Guillaume Melquiond wrote:
Le samedi 08 septembre 2007 ? 23:10 -0400, Daryle Walker a ?crit :
Maybe we should add a header to our detail (and/or pending) folder encapsulating the full versions of built-in integer arithmetic operations, that most(?) processors have, but neither C nor C++ let us access. I'm talking about functions like:
std::pair<unsigned, bool> full_add( unsigned a1, unsigned a2 );
The processor has a "carry" flag whenever an integer addition overflows, but we can't access it (without assembly-language hacks).
Is it really worth it? Nowadays, compilers are smart enough. They generate better code when details are not obfuscated by hand-crafted assembly code. I tested the following code with GCC:
#include <algorithm>
std::pair<unsigned, bool> full_add(unsigned a, unsigned b) { return std::make_pair(a + b, a + b < a); }
bool no_overflow(unsigned a, unsigned b) { return !full_add(a, b).second; }
Here's what I get on ARM: _Z8full_addjj: adds r2, r1, r2 movcc r1, #0 movcs r1, #1 str r2, [r0, #0] strb r1, [r0, #4] mov pc, lr _Z11no_overflowjj: cmn r0, r1 movcs r0, #0 movcc r0, #1 mov pc, lr which looks pretty good, and probably optimal. But I have encountered cases where compilers don't (can't?) do a very good job; for example, I don't think I've seen a way to get optimal code for 128-bit addition. Even 64-bit arithmetic is not so great on 32-bit processors, with library functions used rather than inline adds/adc sequences. But maybe this is better fixed by improving the compiler. Daryle, what application did you have in mind? Phil.