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* [PATCH] libgcc, emutls: Allow building weak definitions of the emutls functions.
@ 2021-09-20  8:25 Iain Sandoe
  2021-09-24 16:57 ` [PING}[PATCH] " Iain Sandoe
  0 siblings, 1 reply; 4+ messages in thread
From: Iain Sandoe @ 2021-09-20  8:25 UTC (permalink / raw)
  To: GCC Patches

Hi,

The non-Darwin part of this patch is trivial but raises a couple of questions

A/
We define builtins to support emulated TLS.
These are defined with void * pointers
The implementation (in libgcc) uses the correct type (struct __emutls_object *)
in both a forward declaration of the functions and in thier eventual implementation.

This leads to a (long-standing, nothing new) complaint at build-time about
the mismatch in the builtin/implementation decls.

AFAICT, there’s no way to fix that unless we introduce struct __emutls_object *
as a built-in type?

B/ 
It seems that a consequence of the mismatch in decls means that if I apply
attributes to the decl (in the implementation file), they are ignored and I have
to apply them to the definition in order for this to work.

This (B) is what the patch below does.

tested on powerpc,i686,x86_64-darwin, x86_64-linux
OK for master?
thanks,
Iain

If the current situation is that A or B indicates “there’s a bug”, please could that
be considered as distinct from the current patch (which doesn’t alter this in any
way) so that we can make progress on fixing Darwin libgcc issues.

===== commit log

In order to better support use of the emulated TLS between objects with
DSO dependencies and static-linked libgcc, allow a target to make weak
definitions.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

libgcc/ChangeLog:

	* config.host: Add weak-defined emutls crt.
	* config/t-darwin: Build weak-defined emutls objects.
	* emutls.c (__emutls_get_address): Add optional attributes.
	(__emutls_register_common): Likewise.
	(EMUTLS_ATTR): New.
---
 libgcc/config.host     |  2 +-
 libgcc/config/t-darwin | 13 +++++++++++++
 libgcc/emutls.c        | 17 +++++++++++++++--
 3 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/libgcc/config.host b/libgcc/config.host
index 6c34b13d611..a447ac7ae30 100644
--- a/libgcc/config.host
+++ b/libgcc/config.host
@@ -215,7 +215,7 @@ case ${host} in
 *-*-darwin*)
   asm_hidden_op=.private_extern
   tmake_file="$tmake_file t-darwin ${cpu_type}/t-darwin t-libgcc-pic t-slibgcc-darwin"
-  extra_parts="crt3.o libd10-uwfef.a crttms.o crttme.o"
+  extra_parts="crt3.o libd10-uwfef.a crttms.o crttme.o libemutls_w.a"
   ;;
 *-*-dragonfly*)
   tmake_file="$tmake_file t-crtstuff-pic t-libgcc-pic t-eh-dw2-dip"
diff --git a/libgcc/config/t-darwin b/libgcc/config/t-darwin
index 14ae6b35a4e..d6f688d66d5 100644
--- a/libgcc/config/t-darwin
+++ b/libgcc/config/t-darwin
@@ -15,6 +15,19 @@ crttme.o: $(srcdir)/config/darwin-crt-tm.c
 LIB2ADDEH = $(srcdir)/unwind-dw2.c $(srcdir)/config/unwind-dw2-fde-darwin.c \
   $(srcdir)/unwind-sjlj.c $(srcdir)/unwind-c.c
 
+# Make emutls weak so that we can deal with -static-libgcc, override the
+# hidden visibility when this is present in libgcc_eh.
+emutls.o: HOST_LIBGCC2_CFLAGS += \
+  -DEMUTLS_ATTR='__attribute__((__weak__,__visibility__("default")))'
+emutls_s.o: HOST_LIBGCC2_CFLAGS += \
+  -DEMUTLS_ATTR='__attribute__((__weak__,__visibility__("default")))'
+
+# Make the emutls crt as a convenience lib so that it can be linked
+# optionally, use the shared version so that we can link with DSO.
+libemutls_w.a: emutls_s.o
+	$(AR_CREATE_FOR_TARGET) $@ $<
+	$(RANLIB_FOR_TARGET) $@
+
 # Patch to __Unwind_Find_Enclosing_Function for Darwin10.
 d10-uwfef.o: $(srcdir)/config/darwin10-unwind-find-enc-func.c
 	$(crt_compile) -mmacosx-version-min=10.6 -c $<
diff --git a/libgcc/emutls.c b/libgcc/emutls.c
index ed2658170f5..d553a74728f 100644
--- a/libgcc/emutls.c
+++ b/libgcc/emutls.c
@@ -50,7 +50,16 @@ struct __emutls_array
   void **data[];
 };
 
+/* EMUTLS_ATTR is provided to allow targets to build the emulated tls
+   routines as weak definitions, for example.
+   If there is no definition, fall back to the default.  */
+#ifndef EMUTLS_ATTR
+#  define EMUTLS_ATTR
+#endif
+
+EMUTLS_ATTR
 void *__emutls_get_address (struct __emutls_object *);
+EMUTLS_ATTR
 void __emutls_register_common (struct __emutls_object *, word, word, void *);
 
 #ifdef __GTHREADS
@@ -123,7 +132,11 @@ emutls_alloc (struct __emutls_object *obj)
   return ret;
 }
 
-void *
+/* Despite applying the attribute to the declaration, in this case the mis-
+   match between the builtin's declaration [void * (*)(void *)] and the
+   implementation here, causes the decl. attributes to be discarded.  */
+
+EMUTLS_ATTR void *
 __emutls_get_address (struct __emutls_object *obj)
 {
   if (! __gthread_active_p ())
@@ -187,7 +200,7 @@ __emutls_get_address (struct __emutls_object *obj)
 #endif
 }
 
-void
+EMUTLS_ATTR void
 __emutls_register_common (struct __emutls_object *obj,
 			  word size, word align, void *templ)
 {
-- 


^ permalink raw reply	[flat|nested] 4+ messages in thread
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2021-09-20  8:25 [PATCH] libgcc, emutls: Allow building weak definitions of the emutls functions Iain Sandoe
2021-09-24 16:57 ` [PING}[PATCH] " Iain Sandoe
2021-10-01 18:07   ` [PING^2][PATCH] " Iain Sandoe
2021-10-04 16:06     ` Richard Sandiford

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