util: Remove util_cpu_detect

util_cpu_detect is an anti-pattern: it relies on callers high up in the call
chain initializing a local implementation detail. As a real example, I added:

...a Mali compiler unit test
...that called bi_imm_f16() to construct an FP16 immediate
...that calls _mesa_float_to_half internally
...that calls util_get_cpu_caps internally, but only on x86_64!
...that relies on util_cpu_detect having been called before.

As a consequence, this unit test:

...crashes on x86_64 with USE_X86_64_ASM set
...passes on every other architecture
...works on my local arm64 workstation and on my test board
...failed CI which runs on x86_64
...needed to have a random util_cpu_detect() call sprinkled in.

This is a bad design decision. It pollutes the tree with magic, it causes
mysterious CI failures especially for non-x86_64 developers, and it is not
justified by a micro-optimization.

Instead, let's call util_cpu_detect directly from util_get_cpu_caps, avoiding
the footgun where it fails to be called.  This cleans up Mesa's design,
simplifies the tree, and avoids a class of a (possibly platform-specific)
failures. To mitigate the added overhead, wrap it all in a (fast) atomic
load check and declare the whole thing as ATTRIBUTE_CONST so the
compiler will CSE calls to util_cpu_detect.

Co-authored-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15580>

src/compiler/glsl_types.cpp

@@ -26,7 +26,6 @@
 #include "compiler/glsl/glsl_parser_extras.h"
 #include "glsl_types.h"
 #include "util/hash_table.h"
-#include "util/u_cpu_detect.h"
 #include "util/u_string.h"
 
 
@@ -520,11 +519,6 @@ hash_free_type_function(struct hash_entry *entry)
 void
 glsl_type_singleton_init_or_ref()
 {
-   /* This is required for _mesa_half_to_float() which is
-    * required for constant-folding 16-bit float ops.
-    */
-   util_cpu_detect();
-
    mtx_lock(&glsl_type::hash_mutex);
    glsl_type_users++;
    mtx_unlock(&glsl_type::hash_mutex);

src/compiler/isaspec/decode.c

@@ -773,8 +773,6 @@ isa_decode(void *bin, int sz, FILE *out, const struct isa_decode_options *option
 	if (!options)
 		options = &default_options;
 
-	util_cpu_detect();  /* needed for _mesa_half_to_float() */
-
 	state = rzalloc_size(NULL, sizeof(*state));
 	state->options = options;
 	state->num_instr = sz / (BITMASK_WORDS * sizeof(BITSET_WORD));