Re: [PATCH] tcg/ppc: Fix building with Clang

From: Richard Henderson <richard.henderson@linaro.org>
To: Peter Maydell <peter.maydell@linaro.org>, Brad Smith <brad@comstyle.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>
Subject: Re: [PATCH] tcg/ppc: Fix building with Clang
Date: Wed, 21 Apr 2021 22:18:22 -0700	[thread overview]
Message-ID: <1ebc5a66-2763-1379-24e0-774739f2edbe@linaro.org> (raw)
In-Reply-To: <CAFEAcA995L--csz+fX4MqkSPTPxQ7Nx=q-Lh70bi_zQGoj54mw@mail.gmail.com>

On 4/21/21 2:03 AM, Peter Maydell wrote:
>> +/* Clang does not define _CALL_* */
>> +#if defined(__clang__) && defined(__ELF__) && !defined(_CALL_SYSV)
>> +#define _CALL_SYSV 1
>> +#endif
> 
> This is trying to identify the calling convention used by the OS.
> That's not purely compiler specific (ie it is not the case that
> all ELF output from clang is definitely using the calling convention
> that _CALL_SYSV implies), so settign it purely based on "this is clang
> producing ELF files" doesn't seem right.

We can get pretty close though.  There are three ppc32 calling conventions: 
AIX, DARWIN, SYSV.  The _CALL_ELF symbol is a 64-bit thing, and AIX itself 
doesn't use ELF.

> I guess if clang doesn't reliably tell us the calling convention
> maybe we should scrap the use of _CALL_SYSV and _CALL_ELF and
> use the host OS defines to guess the calling convention ?

No, I'd rely on _CALL_* first, and only fall back to something else if they're 
not present.

I'm thinking something like

#if !defined(_CALL_SYSV) && \
     !defined(_CALL_DARWIN) && \
     !defined(_CALL_AIX) && \
     !defined(_CALL_ELF)
# if defined(__APPLE__)
#  define _CALL_DARWIN
# elif defined(__ELF__) && TCG_TARGET_REG_BITS == 32
#  define _CALL_SYSV
# else
#  error "Unknown ABI"
# endif
#endif

r~