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On Fri, Jan 27, 2012 at 10:40 AM, David Brown<david@westcontrol.com> wrote:On 27/01/2012 10:02, Richard Guenther wrote:
On Thu, Jan 26, 2012 at 4:58 PM, David Brown<david@westcontrol.com> wrote:
On 26/01/2012 12:53, Konstantin Vladimirov wrote:
Hi,
If I know what I am doing, and my code itself guarantees, that there will be no overflows and UB here, can I switch off this signed char to unsigned char expansion in favor of signed char to signed int expansion?
The big question here is why you are using an unqualified "char" for arithmetic in the first place. The signedness of plain "char" varies by target (some default to signed, some to unsigned) and by compiler flags. If you want to write code that uses signed chars, use "signed char". Or even better, use<stdint.h> type "int8_t".
However, as has been pointed out, the problem is that signed arithmetic doesn't wrap - it must be turned into unsigned arithmetic to make it safe. An alternative is to tell gcc that signed arithmetic is 2's compliment and wraps, by using the "-fwrapv" flag or "int8_t char sum_A_B(void) __attribute__((optimize("wrapv")));" on the specific function.
Note that semantic changing optimize attributes do not work reliably.
Richard.
Is there any more information about that somewhere? Certainly I expect there to be issues when trying to turn on and off options like "-fshort-enums" or "-freg-struct-return" - just as you can expect problems linking modules that have been compiled with different flags like that. But others like "-fwrapv", "-fargument-alias", or "-finstrument-functions" can logically be applied to a single function. If there are problems with changing these (with attributes or pragmas), then perhaps they should be disabled, or the potential problems documented in the manual?
Well, the implementation is simply a bit naiive in accepting all options and not all code is prepared to handle functions which differ in optimization options. For example inlining happily will inline a -fwrapv function into a -fno-wrapv function and later assume the inlined copy does have -fno-wrapv semantics. The safe implementation here would have been to disallow any inlining between functions that have any optimize attribute/pragma, but that of course would defeat most of its use.
That said, using the attribute for debugging might be of help, but I would not rely on it in any way.
Richard.
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