runtime: Remove usages of weak symbols

Inspired by a bug I hit on MinGW recently with weak symbol resolution.
I'm not sure why we started using these in
70000ac. We should be able to lookup
these symbols just fine in our `jl_exe_handle` as long as it was
linked / compiled with `-rdynamic`

Migrating away from weak symbols seems a good idea, given their uneven
implementation across platforms.

Author: topolarity

src/Makefile

@@ -56,7 +56,7 @@ SRCS := \
 	jltypes gf typemap smallintset ast builtins module interpreter symbol \
 	dlload sys init task array genericmemory staticdata toplevel jl_uv datatype \
 	simplevector runtime_intrinsics precompile jloptions mtarraylist \
-	threading scheduler stackwalk \
+	threading scheduler stackwalk null_sysimage \
 	method jlapi signal-handling safepoint timing subtype rtutils \
 	crc32c APInt-C processor ircode opaque_closure codegen-stubs coverage runtime_ccall engine \
 	$(GC_SRCS)
@@ -184,7 +184,7 @@ endif
 CLANG_LDFLAGS := $(LLVM_LDFLAGS)
 ifeq ($(OS), Darwin)
 CLANG_LDFLAGS += -Wl,-undefined,dynamic_lookup
-OSLIBS += -Wl,-U,__dyld_atfork_parent -Wl,-U,__dyld_atfork_prepare -Wl,-U,__dyld_dlopen_atfork_parent -Wl,-U,__dyld_dlopen_atfork_prepare -Wl,-U,_jl_image_pointers -Wl,-U,_jl_system_image_data -Wl,-U,_jl_system_image_size
+OSLIBS += -Wl,-U,__dyld_atfork_parent -Wl,-U,__dyld_atfork_prepare -Wl,-U,__dyld_dlopen_atfork_parent -Wl,-U,__dyld_dlopen_atfork_prepare
 LIBJULIA_PATH_REL := @rpath/libjulia
 else
 LIBJULIA_PATH_REL := libjulia

src/julia_internal.h

@@ -1912,17 +1912,6 @@ jl_sym_t *_jl_symbol(const char *str, size_t len) JL_NOTSAFEPOINT;
   #define JL_GC_ASSERT_LIVE(x) (void)(x)
 #endif
 
-#ifdef _OS_WINDOWS_
-// On Windows, weak symbols do not default to 0 due to a GCC bug
-// (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90826), use symbol
-// aliases with a known value instead.
-#define JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(sym) __attribute__((weak,alias(#sym)))
-#define JL_WEAK_SYMBOL_DEFAULT(sym) &sym
-#else
-#define JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(sym) __attribute__((weak))
-#define JL_WEAK_SYMBOL_DEFAULT(sym) NULL
-#endif
-
 //JL_DLLEXPORT float julia__gnu_h2f_ieee(half param) JL_NOTSAFEPOINT;
 //JL_DLLEXPORT half julia__gnu_f2h_ieee(float param) JL_NOTSAFEPOINT;
 //JL_DLLEXPORT half julia__truncdfhf2(double param) JL_NOTSAFEPOINT;

src/null_sysimage.c

@@ -0,0 +1,13 @@
+// This file is a part of Julia. License is MIT: https://julialang.org/license
+
+#include <stddef.h>
+
+/**
+ * These symbols support statically linking the sysimage with libjulia-internal.
+ *
+ * Here we provide dummy definitions that are used when these are not linked
+ * together (the default build configuration). The 0 value of jl_system_image_size
+ * is used as a sentinel to indicate that the sysimage should be loaded externally.
+ **/
+size_t jl_system_image_size = 0;
+char jl_system_image_data = 0;

src/processor.cpp

@@ -619,11 +619,6 @@ static inline llvm::SmallVector<TargetData<n>, 0> &get_cmdline_targets(F &&featu
     return targets;
 }
 
