From: Jonathan Nieder <jrnieder@gmail.com>
Date: Mon, 04 Oct 2010 22:33:31 GMT
Subject: Re: [PATCH v2] do not depend on signed integer overflow
Message-ID: <20101004223331.GB9994@burratino>
In-Reply-To: <1286227522-5468-1-git-send-email-kusmabite@gmail.com>

Erik Faye-Lund wrote:

> +++ b/git-compat-util.h
> @@ -28,6 +28,15 @@
>  #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
>  #define bitsizeof(x)  (CHAR_BIT * sizeof(x))
>  
> +/*
> + * Signed integer overflow is undefined in C, so here's a helper macro
> + * to detect if the sum of two integers will overflow. The bitsize to
> + * overflow at is taken from the first parameter, which must be zero
> + * or positive.
> + */
> +#define signed_add_overflows(a, b) \
> +    ((b) > ((INTMAX_MAX >> (bitsizeof(intmax_t) - bitsizeof(a))) - (a)))

Yes, I still like it.  This could be made closer to self-documenting
like so:

#define maximum_signed_value_of_type(a) \
	(INTMAX_MAX >> (bitsizeof(intmax_t) - bitsizeof(a)))

/*
 * Signed overflow is undefined in C, so here's a helper macro
 * to detect if the sum of two signed integers will overflow.
 *
 * Requires: a >= 0, typeof(a) equals typeof(b)
 */
#define signed_add_overflows(a, b) \
	((b) > maximum_signed_value_of_type(a) - (a))