Re: [lmi] [PATCH] Better support for non-x87 platforms

[Greg Chicares]

On 2017-01-05 16:34, Vadim Zeitlin wrote:
> On Thu, 5 Jan 2017 15:17:37 +0000 Greg Chicares <address@hidden> wrote:
[...]
> GC>   http://stackoverflow.com/questions/22581350/clang-5-1-fpmath
> GC> | for Clang to accept -mfpmath=387, I had to also pass -mno-sse

[...but you tried, and got...]

> fatal error: error in backend: SSE register return with SSE disabled
> clang: error: clang frontend command failed with exit code 70 (use -v to see 
> invocation)
> 
> I found several bug reports about this error message, but they also seem to
> be already fixed in the (very recent) version I'm using or not relevant, so
> I'm not sure what's going on here. I'm even less sure whether it makes
> sense to spend any more time on this though, I don't see any good reason to
> use x87 instructions with clang, do you?

Thanks for trying. I don't think it's worth any further effort for clang.

> GC> I'm not necessarily adamant about (1), but it doesn't seem ideal
> GC> to let clang (e.g.) default silently to SSE.
> 
>  There are two objections to this: first, clang definitely does default to
> SSE (and my experience seems to indicate that it's not that simple to avoid
> using SSE with it) and, second, clang predefines __GNUC__ anyhow, so it's
> already covered by gcc checks.

Okay, then the lack of ideality is immanent in clang, and there's
nothing we can do about that. Let me ask, just to be sure--does
the x87 test work with clang? In relevant part:

#if defined LMI_X86
#   if defined __GNUC__
#       if !defined __SSE_MATH__
#           define LMI_X87
#       endif // !defined __SSE_MATH__
#   elif defined _MSC_VER

Now, clang defines __GNUC__, and it can't handle x87, so we rely
on it not defining __SSE_MATH__. Would it be more robust to test
for clang explicitly? E.g.:

 #if defined LMI_X86
+#   if defined __clang__
+       // Do nothing: clang doesn't support x87.
+#   elif defined __GNUC__
-#   if defined __GNUC__
 #       if !defined __SSE_MATH__
 #           define LMI_X87
 #       endif // !defined __SSE_MATH__

> GC> I just pushed a change that looks very much like your rewrite
> GC> above, and moves the LMI_X86 and LMI_X87 tests into propinquity.
> 
>  Thanks! This does (mostly) compiles now but still stops further on with
> 
> fenv_lmi.cpp:94:41: error: unused parameter 'precision_mode' 
> [-Werror=unused-parameter]
> 
> The following trivial patch allows the compilation to succeed

Applied and pushed. Thanks.