-extern "C" {
-void *image_pointers_unavailable;
-extern void * JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(image_pointers_unavailable) jl_image_pointers;
-}
-
 // Load sysimg, use the `callback` for dispatch and perform all relocations
 // for the selected target.
 template<typename F>
@@ -633,10 +628,7 @@ static inline jl_image_t parse_sysimg(void *hdl, F &&callback)
     jl_image_t res{};
 
     const jl_image_pointers_t *pointers;
-    if (hdl == jl_exe_handle && &jl_image_pointers != JL_WEAK_SYMBOL_DEFAULT(image_pointers_unavailable))
-        pointers = (const jl_image_pointers_t *)&jl_image_pointers;
-    else
-        jl_dlsym(hdl, "jl_image_pointers", (void**)&pointers, 1);
+    jl_dlsym(hdl, "jl_image_pointers", (void**)&pointers, 1);
 
     const void *ids = pointers->target_data;
     jl_value_t* rejection_reason = nullptr;

src/staticdata.c

@@ -645,24 +645,28 @@ JL_DLLEXPORT int jl_running_on_valgrind(void)
     return RUNNING_ON_VALGRIND;
 }
 
-void *system_image_data_unavailable;
-extern void * JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(system_image_data_unavailable) jl_system_image_data;
-extern void * JL_WEAK_SYMBOL_OR_ALIAS_DEFAULT(system_image_data_unavailable) jl_system_image_size;
+extern size_t jl_system_image_size;
+extern char jl_system_image_data;
+
 static void jl_load_sysimg_so(void)
 {
-    const char *sysimg_data;
     assert(sysimage.fptrs.ptrs); // jl_init_processor_sysimg should already be run
-    if (jl_sysimg_handle == jl_exe_handle &&
-            &jl_system_image_data != JL_WEAK_SYMBOL_DEFAULT(system_image_data_unavailable))
-        sysimg_data = (const char*)&jl_system_image_data;
-    else
-        jl_dlsym(jl_sysimg_handle, "jl_system_image_data", (void **)&sysimg_data, 1);
+
     size_t *plen;
-    if (jl_sysimg_handle == jl_exe_handle &&
-            &jl_system_image_size != JL_WEAK_SYMBOL_DEFAULT(system_image_data_unavailable))
-        plen = (size_t *)&jl_system_image_size;
-    else
+    const char *sysimg_data;
+
+    if (jl_system_image_size == 0) {
+        // in the usual case, the sysimage was not statically linked to libjulia-internal
+        // look up the external sysimage symbols via the dynamic linker
         jl_dlsym(jl_sysimg_handle, "jl_system_image_size", (void **)&plen, 1);
+        jl_dlsym(jl_sysimg_handle, "jl_system_image_data", (void **)&sysimg_data, 1);
+    } else {
+        // the sysimage was statically linked directly against libjulia-internal
+        // use the internal symbols
+        plen = &jl_system_image_size;
+        sysimg_data = &jl_system_image_data;
+    }
+
     jl_gc_notify_image_load(sysimg_data, *plen);
     jl_restore_system_image_data(sysimg_data, *plen);
 }

src/threading.c

@@ -228,10 +228,6 @@ void jl_set_pgcstack(jl_gcframe_t **pgcstack) JL_NOTSAFEPOINT
 {
     *jl_pgcstack_key() = pgcstack;
 }
-#  if JL_USE_IFUNC
-JL_DLLEXPORT __attribute__((weak))
-void jl_register_pgcstack_getter(void);
-#  endif
 static jl_gcframe_t **jl_get_pgcstack_init(void);
 static jl_get_pgcstack_func *jl_get_pgcstack_cb = jl_get_pgcstack_init;
 static jl_gcframe_t **jl_get_pgcstack_init(void)
@@ -244,15 +240,8 @@ static jl_gcframe_t **jl_get_pgcstack_init(void)
     // This is clearly not thread-safe but should be fine since we
     // make sure the tls states callback is finalized before adding
     // multiple threads
-#  if JL_USE_IFUNC
-    if (jl_register_pgcstack_getter)
-        jl_register_pgcstack_getter();
-    else
-#  endif
-    {
-        jl_get_pgcstack_cb = jl_get_pgcstack_fallback;
-        jl_pgcstack_key = &jl_pgcstack_addr_fallback;
-    }
+    jl_get_pgcstack_cb = jl_get_pgcstack_fallback;
+    jl_pgcstack_key = &jl_pgcstack_addr_fallback;
     return jl_get_pgcstack_cb();
 }