pavel wrote:
i think that ideally a feature like 'add_with_carry()' should be available as a compiler intrinsic
I had a look at this for the XInt review, when I wrote some ARM assembler here: http://article.gmane.org/gmane.comp.lib.boost.devel/215565 The relevant code is this snippet to do a 96-bit add: "adds %0, %3, %6\n" // Add and set carry flag "adcs %1, %4, %7\n" // Add with carry-in and set carry flag "adc %2, %5, %8\n" // Add with carry-in. The difficulty is that there is only one carry flag and that it is implicit. So in a compiler intrinsic for add-with-carry you need to either: - Keep the carry implicit, and somehow rely on the compiler not inserting any instructions that change it in the generated instruction stream (which seems impractical), or - Make it explicit, but since there is only one place that it can be stored, the compiler will have a challenge to generate code, or - Transfer it to and from a general-purpose register, which will greatly reduce the speed (back to what you can get without assembler or intrinsics). I came to the conclusion that it is better to write multi-word addition code (like the above) in assembler for each platform. I believe that the issues are similar on other architectures that have a flag register, but maybe others can confirm or deny that. Any thoughts anyone? Phil.