On Fri, Mar 29, 2019 at 01:07:07PM +0000, George Spelvin wrote: > (Cross-posted in case there are generic issues; please trim if > discussion wanders into single-architecture details.) > > I was working on some scaling code that can benefit from 64x64->128-bit > multiplies. GCC supports an __int128 type on processors with hardware > support (including z/Arch and MIPS64), but the support was broken on > early compilers, so it's gated behind CONFIG_ARCH_SUPPORTS_INT128. > > Currently, of the ten 64-bit architectures Linux supports, that's > only enabled on x86, ARM, and RISC-V. > > SPARC and HP-PA don't have support. > > But that leaves Alpha, Mips, PowerPC, and S/390x. > > Current mips64, powerpc64, and s390x gcc seems to generate sensible code > for mul_u64_u64_shr() in <linux/math64.h> if I cross-compile them. > > I don't have easy access to an Alpha cross-compiler to test, but > as it has UMULH, I suspect it would work, too. > > Is there a reason it hasn't been enabled on these platforms? It hasn't been enabled on s390 simply because at least I wasn't aware of this config option. Feel free to send a patch, otherwise I will enable this. Whatever you prefer. Thanks for pointing this out!
