Allow configuration of JANET_THREAD_LOCAL. Address #1595

This is to better allow configuration on various, unknown compilers.
Previously, we hardcoded how thread local storage was specified for a
few different compilers, but we were not following and C standard. In
C11, there is a standardized storage specifier _Thread_local for this
storage class, however this is now deprecated in various C++ compilers
for a new keyword, confusingly. Janet also does not claim to require the
C11 standard, so for maximum flexibilty, the storage specifier must be
specified at configure time.

CHANGELOG.md

@@ -2,6 +2,7 @@
 All notable changes to this project will be documented in this file.
 
 ## Unreleased - ???
+- Allow configuring `JANET_THREAD_LOCAL` during builds to allow multi-threading on unknown compilers.
 - Make `ffi/write` append to a buffer instead of insert at 0 by default.
 - Add `os/getpid` to get the current process id.
 - Add `:out` option to `os/spawn` to be able to redirect stderr to stdout with pipes.

meson.build

@@ -105,6 +105,9 @@ endif
 if get_option('arch_name') != ''
   conf.set('JANET_ARCH_NAME', get_option('arch_name'))
 endif
+if get_option('thread_local_prefix') != ''
+  conf.set('JANET_THREAD_LOCAL', get_option('thread_local_prefix'))
+endif
 jconf = configure_file(output : 'janetconf.h',
   configuration : conf)
 

meson_options.txt

@@ -30,6 +30,7 @@ option('max_macro_expand', type : 'integer', min : 1, max : 8000, value : 200)
 option('stack_max', type : 'integer', min : 8096, max : 0x7fffffff, value : 0x7fffffff)
 
 option('arch_name', type : 'string', value: '')
+option('thread_local_prefix', type : 'string', value: '')
 option('os_name', type : 'string', value: '')
 option('shared', type : 'boolean', value: true)
 option('cryptorand', type : 'boolean', value: true)

src/conf/janetconf.h

@@ -13,6 +13,7 @@
 
 /* These settings all affect linking, so use cautiously. */
 /* #define JANET_SINGLE_THREADED */
+/* #define JANET_THREAD_LOCAL _Thread_local */
 /* #define JANET_NO_DYNAMIC_MODULES */
 /* #define JANET_NO_NANBOX */
 /* #define JANET_API __attribute__((visibility ("default"))) */

src/include/janet.h

@@ -170,14 +170,12 @@ extern "C" {
 /* Also enable the thread library only if not single-threaded */
 #ifdef JANET_SINGLE_THREADED
 #define JANET_THREAD_LOCAL
-#undef JANET_THREADS
-#elif defined(__GNUC__)
+#elif !(defined(JANET_THREAD_LOCAL)) && defined(__GNUC__)
 #define JANET_THREAD_LOCAL __thread
-#elif defined(_MSC_BUILD)
+#elif !(defined(JANET_THREAD_LOCAL)) && defined(_MSC_BUILD)
 #define JANET_THREAD_LOCAL __declspec(thread)
-#else
+#elif !(defined(JANET_THREAD_LOCAL))
 #define JANET_THREAD_LOCAL
-#undef JANET_THREADS
 #endif
 
 /* Enable or disable dynamic module loading. Enabled by default. */