(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? There might be a MIPS64r6 issue, since r6 changed from DMULTU writing the lo and hi registers to DMULU/DMUHU, and gcc 8.3, at least, doesn't know how to generate inline code for the latter. (Note that users *also* check __INT128__, which is defined if GCC claims to support __int128, so you don't have to worry about 32-bit compiles or ancient compilers. It only has to be conditional on *broken* support.) FWIW, the code I'm working on has this inner loop: (https://arxiv.org/abs/1805.10941 for details) 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, like the following hypothetical MIPS R6 code: .L7: balc get_random_u64 dmulu $3, $2, $17 sltu $3, $3, $16 bnezc $3, .L7 dmuhu $2, $2, $17 Or this handwritten Alpha code: 1: bsr $26, get_random_u64 mulq $0, $9, $1 # $9 is range cmpult $1, $10, $1 # $10 is lim bne $1, 1b umulh $0, $9, $0
