Re: [PATCH v4] do not depend on signed integer overflow

[Nicolas Pitre <nico@fluxnic.net>]

On Tue, 5 Oct 2010, Erik Faye-Lund wrote:

> Signed integer overflow is not defined in C, so do not depend on it.
> 
> This fixes a problem with GCC 4.4.0 and -O3 where the optimizer would
> consider "consumed_bytes > consumed_bytes + bytes" as a constant
> expression, and never execute the die()-call.
> 
> Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>

Acked-by: Nicolas Pitre <nico@fluxnic.net>

> ---
>  builtin/index-pack.c     |    2 +-
>  builtin/pack-objects.c   |    2 +-
>  builtin/unpack-objects.c |    2 +-
>  git-compat-util.h        |   12 ++++++++++++
>  4 files changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/builtin/index-pack.c b/builtin/index-pack.c
> index 2e680d7..e243d9d 100644
> --- a/builtin/index-pack.c
> +++ b/builtin/index-pack.c
> @@ -161,7 +161,7 @@ static void use(int bytes)
>  	input_offset += bytes;
>  
>  	/* make sure off_t is sufficiently large not to wrap */
> -	if (consumed_bytes > consumed_bytes + bytes)
> +	if (signed_add_overflows(consumed_bytes, bytes))
>  		die("pack too large for current definition of off_t");
>  	consumed_bytes += bytes;
>  }
> diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
> index 3756cf3..d5a8db1 100644
> --- a/builtin/pack-objects.c
> +++ b/builtin/pack-objects.c
> @@ -431,7 +431,7 @@ static int write_one(struct sha1file *f,
>  	written_list[nr_written++] = &e->idx;
>  
>  	/* make sure off_t is sufficiently large not to wrap */
> -	if (*offset > *offset + size)
> +	if (signed_add_overflows(*offset, size))
>  		die("pack too large for current definition of off_t");
>  	*offset += size;
>  	return 1;
> diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
> index 685566e..f63973c 100644
> --- a/builtin/unpack-objects.c
> +++ b/builtin/unpack-objects.c
> @@ -83,7 +83,7 @@ static void use(int bytes)
>  	offset += bytes;
>  
>  	/* make sure off_t is sufficiently large not to wrap */
> -	if (consumed_bytes > consumed_bytes + bytes)
> +	if (signed_add_overflows(consumed_bytes, bytes))
>  		die("pack too large for current definition of off_t");
>  	consumed_bytes += bytes;
>  }
> diff --git a/git-compat-util.h b/git-compat-util.h
> index 81883e7..2af8d3e 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -28,6 +28,18 @@
>  #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
>  #define bitsizeof(x)  (CHAR_BIT * sizeof(x))
>  
> +#define maximum_signed_value_of_type(a) \
> +    (INTMAX_MAX >> (bitsizeof(intmax_t) - bitsizeof(a)))
> +
> +/*
> + * Signed integer overflow is undefined in C, so here's a helper macro
> + * to detect if the sum of two 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))
> +
>  #ifdef __GNUC__
>  #define TYPEOF(x) (__typeof__(x))
>  #else
> -- 
> 1.7.3.1.51.ge462f.dirty
>