Hi! On Fri, Mar 29, 2019 at 01:07:07PM +0000, George Spelvin wrote: > 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. Yup. > I don't have easy access to an Alpha cross-compiler to test, but > as it has UMULH, I suspect it would work, too. https://mirrors.edge.kernel.org/pub/tools/crosstool/ > u64 get_random_u64(void); > u64 get_random_max64(u64 range, u64 lim) > { > unsigned __int128 prod; > do { > prod = (unsigned __int128)get_random_u64() * range; > } while (unlikely((u64)prod < lim)); > return prod >> 64; > } > Which turns into these inner loops: > MIPS: > .L7: > jal get_random_u64 > nop > dmultu $2,$17 > mflo $3 > sltu $4,$3,$16 > bne $4,$0,.L7 > mfhi $2 > > PowerPC: > .L9: > bl get_random_u64 > nop > mulld 9,3,31 > mulhdu 3,3,31 > cmpld 7,30,9 > bgt 7,.L9 > > s/390: > .L13: > brasl %r14,get_random_u64@PLT > lgr %r5,%r2 > mlgr %r4,%r10 > lgr %r2,%r4 > clgr %r11,%r5 > jh .L13 > > I like that the MIPS code leaves the high half of the product in > the hi register until it tests the low half; I wish PowerPC would > similarly move the mulhdu *after* the loop, The MIPS code has the multiplication inside the loop as well, and even the mfhi I think: MIPS has delay slots. GCC treats the int128 as one register until it has expanded to RTL, and it does not do such loop optimisations after that, apparently. File a PR please? https://gcc.gnu.org/bugzilla/ Segher
