nir: eliminate most ralloc/malloc for nir_variable names

Store small names in a fixed-sized string in nir_variable.
GLSL IR does the same thing.

When compiling my shader-db with the gallium noop driver, it improves GLSL
compile times by 0.7% (much lower than anticipated).

For Unigine Heaven shaders:
- it eliminates 95.6% ralloc calls for nir_variable names
- the total number of ralloc calls is reduced by 11%

It also adds only 16B to nir_variable, while just the ralloc header
for the name would occupy 40B.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36538>

src/compiler/nir/nir.c

@@ -32,6 +32,7 @@
 #include "util/half_float.h"
 #include "util/macros.h"
 #include "util/u_math.h"
+#include "util/u_printf.h"
 #include "util/u_qsort.h"
 #include "nir_builder.h"
 #include "nir_control_flow_private.h"
@@ -270,21 +271,42 @@ nir_shader_add_variable(nir_shader *shader, nir_variable *var)
 void
 nir_variable_set_name(nir_variable *var, const char *name)
 {
-   if (var->name)
+   if (var->name && var->name != var->_name_storage)
       ralloc_free(var->name);
 
-   var->name = ralloc_strdup(var, name);
+   if (!name) {
+      var->name = NULL;
+      return;
+   }
+
+   /* Use _name_storage if the name fits in it. */
+   if (strlen(name) < ARRAY_SIZE(var->_name_storage)) {
+      var->name = var->_name_storage;
+      strcpy(var->name, name);
+   } else {
+      var->name = ralloc_strdup(var, name);
+   }
 }
 
 void
 nir_variable_set_namef(nir_variable *var, const char *fmt, ...)
 {
-   if (var->name)
+   if (var->name && var->name != var->_name_storage)
       ralloc_free(var->name);
 
    va_list args;
    va_start(args, fmt);
-   var->name = ralloc_vasprintf(var, fmt, args);
+   size_t name_size = u_printf_length(fmt, args) + 1;
+
+   /* Use _name_storage if the name fits in it. */
+   if (name_size <= ARRAY_SIZE(var->_name_storage))
+      var->name = var->_name_storage;
+   else
+      var->name = ralloc_size(var, name_size);
+
+   if (var->name)
+      vsnprintf(var->name, name_size, fmt, args);
+
    va_end(args);
 }
 
@@ -293,15 +315,61 @@ nir_variable_append_namef(nir_variable *var, const char *fmt, ...)
 {
    va_list args;
    va_start(args, fmt);
-   ralloc_vasprintf_append(&var->name, fmt, args);
+   size_t old_len = var->name ? strlen(var->name) : 0;
+   size_t append_size = u_printf_length(fmt, args) + 1;
+   size_t name_size = old_len + append_size;
+
+   if (append_size == 1) {
+      /* No change. */
+      va_end(args);
+      return;
+   }
+
+   /* Use _name_storage if the name fits in it. */
+   if (name_size <= ARRAY_SIZE(var->_name_storage)) {
+      if (!var->name) {
+         var->name = var->_name_storage;
+      } else if (var->name != var->_name_storage) {
+         /* Move the name to _name_storage. */
+         memcpy(var->_name_storage, var->name, old_len + 1);
+         ralloc_free(var->name);
+         var->name = var->_name_storage;
+      }
+   } else {
+      /* ralloc the appended name. */
+      if (var->name == var->_name_storage) {
+         /* Move the name from _name_storage to ralloc'd. */
+         var->name = ralloc_size(var, name_size);
+         if (var->name)
+            memcpy(var->name, var->_name_storage, old_len + 1);
+      } else {
+         var->name = reralloc_size(var, var->name, name_size);
+      }
+   }
+
+   /* Append the name. */
+   if (var->name)
+      vsnprintf(var->name + old_len, append_size, fmt, args);
    va_end(args);
 }
 
 void
 nir_variable_steal_name(nir_variable *dst, nir_variable *src)
 {
-   ralloc_steal(dst, src->name);
-   dst->name = src->name;
+   if (!src->name) {
+      dst->name = NULL;
+      return;
+   }
+
+   /* Use _name_storage if the name fits in it. */
+   if (src->name == src->_name_storage) {
+      dst->name = dst->_name_storage;
+      strcpy(dst->name, src->name);
+   } else {
+      ralloc_steal(dst, src->name);
+      dst->name = src->name;
+   }
+
    src->name = NULL;
 }
 

src/compiler/nir/nir.h

@@ -865,6 +865,15 @@ typedef struct nir_variable {
     * variables with a struct or array of array of struct type.
     */
    nir_variable_data *members;
+
+   /**
+    * This is the backing storage for "name" if the name is short enough
+    * to fit in it, so that we don't have to ralloc the name.
+    *
+    * It eliminates 95.6% ralloc calls for variable names when compiling
+    * my GLSL shader-db.
+    */
+   char _name_storage[16];
 } nir_variable;
 
 static inline bool