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Re: atomic update of profile counters (issue7000044)


Joseph and Andrew, thanks for the suggestion. That's really helpful.

Here is the new patch for gcc.c.
Basically, it's just what you have suggested: enclosing -latomic with
--as-needed, and using macros.
For the case of no --as-needed support, I use static link. (just found
that some code already using this in the SPEC).
I'm flexible on this part -- if you think this is unnecessary, I can remove.

Thanks,

-Rong

Index: gcc.c
===================================================================
--- gcc.c       (revision 205053)
+++ gcc.c       (working copy)
@@ -748,6 +748,23 @@ proper position among the other output files.  */
     %{fvtable-verify=preinit: -lvtv -u_vtable_map_vars_start
-u_vtable_map_vars_end}}"
 #endif

+/* This spec is for linking in libatomic in gcov atomic counter update.
+   We will use the atomic functions defined in libatomic, only when the builtin
+   versions are not available. In the case of no LD_AS_NEEDED support, we
+   link libatomic statically.  */
+
+#ifndef GCOV_ATOMIC_SPEC
+#if USE_LD_AS_NEEDED
+#define GCOV_ATOMIC_SPEC "%{fprofile-generate-atomic=*:" LD_AS_NEEDED_OPTION \
+  " -latomic} " LD_NO_AS_NEEDED_OPTION
+#elif defined(HAVE_LD_STATIC_DYNAMIC)
+#define GCOV_ATOMIC_SPEC "%{fprofile-generate-atomic=*:" LD_STATIC_OPTION \
+                    " -latomic " LD_DYNAMIC_OPTION "}"
+#else /* !USE_LD_AS_NEEDED && !HAVE_LD_STATIC_DYNAMIC  */
+#define GCOV_ATOMIC_SPEC "%{fprofile-generate-atomic=*:-latomic}"
+#endif
+#endif
+
 /* -u* was put back because both BSD and SysV seem to support it.  */
 /* %{static:} simply prevents an error message if the target machine
    doesn't handle -static.  */
@@ -771,7 +788,8 @@ proper position among the other output files.  */
     %{fopenmp|ftree-parallelize-loops=*:%:include(libgomp.spec)%(link_gomp)}\
     %{fgnu-tm:%:include(libitm.spec)%(link_itm)}\
     %(mflib) " STACK_SPLIT_SPEC "\
-    %{fprofile-arcs|fprofile-generate*|coverage:-lgcov} " SANITIZER_SPEC " \
+    %{fprofile-arcs|fprofile-generate*|coverage:-lgcov\
+      " GCOV_ATOMIC_SPEC "} " SANITIZER_SPEC " \
     %{!nostdlib:%{!nodefaultlibs:%(link_ssp) %(link_gcc_c_sequence)}}\
     %{!nostdlib:%{!nostartfiles:%E}} %{T*} }}}}}}"



On Wed, Nov 20, 2013 at 1:04 PM, Joseph S. Myers
<joseph@codesourcery.com> wrote:
> On Wed, 20 Nov 2013, Rong Xu wrote:
>
>> I could do this in the SPEC
>>   -Wl,-Bstatic -latomic -Wl,-Bdynamic
>> which would link libatomic statically.
>> I works for me. But it looks a little weird in gcc driver.
>
> I think we should generally link libatomic with --as-needed by default on
> platforms supporting --as-needed, in line with the general principle that
> C code just using language not library facilities (_Atomic in this case)
> shouldn't need any special options to link it (libatomic is like libgcc,
> which is linked in automatically); the trickier question is what to do
> with it on any systems supporting shared libraries but not --as-needed.
>
> --
> Joseph S. Myers
> joseph@codesourcery.com


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