Subject: Re: CONFIG_ARCH_SUPPORTS_INT128: Why not mips, s390, powerpc, and alpha?

From: Segher Boessenkool <[email-masked]>
Date: Fri, 29 Mar 2019 15:25:58 -0500

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