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References: <20190106013802.3645-1-eblake@redhat.com>
From: Richard Henderson <rth@twiddle.net>
Message-ID: <657598d6-0cea-91dc-a975-f716522a49b9@twiddle.net>
Date: Sun, 6 Jan 2019 18:32:03 +1000
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Subject: Re: [Qemu-devel] [PATCH v2] osdep: Make MIN/MAX evaluate arguments
 only once
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To: Eric Blake <eblake@redhat.com>, qemu-devel@nongnu.org
Cc: dirty.ice.hu@gmail.com, f4bug@amsat.org, Gerd Hoffmann <kraxel@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, Juan Quintela <quintela@redhat.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>

On 1/6/19 11:38 AM, Eric Blake wrote:
> +/*
> + * Automatic type deduction, to be used as:
> + * QEMU_TYPEOF(expr) name = expr;
> + */
> +#if QEMU_GNUC_PREREQ(4, 9)
> +# define QEMU_TYPEOF(a) __auto_type
> +#else
> +# define QEMU_TYPEOF(a) typeof(a)
> +#endif

What's wrong with always using typeof?  This seems like it leaves potential odd
bugs affecting gcc-4.8.

> +#undef MIN
> +#define MIN(a, b)                            \
> +    ({                                       \
> +        QEMU_TYPEOF((a) + 0) _a = (a) + 0;   \
> +        QEMU_TYPEOF((b) + 0) _b = (b) + 0;   \

If you're promoting the type, why don't you want to promote to the common type
between A and B?  E.g.

  __typeof((a) + (b)) _a = (a), _b = (b);

After all, that's what the result type of (p ? _a : _b) will be.


r~