nir: Add some notes about const/uniform array access rules in GL.

I was doing some RE on freedreno and we had some questions about when the
hardware might need non-uniform or non-constant array access for various
descriptor types, so let's leave some notes for the next person.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13621>

src/compiler/nir/nir.h

@@ -1981,7 +1981,10 @@ typedef enum {
     *
     * This is added to nir_tex_instr::sampler_index.  Unless
     * nir_tex_instr::sampler_non_uniform is set, this is guaranteed to be
-    * dynamically uniform.
+    * dynamically uniform.  This should not be present until GLSL ES 3.20, GLSL
+    * 4.00, or ARB_gpu_shader5, because in ES 3.10 and GL 3.30 samplers said
+    * "When aggregated into arrays within a shader, samplers can only be indexed
+    * with a constant integral expression."
     */
    nir_tex_src_sampler_offset,
 
@@ -2144,7 +2147,18 @@ typedef struct {
    /** True if the texture index or handle is not dynamically uniform */
    bool texture_non_uniform;
 
-   /** True if the sampler index or handle is not dynamically uniform */
+   /** True if the sampler index or handle is not dynamically uniform.
+    *
+    * This may be set when VK_EXT_descriptor_indexing is supported and the
+    * appropriate capability is enabled.
+    *
+    * This should always be false in GLSL (GLSL ES 3.20 says "When aggregated
+    * into arrays within a shader, opaque types can only be indexed with a
+    * dynamically uniform integral expression", and GLSL 4.60 says "When
+    * aggregated into arrays within a shader, [texture, sampler, and
+    * samplerShadow] types can only be indexed with a dynamically uniform
+    * expression, or texture lookup will result in undefined values.").
+    */
    bool sampler_non_uniform;
 
    /** The texture index

src/compiler/nir/nir_intrinsics.py

@@ -540,9 +540,15 @@ intrinsic("rt_trace_ray", src_comp=[-1, 1, 1, 1, 1, 1, 3, 1, 3, 1, -1],
 
 # Atomic counters
 #
-# The *_var variants take an atomic_uint nir_variable, while the other,
-# lowered, variants take a constant buffer index and register offset.
-
+# The *_deref variants take an atomic_uint nir_variable, while the other,
+# lowered, variants take a buffer index and register offset.  The buffer index
+# is always constant, as there's no way to declare an array of atomic counter
+# buffers.
+#
+# The register offset may be non-constant but must by dynamically uniform
+# ("Atomic counters aggregated into arrays within a shader can only be indexed
+# with dynamically uniform integral expressions, otherwise results are
+# undefined.")
 def atomic(name, flags=[]):
     intrinsic(name + "_deref", src_comp=[-1], dest_comp=1, flags=flags)
     intrinsic(name, src_comp=[1], dest_comp=1, indices=[BASE], flags=flags)
@@ -576,7 +582,9 @@ atomic3("atomic_counter_comp_swap")
 # In the first version, the image variable contains the memory and layout
 # qualifiers that influence the semantics of the intrinsic.  In the second and
 # third, the image format and access qualifiers are provided as constant
-# indices.
+# indices.  Up through GLSL ES 3.10, the image index source may only be a
+# constant array access.  GLSL ES 3.20 and GLSL 4.00 allow dynamically uniform
+# indexing.
 #
 # All image intrinsics take a four-coordinate vector and a sample index as
 # 2nd and 3rd sources, determining the location within the image that will be
@@ -661,7 +669,8 @@ intrinsic("load_vulkan_descriptor", src_comp=[-1], dest_comp=0,
 # All SSBO operations take 3 sources except CompSwap that takes 4. These
 # sources represent:
 #
-# 0: The SSBO buffer index.
+# 0: The SSBO buffer index (dynamically uniform in GLSL, possibly non-uniform
+#    with VK_EXT_descriptor_indexing).
 # 1: The offset into the SSBO buffer of the variable that the atomic
 #    operation will operate on.
 # 2: The data parameter to the atomic function (i.e. the value to add

src/compiler/shader_enums.h

@@ -944,7 +944,32 @@ enum gl_access_qualifier
    /* The memory used by the access/variable is not written. */
    ACCESS_NON_WRITEABLE = (1 << 4),
 
-   /** The access may use a non-uniform buffer or image index */
+   /**
+    * The access may use a non-uniform buffer or image index.
+    *
+    * This is not allowed in either OpenGL or OpenGL ES, or Vulkan unless
+    * VK_EXT_descriptor_indexing is supported and the appropriate capability is
+    * enabled.
+    *
+    * Some GL spec archaeology justifying this:
+    *
+    * Up through at least GLSL ES 3.20 and GLSL 4.50,  "Opaque Types" says "When
+    * aggregated into arrays within a shader, opaque types can only be indexed
+    * with a dynamically uniform integral expression (see section 3.9.3) unless
+    * otherwise noted; otherwise, results are undefined."
+    *
+    * The original GL_AB_shader_image_load_store specification for desktop GL
+    * didn't have this restriction ("Images may be aggregated into arrays within
+    * a shader (using square brackets [ ]) and can be indexed with general
+    * integer expressions.")  At the same time,
+    * GL_ARB_shader_storage_buffer_objects *did* have the uniform restriction
+    * ("A uniform or shader storage block array can only be indexed with a
+    * dynamically uniform integral expression, otherwise results are
+    * undefined"), just like ARB_gpu_shader5 did when it first introduced a
+    * non-constant indexing of an opaque type with samplers.  So, we assume that
+    * this was an oversight in the original image_load_store spec, and was
+    * considered a correction in the merge to core.
+    */
    ACCESS_NON_UNIFORM   = (1 << 5),
 
    /* This has the same semantics as NIR_INTRINSIC_CAN_REORDER, only to be