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nir: Introduce nir_lower_vars_to_scratch_global().

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This lets the driver make a more informed decision about which vars to
lower to scratch based on the vars available to spill.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37245>
This commit is contained in:
Emma Anholt 2025-09-05 10:00:33 -07:00 • committed by Marge Bot
parent 059d301c79
commit 5a09abe890
2 changed files with 83 additions and 9 deletions
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6
src/compiler/nir/nir.h
View file

@ -5194,6 +5194,12 @@ bool nir_lower_vars_to_scratch(nir_shader *shader,
glsl_type_size_align_func variable_size_align,
glsl_type_size_align_func scratch_layout_size_align);
typedef void (*nir_lower_vars_to_scratch_cb)(struct set *, void *);
bool nir_lower_vars_to_scratch_global(nir_shader *shader,
glsl_type_size_align_func scratch_layout_size_align,
nir_lower_vars_to_scratch_cb cb, void *data);
bool nir_lower_scratch_to_var(nir_shader *nir);
bool nir_lower_clip_halfz(nir_shader *shader);

86
src/compiler/nir/nir_lower_scratch.c
View file

@ -91,11 +91,37 @@ only_used_for_load_store(nir_deref_instr *deref)
return true;
}
/**
* Lowers indirect-addressed function temporary variables to scratch accesses
* based on a driver-provided callback selecting which variables to lower.
*
* Most drivers need this in some form -- a large array may be larger than the
* register space, so for an indirect store (not lowered to a series of csels
* using nir_lower_indirect_derefs) you would simply not be able to register
* allocate for the instruction. In that case you want to move the whole array
* to scratch memory and have the load/stores be handled using NIR scratch
* intrinsics.
*
* The callback lets you make a global decision of which vars to spill based on
* the set of indirect-addressed function temps. If scheduling an instruction
* could mean more than one array must be fully unspilled, then you might want
* to decide which variables to spill as a maximum register pressure calculation
* of variables you're going to leave as function temps.
*
* @scratch_layout_size_align function to use to compute the size and alignment
* of the values in scratch space.
* @cb driver callback that will be called if there are any candidates to spill.
* It will be passed the set of candidate nir_variables, along with the
* driver-provided @data, and any variables left in the set after the
* callback will be spilled to scratch.
* @data driver data to pass to the callback The callback is passed the set of
* nir_variable pointers to consider. Any variables not removed from the
* set will be spilled to scratch after the callback.
*/
bool
nir_lower_vars_to_scratch(nir_shader *shader,
int size_threshold,
glsl_type_size_align_func variable_size_align,
glsl_type_size_align_func scratch_layout_size_align)
nir_lower_vars_to_scratch_global(nir_shader *shader,
glsl_type_size_align_func scratch_layout_size_align,
nir_lower_vars_to_scratch_cb cb, void *data)
{
struct set *set = _mesa_pointer_set_create(NULL);
@ -130,16 +156,15 @@ nir_lower_vars_to_scratch(nir_shader *shader,
if (var->data.mode == 0)
continue;
unsigned var_size, var_align;
variable_size_align(var->type, &var_size, &var_align);
if (var_size <= size_threshold)
continue;
_mesa_set_add(set, var);
}
}
}
/* Have the driver pick which variables to lower (if any) */
if (set->entries != 0)
cb(set, data);
if (set->entries == 0) {
_mesa_set_destroy(set, NULL);
return false;
@ -227,3 +252,46 @@ nir_lower_vars_to_scratch(nir_shader *shader,
return progress;
}
struct nir_lower_vars_to_scratch_state {
int size_threshold;
glsl_type_size_align_func variable_size_align;
};
/**
* Callback for nir_lower_vars_to_scratch: Remove any vars from the set to spill
* that are under the size threshold.
*/
static void
nir_lower_vars_to_scratch_size_cb(struct set *set, void *data)
{
struct nir_lower_vars_to_scratch_state *state = data;
set_foreach(set, entry) {
nir_variable *var = (void *)entry->key;
unsigned var_size, var_align;
state->variable_size_align(var->type, &var_size, &var_align);
if (var_size <= state->size_threshold)
_mesa_set_remove(set, entry);
}
}
/**
* Lowers indirect-addressed function temporary variables to scratch accesses
* based on a size threshold for variables to lower.
*
* See nir_lower_vars_to_scratch_global for more explanation.
*/
bool
nir_lower_vars_to_scratch(nir_shader *shader,
int size_threshold,
glsl_type_size_align_func variable_size_align,
glsl_type_size_align_func scratch_layout_size_align)
{
struct nir_lower_vars_to_scratch_state state = {
.size_threshold = size_threshold,
.variable_size_align = variable_size_align,
};
return nir_lower_vars_to_scratch_global(shader, scratch_layout_size_align,
nir_lower_vars_to_scratch_size_cb, &state);
}