Re: [PATCH v3] osdep: Make MIN/MAX evaluate arguments only once

[Richard Henderson <richard.henderson@linaro.org>]

On 6/2/20 7:29 PM, Eric Blake wrote:
> Because:
> 
> #if MIN(...)
> 
> now fails to compile (you can't have { in a preprocessor expression), and:
> 
> #if MIN_CONST(...)
> 
> fails to compile (__builtin_constant_p() is not a preprocessor macro, so it
> warns that it is being treated as 0).  The only fix is to move the MIN() out of
> the #if and into the #define.

Ah, right.  Thanks.

>> Is it possible to use qemu_build_not_reached?
> 
> Possibly.
> 
> /me goes and recompiles; touching osdep.h recompiles the world...
> 
> No, it blows up hard, because qemu_build_not_reached() is not embeddable in an
> expression:

Ah, right, because without -O, qemu_build_not_reached expands to
g_assert_not_reached and not to a symbol marked with QEMU_ERROR.

>> I'd prefer we generate a compile-time error than a runtime trap (or nothing,
>> depending on compiler flags controlling __builtin_unreachable).
> 
> What we have DOES produce a compile-time error.  If either expression to
> MIN_CONST() is not actually const, the fact that __builtin_unreachable()
> returns void causes a compilation failure because a value is expected.

Ah!  Well, that's good and certainly sufficient for my needs.

I do now wonder if it wouldn't be clearer to use "(void)0"
instead of __builtin_unreachable, and add a note to the comment just above.


r~