FreeBSD build broken following _Static_assert commit

On FreeBSD 10.1 with clang 3.4.1, a standard "gmake" build now fails with:

libfio.c:309:21: error: static_assert expression is not an integral constant expression
compiletime_assert((offsetof(struct thread_stat,
percentile_list) % 8) == 0, "stat percentile_list"); ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'

#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^
libfio.c:309:22: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
        compiletime_assert((offsetof(struct thread_stat, 
percentile_list) % 8) == 0, "stat percentile_list");
                            ^
./flist.h:10:33: note: expanded from macro 'offsetof'
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
                                ^
./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'
#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^

libfio.c:310:21: error: static_assert expression is not an integral constant expression
compiletime_assert((offsetof(struct thread_stat,
total_run_time) % 8) == 0, "total_run_time"); ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'

#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^
libfio.c:310:22: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
        compiletime_assert((offsetof(struct thread_stat, 
total_run_time) % 8) == 0, "total_run_time");
                            ^
./flist.h:10:33: note: expanded from macro 'offsetof'
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
                                ^
./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'
#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^

libfio.c:311:21: error: static_assert expression is not an integral constant expression
compiletime_assert((offsetof(struct thread_stat,
total_err_count) % 8) == 0, "total_err_count"); ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'

#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^
libfio.c:311:22: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
        compiletime_assert((offsetof(struct thread_stat, 
total_err_count) % 8) == 0, "total_err_count");
                            ^
./flist.h:10:33: note: expanded from macro 'offsetof'
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
                                ^
./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'
#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^

libfio.c:312:21: error: static_assert expression is not an integral constant expression
compiletime_assert((offsetof(struct thread_stat,
latency_percentile) % 8) == 0, "stat latency_percentile"); ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'

#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^
libfio.c:312:22: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
        compiletime_assert((offsetof(struct thread_stat, 
latency_percentile) % 8) == 0, "stat latency_percentile");
                            ^
./flist.h:10:33: note: expanded from macro 'offsetof'
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
                                ^
./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'
#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^

libfio.c:313:21: error: static_assert expression is not an integral constant expression
compiletime_assert((offsetof(struct thread_options_pack,
zipf_theta) % 8) == 0, "zipf_theta"); ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'

#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^
libfio.c:313:22: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
        compiletime_assert((offsetof(struct thread_options_pack, 
zipf_theta) % 8) == 0, "zipf_theta");
                            ^
./flist.h:10:33: note: expanded from macro 'offsetof'
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
                                ^
./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'
#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^

libfio.c:314:21: error: static_assert expression is not an integral constant expression
compiletime_assert((offsetof(struct thread_options_pack,
pareto_h) % 8) == 0, "pareto_h"); ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'

#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^
libfio.c:314:22: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
        compiletime_assert((offsetof(struct thread_options_pack, 
pareto_h) % 8) == 0, "pareto_h");
                            ^
./flist.h:10:33: note: expanded from macro 'offsetof'
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
                                ^
./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'
#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^

libfio.c:315:21: error: static_assert expression is not an integral constant expression
compiletime_assert((offsetof(struct thread_options_pack,
percentile_list) % 8) == 0, "percentile_list"); ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'

#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^
libfio.c:315:22: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
        compiletime_assert((offsetof(struct thread_options_pack, 
percentile_list) % 8) == 0, "percentile_list");
                            ^
./flist.h:10:33: note: expanded from macro 'offsetof'
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
                                ^
./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'
#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^

libfio.c:316:21: error: static_assert expression is not an integral constant expression
compiletime_assert((offsetof(struct thread_options_pack,
latency_percentile) % 8) == 0, "latency_percentile"); ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'

#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^
libfio.c:316:22: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
        compiletime_assert((offsetof(struct thread_options_pack, 
latency_percentile) % 8) == 0, "latency_percentile");
                            ^
./flist.h:10:33: note: expanded from macro 'offsetof'
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
                                ^
./compiler/compiler.h:39:59: note: expanded from macro 'compiletime_assert'
#define compiletime_assert(condition, msg) _Static_assert(condition, msg)
                                                          ^

8 errors generated.

Bruce

Thanks for reporting this, Bruce. No idea why clang complains about that, it's definitely a constant expression. Anyway, committed this:

http://git.kernel.dk/cgit/fio/commit/?id=94815a5c8366a9290167e8539f29994c2d43d15c

which adds this sort of check to the configure _Static_assert() check.

Jens Axboe

It seems the relevant text is:

note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression

Is there something about our use of offsetof that means we can't depend on the version from <stddef.h>?

Bruce

But it's bogus, it's nothing but static pointer math.

No other reason than having a generic solution is superior to depending on whatever might be supplied with the given system.

Jens Axboe

Apparently it's a known bug: https://llvm.org/bugs/show_bug.cgi?id=17410 .
It looks like on FreeBSD there's no __compiler_offsetof (which on Linux ends up being defined as __builtin_offsetof) but clang _does_ provide __builtin_offsetof.

Bruce

I used clang 3.6.0 to reproduce it, so apparently still not fixed. If you want to do a cleanup around what I committed, feel free, but as far as I'm concerned, it's resolved.

Jens Axboe