Cross-compilation for arm-none-eabi target

[danngreen]

Hi, very nice project!

I've found that two changes need to be made in order to cross-compile for a "bare-metal" ARM target using arm-none-eabi-gcc (that is, the gcc arm embedded toolchain).

1. In float_common.h, there is this block:

fast_float/include/fast_float/float_common.h

Lines 47 to 53 in b15abc9

	#if defined(__APPLE__) || defined(__FreeBSD__)
	#include <machine/endian.h>
	#elif defined(sun) || defined(__sun)
	#include <sys/byteorder.h>
	#else
	#include <endian.h>
	#endif

With every version of the toolchain from 10.3.1 going back to at least 5.4, endian.h is located in machine/endian.h, so line 47 needs to match a macro that for the ARM target. I've experimented and found both || defined (__arm__) and || defined (__ARM_EABI__) work, but I'm not sure which is proper (nor do I know where this file is located with propriety non-gcc-based ARM compilers).

2. In digit_comparison.h:
   

   fast_float/include/fast_float/digit_comparison.h

   Lines 88 to 89 in b15abc9

   	int32_t shift = -am.power2 + 1;
   	cb(am, std::min(shift, 64));

The arm-none-eabi-g++ compiler complains no matching function for call to 'min(int32_t&, int)'. Explicitly casting the integer constant to the same type as the variable works, and seems to be stylistically similar to other casts in this file:

cb(am, std::min(shift, int32_t(64)));

If these changes seem helpful to others, I'm happy to issue a pull request. Or try different solutions if this breaks someone else's build. In any case, thanks for the great project!

[lemire]

@danngreen Please do submit a PR. It seems entirely reasonable.

[jwakely]

See #122 and #123 which fix both these.

[jwakely]

In #122 I fixed the min deduction with std::min<int32_t>(shift, 64) but I can change it to the cast if you prefer that.

For the header, it's newlib that provides the <machine/endian.h> header, not the CPU, so it would be better to detect __NEWLIB__ not just __arm__ or __ARM_EABI__. Detecting newlib would also fix it for risc-v, cris-elf, and countless other targets that don't match the __arm__ macro. But #123 has what I think is a better fix anyway.

[danngreen]

Excellent, yes it did occur to me to do something like what you did in #123 (looking for the required symbols first), so I'm glad to see you did that. I can confirm this compiles with the arm-none-eabi. Thanks!

[jwakely]

Those PRs have been merged, so I hope arm-none-eabi works now and this can be closed, @danngreen

[lemire]

Please verify.

[danngreen]

Yes, thank you, it builds now.