Re: [PATCH v4 4/4] target/ppc: fix warning with clang-15

[Pierrick Bouvier]

On 2/22/23 00:43, Richard Henderson wrote:
> On 2/21/23 12:30, Philippe Mathieu-Daudé wrote:
> > On 21/2/23 16:30, Pierrick Bouvier wrote:
> > > When compiling for windows-arm64 using clang-15, it reports a sometimes
> > > uninitialized variable. This seems to be a false positive, as a default
> > > case guards switch expressions, preventing to return an uninitialized
> > > value, but clang seems unhappy with assert(0) definition.
> >
> > $ git grep 'assert(0)' | wc -l
> >         96
> >
> > Should we mass-update and forbid 'assert(0)' adding a check in
> > scripts/checkpatch.pl? Otherwise we'll keep getting similar clang
> > warnings...
>
> I just think assert(0) produces a less clean error message, so on that basis 
> yes, we
> should replace them all.  Perhaps abort() as well, unless there's an 
> error_report
> immediately preceding.

If we start exploring this way, why not define explicit noreturn 
functions for this: unreachable() and todo()/unimplemented(). Names are 
inspired from the same Rust macros [1]. IMHO, all assert(0) fall into 
one of those two categories. It's easy to grep, and explicit.

The advantage, besides clarity, would be to guarantee it is never 
removed. I noticed qemu can't be compiled with NDEBUG for now, but with 
this, it would remove all assert invocations, including assert(0).

[1] https://doc.rust-lang.org/std/index.html#macros

> The fact that assert(0) was seen to fall through is a system header bug.  I see 
> we have a
> workaround in include/qemu/osdep.h for __MINGW32__, but I guess this doesn't 
> trigger for
> arm64?  Pierrick, would you mind testing a change there?

I can confirm this workaround is enabled for windows-arm64.

Indeed, from msys2 clangarm64 env:
$ echo | cc -dM -E - | grep MINGW
#define __MINGW32__ 1
#define __MINGW64__ 1
$ cc --version
clang version 15.0.7
Target: aarch64-w64-windows-gnu

In more, it is needed. Without the workaround in osdep.h, compilation 
fails with this message:

qemu/include/exec/exec-all.h:619:5: error: call to 
qemu_build_not_reached_always declared with 'error' attribute: code path 
is reachable
    qemu_build_not_reached();
    ^
qemu/include/qemu/osdep.h:242:35: note: expanded from macro 
'qemu_build_not_reached'
#define qemu_build_not_reached()  qemu_build_not_reached_always()
                                  ^
1 error generated.

> r~