FLT16_MAX is defined but _Float16 is not supported: confused

[llvmbot]

Bugzilla Link	52532
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@zygoloid,@xtkoba

Extended Description

Notes:

1. This may not be a bug. Just want to draw attention to it.
2. This may not be a bug in clang itself, but rather in the system / C standard library headers. Feel free to move it.

This code:
#define STDC_WANT_IEC_60559_TYPES_EXT
#include <float.h>
#ifdef FLT16_MAX
_Float16 f16;
#endif

compiled with [1]:
clang -std=c11 -Wall -Wextra

leads to:
t0.c:4:1: error: _Float16 is not supported on this target

which is confusing because FLT16_MAX is defined.

Per IEC 60559 interchange and extended types [2] it seems that if FLT16_MAX is defined, then _Float16 is supported. Otherwise, how to correctly determine at compile time that _Float16 is supported?

Thanks for the attention.

[1] Version: clang trunk on linux on x86_64
[2] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2601.pdf

[xtkoba]

I suppose this is actually a bug, derived from https://reviews.llvm.org/rGc05609ca36e7269b73f982725b59c48a5fa68a2c.

A workaround seems to be to use defined(FLT16_MAX) instead of defined(FLT16_MAX).

(FWIW, in the following C code, BAR is defined even if FOO is not defined.)

#undef FOO
#define BAR FOO
#ifdef BAR
#warning BAR is defined.
#endif

[xtkoba]

Fix #defines of IEC 60559 FLT16_* identifiers
Does this work?

[arsenm]

Fix #defines of IEC 60559 FLT16_* identifiers

Can you resubmit this as PR?