Add a switch in the CMake build to allow falling back to the system swapcontext.

Author: insertinterestingnamehere

config/qthread_check_swapcontext.m4

@@ -83,7 +83,7 @@ int main()
 				  [qthread_cv_swapcontext_works=yes])],
 	       [qthread_cv_swapcontext_works=yes])])])
 AS_IF([test "$qthread_cv_swapcontext_works" = yes],
-	[AC_DEFINE([HAVE_NATIVE_MAKECONTEXT], [1], [The system provides functional native make/swap/get-context functions])
+	[AC_DEFINE([USE_SYSTEM_SWAPCONTEXT], [1], [Use the system provided functional native make/swap/get-context functions])
 	 QTHREAD_CHECK_MAKECONTEXT_SPLIT_ARGS(
 	 	[AC_DEFINE([QTHREAD_MAKECONTEXT_SPLIT],[1],[makecontext()passes args as int-size, not long-size])])
 	 $1],

include/fastcontext/386-ucontext.h

@@ -2,9 +2,7 @@
 #include <config.h>
 #endif
 
-#ifdef HAVE_STDARG_H
 #include <stdarg.h> /* for the qt_makectxt prototype */
-#endif
 #include <stddef.h> /* for size_t, per C89 */
 #include <stdint.h>
 

include/fastcontext/power-ucontext.h

@@ -1,6 +1,4 @@
-#ifdef HAVE_STDARG_H
 #include <stdarg.h> /* for the qt_makectxt prototype */
-#endif
 
 #include "qt_visibility.h"
 

include/fastcontext/taskimpl.h

@@ -22,16 +22,12 @@
 #define NEEDSWAPCONTEXT
 #include "power-ucontext.h"
 #elif (QTHREAD_ASSEMBLY_ARCH == QTHREAD_ARM)
-#ifdef HAVE_STDARG_H
 #include <stdarg.h>
-#endif
 #define NEEDARMMAKECONTEXT
 #define NEEDSWAPCONTEXT
 #include "arm-ucontext.h"
 #elif (QTHREAD_ASSEMBLY_ARCH == QTHREAD_ARMV8_A64)
-#ifdef HAVE_STDARG_H
 #include <stdarg.h>
-#endif
 #define NEEDARMA64MAKECONTEXT
 #define NEEDSWAPCONTEXT
 #include "arm-ucontext.h"

include/qt_context.h

@@ -1,7 +1,7 @@
 #ifndef QT_CONTEXT_H
 #define QT_CONTEXT_H
 
-#if defined(HAVE_UCONTEXT_H) && defined(HAVE_NATIVE_MAKECONTEXT)
+#if defined(USE_SYSTEM_SWAPCONTEXT)
 #include <ucontext.h> /* for ucontext_t */
 typedef ucontext_t qt_context_t;
 #else

src/CMakeLists.txt

@@ -8,6 +8,7 @@ set(QTHREADS_DEFAULT_STACK_SIZE 32768 CACHE STRING "Default qthread stack size."
 set(QTHREADS_HASHMAP hashmap CACHE STRING "Which hashmap implementation to use. Valid values are \"hashmap\" and \"lf_hashmap\".")
 set(QTHREADS_DICT_TYPE shavit CACHE STRING "Which dictionary implementation to use. Valid values are \"shavit\", \"trie\", and \"simple\".")
 set(QTHREADS_TIMER_TYPE gettimeofday CACHE STRING "Which timer implementation to use. Valid values are \"clock_gettime\", \"mach\", \"gettimeofday\", and \"gethrtime\".")
+set(QTHREADS_CONTEXT_SWAP_IMPL fastcontext CACHE STRING "Which context swap implementation to use. Valid values are \"system\" and \"fastcontext\".")
 
 set(QTHREADS_SOURCES
   cacheline.c
@@ -34,8 +35,6 @@ set(QTHREADS_SOURCES
   touch.c
   tls.c
   teams.c
-  fastcontext/asm.S
-  fastcontext/context.c
   ${QTHREADS_HASHMAP}.c
   ds/qarray.c
   ds/qdqueue.c
@@ -60,10 +59,16 @@ set(QTHREADS_SOURCES
   patterns/wavefront.c
 )
 
