gcc 5.2.1/6.1.1: error: inconsistent operand constraints in an ‘asm’

[zabereer]

Compilation fails when using benchmark::DoNotOptimize on an array:

In file included from benchmark/include/benchmark/benchmark.h:18:0, from bm_boost_crc.cpp:1: benchmark/include/benchmark/benchmark_api.h: In function ‘void {anonymous}::boost_crc6(benchmark::State&)’: benchmark/include/benchmark/benchmark_api.h:217:54: error: inconsistent operand constraints in an ‘asm’ asm volatile("" : "+rm" (const_cast<Tp&>(value)));

Although gcc documenation on asm contraints indicates they should be adjacent I find inserting a comma ("+r,m") solves this problem. However it then does not work for functions returning an integral like boost::crc_optimal's checksum method:

value_type checksum() const;

I find the best solution to both is to use a template for pointers separately.
I have a patch that seem to work in all cases I have encountered so far.

[EricWF]

Urg. so the "rm" used to mean put this value in a register if possible, otherwise put it in memory. I think there's another way to say this that will avoid the conflicting constraints error.

[EricWF]

I cannot reproduce the error your seeing with GCC. (See https://goo.gl/krlgJM).

Could you please try changing "+rm" to "+g" and tell me if that works?

[EricWF]

OK so I managed to reproduce here: https://goo.gl/CJqGjJ. Unfortunately the change to "+g" doesn't work. Versions of GCC prior to v5 ICE on this code. Changing "+rm" to "+g" still emits the same error. I'll file a GCC bug since "+g" shouldn't have any conflicting constraints since its obviously a single constraint.

[zabereer]

In hindsight I realise it is wrong to try to force a memory constraint on all non-pointer types.
Many thanks for filing the gcc bug, that is kind of you.

[EricWF]

GCC Bug Filed: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71246

[EricWF]

I think this has been fixed in master. Please reopen if I'm incorrect.