[Bug libstdc++/54754] New: [parallel mode] 'make check-parallel' only works on x86-64

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54754

             Bug #: 54754
           Summary: [parallel mode] 'make check-parallel' only works on
                    x86-64
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: redi@gcc.gnu.org


include/parallel/compatibility.h uses an OpenMP critical section on all
platforms except x86_64, which issues messages that break the check-parallel
testsuite:

Running
/home/jwakely/src/gcc/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp ...
ERROR: tcl error sourcing
/home/jwakely/src/gcc/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp.
ERROR: could not compile testsuite_abi.cc
    while executing
"error "could not compile $f""
    (procedure "v3-build_support" line 61)
    invoked from within
"v3-build_support"
    (file
"/home/jwakely/src/gcc/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp"
line 25)
    invoked from within
"source
/home/jwakely/src/gcc/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp"
    ("uplevel" body line 1)
    invoked from within
"uplevel #0 source
/home/jwakely/src/gcc/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp"
    invoked from within
"catch "uplevel #0 source $test_file_name""


testsuite/parallel/libstdc++.log shows:

In file included from
/home/jwakely/build/i686-pc-linux-gnu/libstdc++-v3/include/parallel/find.h:40:0,
                 from
/home/jwakely/build/i686-pc-linux-gnu/libstdc++-v3/include/parallel/algobase.h:42,
                 from
/home/jwakely/build/i686-pc-linux-gnu/libstdc++-v3/include/bits/stl_algobase.h:1222,
                 from
/home/jwakely/build/i686-pc-linux-gnu/libstdc++-v3/include/algorithm:62,
                 from
/home/jwakely/build/i686-pc-linux-gnu/libstdc++-v3/include/i686-pc-linux-gnu/bits/stdc++.h:65,
                 from <command-line>:0:
/home/jwakely/build/i686-pc-linux-gnu/libstdc++-v3/include/parallel/compatibility.h:
In function 'int64_t __gnu_parallel::__fetch_and_add_64(volatile int64_t*,
int64_t)':
/home/jwakely/build/i686-pc-linux-gnu/libstdc++-v3/include/parallel/compatibility.h:167:42:
note: #pragma message: slow __fetch_and_add_64
 #pragma message("slow __fetch_and_add_64")
                                          ^
/home/jwakely/build/i686-pc-linux-gnu/libstdc++-v3/include/parallel/compatibility.h:
In function 'bool __gnu_parallel::__compare_and_swap_64(volatile int64_t*,
int64_t, int64_t)':
/home/jwakely/build/i686-pc-linux-gnu/libstdc++-v3/include/parallel/compatibility.h:320:45:
note: #pragma message: slow __compare_and_swap_64
 #pragma message("slow __compare_and_swap_64")
                                             ^


The parallel/compatibility.h header checks for 

#if defined(__x86_64)
  [use atomics]
#elif defined(__i386) &&                   \
  (defined(__i686) || defined(__pentium4) || defined(__athlon)  \
   || defined(__k8) || defined(__core2))
  [use atomics]
#else
  [use omp critical]
#endif

But __i686 isn't defined on i686, for example, and this doesn't handle
platforms such as power64 which support the necessary 64-bit ops.

Instead of a (broken) whitelist of targets the code could use
__atomic_always_lock_free to check for the availability of atomic ops on the
target.

Another possibility would be to add a new configure option:
   --enable-libstdcxx-parallel-atomics=[omp|libatomic]

The default would be to use the omp criticial section as it is now, the
libatomic option would force use of atomics, which would require libatomic for
platforms where the built-ins call a library function.