-# TODO: switch/checks necessary to include the correct
-# fastcontext version when we need the fallback.
-
 add_library(qthread SHARED ${QTHREADS_SOURCES})
+
+if ("${QTHREADS_CONTEXT_SWAP_IMPL}" STREQUAL "fastcontext")
+  target_sources(qthread PRIVATE fastcontext/asm.S fastcontext/context.c)
+elseif ("${QTHREADS_CONTEXT_SWAP_IMPL}" STREQUAL "system")
+  target_compile_definitions(qthread PRIVATE USE_SYSTEM_SWAPCONTEXT)
+else()
+  message(FATAL_ERROR "The specified context swap implementation does not match any known implementations.")
+endif()
+
 target_include_directories(qthread
   PRIVATE "../include"
 )

src/qthread.c

@@ -439,7 +439,7 @@ static void *qthread_master(void *arg) {
 
         *current = t;
 
-#ifdef HAVE_NATIVE_MAKECONTEXT
+#ifdef USE_SYSTEM_SWAPCONTEXT
         getcontext(&my_context);
 #endif
         qthread_exec(t, &my_context);
@@ -846,7 +846,7 @@ int API_FUNC qthread_initialize(void) { /*{{{ */
 
   qthread_before_swap_to_main();
 
-#ifdef HAVE_NATIVE_MAKECONTEXT
+#ifdef USE_SYSTEM_SWAPCONTEXT
   qassert(
     swapcontext(&qlib->mccoy_thread->rdata->context, &(qlib->master_context)),
     0);
@@ -938,21 +938,21 @@ static inline void qthread_makecontext(qt_context_t *const c,
 #ifdef UCSTACK_HAS_SSFLAGS
   c->uc_stack.ss_flags = 0;
 #endif
-#ifdef HAVE_NATIVE_MAKECONTEXT
+#ifdef USE_SYSTEM_SWAPCONTEXT
   /* the makecontext man page (Linux) says: set the uc_link FIRST.
    * why? no idea */
   c->uc_link = returnc; /* NULL pthread_exit() */
 #endif
-#ifdef HAVE_NATIVE_MAKECONTEXT
+#ifdef USE_SYSTEM_SWAPCONTEXT
 #ifdef QTHREAD_MAKECONTEXT_SPLIT
   makecontext(c, func, 2, high, low);
 #else  /* QTHREAD_MAKECONTEXT_SPLIT */
   makecontext(c, func, 1, arg);
 #endif /* QTHREAD_MAKECONTEXT_SPLIT */
   assert((void *)c->uc_link == (void *)returnc);
-#else  /* ifdef HAVE_NATIVE_MAKECONTEXT */
+#else  /* ifdef USE_SYSTEM_SWAPCONTEXT */
   qt_makectxt(c, func, 1, arg);
-#endif /* ifdef HAVE_NATIVE_MAKECONTEXT */
+#endif /* ifdef USE_SYSTEM_SWAPCONTEXT */
 } /*}}} */
 
 /* this adds a function to the list of cleanup functions to call at finalize;
@@ -1743,7 +1743,7 @@ void INTERNAL qthread_exec(qthread_t *t, qt_context_t *c) { /*{{{ */
                           (void (*)(void))qthread_wrapper,
                           t,
                           c);
-#ifdef HAVE_NATIVE_MAKECONTEXT
+#ifdef USE_SYSTEM_SWAPCONTEXT
     } else {
       t->rdata->context.uc_link = c; /* NULL pthread_exit() */
 #endif
@@ -1766,7 +1766,7 @@ void INTERNAL qthread_exec(qthread_t *t, qt_context_t *c) { /*{{{ */
 
     qthread_before_swap_to_qthread(t);
 
-#ifdef HAVE_NATIVE_MAKECONTEXT
+#ifdef USE_SYSTEM_SWAPCONTEXT
     qassert(swapcontext(atomic_load_explicit(&t->rdata->return_context,
                                              memory_order_relaxed),
                         &t->rdata->context),
@@ -2233,7 +2233,7 @@ void INTERNAL qthread_back_to_master(qthread_t *t) { /*{{{ */
     sizeof(qt_context_t));
 #endif
   qthread_before_swap_from_qthread(t);
-#ifdef HAVE_NATIVE_MAKECONTEXT
+#ifdef USE_SYSTEM_SWAPCONTEXT
   qassert(swapcontext(&t->rdata->context,
                       atomic_load_explicit(&t->rdata->return_context,
                                            memory_order_relaxed)),
@@ -2262,7 +2262,7 @@ void INTERNAL qthread_back_to_master2(qthread_t *t) { /*{{{ */
     atomic_load_explicit(&t->rdata->return_context, memory_order_relaxed),
     sizeof(qt_context_t));
 #endif
-#ifdef HAVE_NATIVE_MAKECONTEXT
+#ifdef USE_SYSTEM_SWAPCONTEXT
   setcontext(
     atomic_load_explicit(&t->rdata->return_context, memory_order_relaxed));
 #else