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!