On 2017-01-20 18:17:30 +0100, Marc Glisse wrote: > On Fri, 20 Jan 2017, Vincent Lefevre wrote: > > > In order to do a multiple-precision subtraction on 3 words, one > > needs to make the compiler recognize subtraction with borrow to > > get optimal code. For instance, on x86_64, one should get a sub > > and 2 sbb's. But how can one do this in C (with no inline asm)? > > You said no asm, but not "no builtins", so I'd recommend > http://clang.llvm.org/docs/LanguageExtensions.html > https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html > Note that gcc does not model carry-in, you'll probably have more success > with clang for that. But the problem is precisely carry-in. > I assume there are already relevant bugs in bugzilla, maybe you can > add your testcase to one (or file a new one if not). There are bugs, but one is about inline asm and others are on simpler, more specific cases. So, I've reported: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79173 -- Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <https://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
