655..fe186c1fa79 100644
--- a/src/util/xmlpool/t_options.h
+++ b/src/util/xmlpool/t_options.h
@@ -279,6 +279,11 @@ DRI_CONF_OPT_BEGIN_B(glsl_zero_init, def) \
         DRI_CONF_DESC(en,gettext("Force uninitialized variables to default to zero")) \
 DRI_CONF_OPT_END
 
+#define DRI_CONF_VS_POSITION_ALWAYS_INVARIANT(def) \
+DRI_CONF_OPT_BEGIN_B(vs_position_always_invariant, def) \
+        DRI_CONF_DESC(en,gettext("Force the vertex shader's gl_Position output to be considered 'invariant'")) \
+DRI_CONF_OPT_END
+
 #define DRI_CONF_ALLOW_RGB10_CONFIGS(def) \
 DRI_CONF_OPT_BEGIN_B(allow_rgb10_configs, def) \
 DRI_CONF_DESC(en,gettext("Allow exposure of visuals and fbconfigs with rgb10a2 formats")) \