nir/lower_system_values: simplify load_helper_invocation lowering

The two backends (ir3, dxil) using the lowering have
info->fs.uses_sample_shading matching set when sample shading is used -- the VK
drivers pass the rasterization state flag into the compiler, while
freedreno and d3d12 have caps->force_persample_interp so the frontend
creates a shader variant with info->fs.uses_sample_shading set.

This means that we can drop the sample-id SHL/AND in the pixel-rate
shading case, which in turn means that drivers don't need to have a
load_sample_id() that doesn't trigger sample-rate shading (which Adreno
doesn't have).

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36429>

src/compiler/nir/nir_lower_system_values.c

@@ -347,11 +347,14 @@ lower_system_value_instr(nir_builder *b, nir_instr *instr, void *_state)
 nir_def *
 nir_build_lowered_load_helper_invocation(nir_builder *b)
 {
-   nir_def *tmp;
-   tmp = nir_ishl(b, nir_imm_int(b, 1),
-                  nir_load_sample_id_no_per_sample(b));
-   tmp = nir_iand(b, nir_load_sample_mask_in(b), tmp);
-   return nir_inot(b, nir_i2b(b, tmp));
+   nir_def *mask = nir_load_sample_mask_in(b);
+
+   if (b->shader->info.fs.uses_sample_shading) {
+      nir_def *id = nir_load_sample_id(b);
+      mask = nir_iand(b, mask, nir_ishl(b, nir_imm_int(b, 1), id));
+   }
+
+   return nir_ieq_imm(b, mask, 0);
 }
 
 bool

src/compiler/nir/nir_shader_compiler_options.h

@@ -370,7 +370,14 @@ typedef struct nir_shader_compiler_options {
    /**
     * If enabled, gl_HelperInvocation will be lowered as:
     *
-    *   !((1 << sample_id) & sample_mask_in))
+    * - non-sample-shading: sample_mask_in == 0.
+    * - sample shading:     !((1 << sample_id) & sample_mask_in))
+    *
+    * For this to be correct, it requires that fs.uses_sample_shading is set to
+    * true when sample shading is enabled.  This means that you need shader
+    * variants to set the flag when Vulkan's
+    * VkPipelineMultisampleStateCreateInfo->sampleShadingEnable or GL's
+    * glMinSampleshading() are enabled.
     *
     * This depends on some possibly hw implementation details, which may
     * not be true for all hw.  In particular that the FS is only executed