nir: Rename is_arb_asm to use_legacy_math_rules and document its meaning.

On iris and crocus, this flag is used to set "alt mode" math on the shader
as a whole.  Some other drivers have a similar mode for DX9/ARB-program
behavior, so document what it does so we can start using it.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16176>

src/compiler/glsl/standalone.cpp

@@ -102,7 +102,7 @@ init_gl_program(struct gl_program *prog, bool is_arb_asm, gl_shader_stage stage)
 {
    prog->RefCount = 1;
    prog->Format = GL_PROGRAM_FORMAT_ASCII_ARB;
-   prog->info.is_arb_asm = is_arb_asm;
+   prog->info.use_legacy_math_rules = is_arb_asm;
    prog->info.stage = stage;
 }
 

src/compiler/shader_info.h

@@ -310,9 +310,28 @@ typedef struct shader_info {
    bool workgroup_size_variable:1;
 
    /**
-     * Is this an ARB assembly-style program.
+     * Set if this shader uses legacy (DX9 or ARB assembly) math rules.
+     *
+     * From the ARB_fragment_program specification:
+     *
+     *    "The following rules apply to multiplication:
+     *
+     *      1. <x> * <y> == <y> * <x>, for all <x> and <y>.
+     *      2. +/-0.0 * <x> = +/-0.0, at least for all <x> that correspond to
+     *         *representable numbers (IEEE "not a number" and "infinity"
+     *         *encodings may be exceptions).
+     *      3. +1.0 * <x> = <x>, for all <x>.""
+     *
+     * However, in effect this was due to DX9 semantics implying that 0*x=0 even
+     * for inf/nan if the hardware generated them instead of float_min/max.  So,
+     * you should not have an exception for inf/nan to rule 2 above.
+     *
+     * One implementation of this behavior would be to flush all generated NaNs
+     * to zero, at which point 0*Inf=Nan=0.  Most DX9/ARB-asm hardware did not
+     * generate NaNs, and the only way the GPU saw one was to possibly feed it
+     * in as a uniform.
      */
-   bool is_arb_asm;
+   bool use_legacy_math_rules;
 
    union {
       struct {

src/gallium/drivers/crocus/crocus_program.c

@@ -1208,7 +1208,7 @@ crocus_compile_vs(struct crocus_context *ice,
    if (key->clamp_pointsize)
       nir_lower_point_size(nir, 1.0, 255.0);
 
-   prog_data->use_alt_mode = nir->info.is_arb_asm;
+   prog_data->use_alt_mode = nir->info.use_legacy_math_rules;
 
    crocus_setup_uniforms(compiler, mem_ctx, nir, prog_data, &system_values,
                          &num_system_values, &num_cbufs);
@@ -1858,7 +1858,7 @@ crocus_compile_fs(struct crocus_context *ice,
 
    nir_shader *nir = nir_shader_clone(mem_ctx, ish->nir);
 
-   prog_data->use_alt_mode = nir->info.is_arb_asm;
+   prog_data->use_alt_mode = nir->info.use_legacy_math_rules;
 
    crocus_setup_uniforms(compiler, mem_ctx, nir, prog_data, &system_values,
                          &num_system_values, &num_cbufs);

src/gallium/drivers/iris/iris_program.c

@@ -1335,7 +1335,7 @@ iris_compile_vs(struct iris_screen *screen,
       nir_shader_gather_info(nir, impl);
    }
 
-   prog_data->use_alt_mode = nir->info.is_arb_asm;
+   prog_data->use_alt_mode = nir->info.use_legacy_math_rules;
 
    iris_setup_uniforms(compiler, mem_ctx, nir, prog_data, 0, &system_values,
                        &num_system_values, &num_cbufs);
@@ -1967,7 +1967,7 @@ iris_compile_fs(struct iris_screen *screen,
    nir_shader *nir = nir_shader_clone(mem_ctx, ish->nir);
    const struct iris_fs_prog_key *const key = &shader->key.fs;
 
-   prog_data->use_alt_mode = nir->info.is_arb_asm;
+   prog_data->use_alt_mode = nir->info.use_legacy_math_rules;
 
    iris_setup_uniforms(compiler, mem_ctx, nir, prog_data, 0, &system_values,
                        &num_system_values, &num_cbufs);

src/gallium/drivers/radeonsi/si_shader_nir.c

@@ -273,7 +273,7 @@ static void si_lower_nir(struct si_screen *sscreen, struct nir_shader *nir)
 
    NIR_PASS_V(nir, nir_lower_discard_or_demote,
               (sscreen->debug_flags & DBG(FS_CORRECT_DERIVS_AFTER_KILL)) ||
-               nir->info.is_arb_asm);
+               nir->info.use_legacy_math_rules);
 
    /* Lower load constants to scalar and then clean up the mess */
    NIR_PASS_V(nir, nir_lower_load_const_to_scalar);

src/mesa/main/uniform_query.cpp

@@ -956,7 +956,7 @@ associate_uniform_storage(struct gl_context *ctx,
          unsigned columns = 0;
 
          int dmul;
-         if (ctx->Const.PackedDriverUniformStorage && !prog->info.is_arb_asm) {
+         if (ctx->Const.PackedDriverUniformStorage && !prog->info.use_legacy_math_rules) {
             dmul = storage->type->vector_elements * sizeof(float);
          } else {
             dmul = 4 * sizeof(float);
@@ -1050,7 +1050,7 @@ associate_uniform_storage(struct gl_context *ctx,
           * initializers in the source code to be copied over.
           */
          unsigned array_elements = MAX2(1, storage->array_elements);
-         if (ctx->Const.PackedDriverUniformStorage && !prog->info.is_arb_asm &&
+         if (ctx->Const.PackedDriverUniformStorage && !prog->info.use_legacy_math_rules &&
              (storage->is_bindless || !storage->type->contains_opaque())) {
             const int dmul = storage->type->is_64bit() ? 2 : 1;
             const unsigned components =

src/mesa/program/program.c

@@ -196,7 +196,7 @@ _mesa_init_gl_program(struct gl_program *prog, gl_shader_stage stage,
    prog->RefCount = 1;
    prog->Format = GL_PROGRAM_FORMAT_ASCII_ARB;
    prog->info.stage = stage;
-   prog->info.is_arb_asm = is_arb_asm;
+   prog->info.use_legacy_math_rules = is_arb_asm;
 
    /* Uniforms that lack an initializer in the shader code have an initial
     * value of zero.  This includes sampler uniforms.