On 26/08/15 14:32, Kostas Savvidis wrote: > I sense there is a consensus that > 1) the 128bit integer is emulated emulated on 64-bit platforms, not > available on 32-bit platforms, and is not native anywhere As far as I know, 128-bit integers are supported natively on the RISC-V architecture, which has a gcc port. I've never used such a device, so I don't know the details - but perhaps it has 128-bit long long's. The point is, there is nothing to stop an architecture having native 128-bit integers. And there is also nothing to stop 32-bit (or smaller) targets supporting __int128. I use gcc on an 8-bit device, and it has full support for 64-bit long long's (emulated by software and library routines, of course). If people find 128-bit integers useful and convenient, then it seems likely that support will be added to more targets - perhaps using SIMD or floating point registers where these are more efficient than general purpose registers. But I'd imagine that they are only useful in a few specialised algorithms (such as in cryptography), and then only if they are noticeably faster than using 64-bit integers. > 2) the long long int is 64-bits everywhere so you can *NEVER* do what > the document seems to suggest one *MIGHT* be able to do — input a > 128-bit constant Again, there is nothing in the C standards saying that a "long long" is limited to 64-bit - only that it is /at least/ 64-bit. Some targets may have longer long long's. > > To me, this would justify rewriting the documentation. The fact that you are asking these questions suggests that the documentation is not as clear as it could be. > > My personal lament is that i still cannot find out anywhere if it is > available on all 64-bit platforms or on intel only. There are a fair number of places where the documentation mentions features that are available or unavailable on some targets, without being explicit. I am sure the gcc developers would be happy with volunteers who can fill in the details :-) David > > KS > >> On Aug 26, 2015, at 3:22 PM, Jonathan Wakely >> <jwakely.gcc@xxxxxxxxx> wrote: >> >> On 26 August 2015 at 12:04, Kostas Savvidis wrote: >>> The online documentation contains the attached passage as part of >>> the "C-Extensions” chapter. There are no actual machines which >>> have an " integer mode wide enough to hold 128 bits” as the >>> document puts it. >> >> It's not talking about machine integers, it's talking about GCC >> integer modes. Several targets support that. >> >>> This would be a harmless confusion if it didn’t go on to say “… >>> long long integer less than 128 bits wide” (???!!!) Whereas in >>> reality "long long int” is 64 bits everywhere i have seen. >> >> >> Read it more carefully, it says you can't express an integer >> constant of type __int128 on such platforms. >> >> So you can't write __int128 i = >> 999999999999999999999999999999999999999999999999999999999999; > >
