If we have thread local storage, use it instead of posix_getspecific().

Declare and use a variable PL_current_context to store a pointer to the
thread's interpreter struct. Use this to implement implicit context undef
ITHREADS, instead of a call to posix_getspecific().

For normal threaded code, this will eliminate function calls. For threaded
code in shared objects (the typical configuration for Linux distribution
builds) there still needs to be a function call to get the pointer to
thread local variables, but there will only be one per function, not one
per read of the variable.

globals.c

@@ -30,6 +30,14 @@
  */
 #include "regcomp.h"
 
+/* We need somewhere to declare this. This file seems a good place.
+ * This is not a regular "global" in that we don't know whether it needs to
+ * exist until we include threads.h, and we don't want it as part of any
+ * global struct (if that or something similar is re-introduced. */
+
+#if defined(USE_ITHREADS) && defined(PERL_THREAD_LOCAL)
+PERL_THREAD_LOCAL void *PL_current_context;
+#endif
 
 /*
  * ex: set ts=8 sts=4 sw=4 et:

makedef.pl

@@ -189,6 +189,9 @@ BEGIN
 # All platforms export boot_DynaLoader unconditionally.
 my %export = ( boot_DynaLoader => 1 );
 
+++$export{PL_current_context}
+    if defined $Config{perl_thread_local} && $define{USE_ITHREADS};
+
 sub try_symbols {
     foreach my $symbol (@_) {
 	++$export{$symbol} unless exists $skip{$symbol};

thread.h

@@ -379,19 +379,39 @@
 #    define PTHREAD_GETSPECIFIC(key) pthread_getspecific(key)
 #endif
 
-#ifndef PERL_GET_CONTEXT
-#  define PERL_GET_CONTEXT	PTHREAD_GETSPECIFIC(PL_thr_key)
-#endif
+#if defined(PERL_THREAD_LOCAL) && !defined(PERL_GET_CONTEXT) && !defined(PERL_SET_CONTEXT)
+/* Use C11 thread-local storage, where possible: */
+#  define PERL_USE_THREAD_LOCAL
+extern PERL_THREAD_LOCAL void *PL_current_context;
+
+#  define PERL_GET_CONTEXT        PL_current_context
+
+/* Set our thread-specific value anyway, in case code is reading it directly. */
+#  define PERL_SET_CONTEXT(t)                                           \
+    STMT_START {                                                        \
+        int _eC_;                                                       \
+        if ((_eC_ = pthread_setspecific(PL_thr_key, PL_current_context = (void *)(t)))) \
+            Perl_croak_nocontext("panic: pthread_setspecific (%d) [%s:%d]", \
+                                 _eC_, __FILE__, __LINE__);             \
+    } STMT_END
+
+#else
+/* else fall back to pthreads */
+
+#  ifndef PERL_GET_CONTEXT
+#    define PERL_GET_CONTEXT	PTHREAD_GETSPECIFIC(PL_thr_key)
+#  endif
 
-#ifndef PERL_SET_CONTEXT
-#  define PERL_SET_CONTEXT(t) \
+#  ifndef PERL_SET_CONTEXT
+#    define PERL_SET_CONTEXT(t) \
     STMT_START {						\
         int _eC_;						\
         if ((_eC_ = pthread_setspecific(PL_thr_key, (void *)(t))))	\
             Perl_croak_nocontext("panic: pthread_setspecific (%d) [%s:%d]",	\
                                  _eC_, __FILE__, __LINE__);	\
     } STMT_END
-#endif /* PERL_SET_CONTEXT */
+#  endif /* PERL_SET_CONTEXT */
+#endif /* PERL_THREAD_LOCAL */
 
 #ifndef INIT_THREADS
 #  ifdef NEED_PTHREAD_INIT