mro_core.c mro.xs actually use static global field struct mro_alg->hash_U32

Perl_mro_register(aTHX_ &dfs_alg); will remember and store
ptr &dfs_alg forever in a SVUV. So do not keep struct mro_alg'es in const
RO memory, and fill in the U32 hash at perl proc start or shared lib
attach time. Other parts of the interp will deref and pass struct member
"struct mro_alg->hash" to hv_common().

PERL_HASH(hash, "c3", (sizeof("c3")-1));
was specially written instead of
PERL_HASH(hash, c3_alg.name, c3_alg.length);
because PERL_HASH() is inlineable, and I've seen MSVC 2022 -O1 do heavy
inlining and heavy const folding of sbox32_hash_with_state(), if it knows
the constant chars in a string, and the constant length of the string,
ahead of time during CC .c->.obj phase.
The machine code of a very heavy const fold of sbox32_hash_with_state(),
superficially looks more like an inlined fixed length memcpy() or a
struct to struct = assignment, than some kind of fancy "hash" or
"cryptographic" or jpg/mpeg algorithm.

I will assume "U32 hash = c3_alg.hash; if (hash == 0) {" and
"c3_alg.hash = hash;" are atomic/multi-core safe.

Aligned R/Ws to types U8/U16/U32 on all CPUs, and U64 on 64b, are supposed
to be atomic safe on all CPUs. Aligned U32 R/Ws in C are never emulated
with 4 separate U8 or 8 U4 R/W ops on any commercial CPU.

Author: bulk88

ext/mro/mro.xs

@@ -7,7 +7,7 @@
 static AV*
 S_mro_get_linear_isa_c3(pTHX_ HV* stash, U32 level);
 
-static const struct mro_alg c3_alg =
+static struct mro_alg c3_alg =
     {S_mro_get_linear_isa_c3, "c3", 2, 0, 0};
 
 /*
@@ -665,4 +665,14 @@ mro__nextcan(...)
     XSRETURN_EMPTY;
 
 BOOT:
+    {
+        U32 hash = c3_alg.hash;
+        if (hash == 0) {
+            assert(c3_alg.name == "c3" && c3_alg.length == (sizeof("c3")-1));
+            /* Using "c3" will aggressively SBOX32 CC const fold.
+               But RW const char * c3_alg.name can not. */
+            PERL_HASH(hash, "c3", (sizeof("c3")-1));
+            c3_alg.hash = hash;
+        }
+    }
     Perl_mro_register(aTHX_ &c3_alg);

mro_core.c

@@ -31,7 +31,7 @@ Also see L<perlmroapi>.
 #define PERL_IN_MRO_CORE_C
 #include "perl.h"
 
-static const struct mro_alg dfs_alg =
+static struct mro_alg dfs_alg =
     {S_mro_get_linear_isa_dfs, "dfs", 3, 0, 0};
 
 SV *
@@ -1429,6 +1429,16 @@ Perl_boot_core_mro(pTHX)
 {
     static const char file[] = __FILE__;
 
+    {
+        U32 hash = dfs_alg.hash;
+        if (hash == 0) {
+            assert(dfs_alg.name == "dfs" && dfs_alg.length == STRLENs("dfs"));
+            /* Using "dfs" will aggressively SBOX32 CC const fold.
+               But RW const char * dfs_alg.name can not. */
+            PERL_HASH(hash, "dfs", STRLENs("dfs"));
+            dfs_alg.hash = hash;
+        }
+    }
     Perl_mro_register(aTHX_ &dfs_alg);
 
     newXSproto("mro::method_changed_in", XS_mro_method_changed_in, file, "$");