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

[George Spelvin]

On Sat, 30 Mar 2019 at 09:43:47 +0100, Heiko Carstens wrote:
> 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!

Here's a draft patch, but obviously it should be tested!

>From 6f3cc608c49dba33a38e81232a2fd46e8fa8dbcd Mon Sep 17 00:00:00 2001
From: George Spelvin <lkml at sdf.org>
Date: Sat, 30 Mar 2019 10:27:14 +0000
Subject: [PATCH] s390: Enable CONFIG_ARCH_SUPPORTS_INT128 on 64-bit builds

If a platform supports a 64x64->128-bit widening multiply,
that allows more efficient scaling of 64-bit values in various
parts of the kernel.  GCC advertises __int128 support with the
__INT128__ #define, but we care about efficient inline
support, so this is a separate flag.

For s390, that was added on 24 March 2017 by
https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=246457
which is part of GCC 7.

It also only applies to TARGET_ARCH12, which I am guessing
corresponds to HAVE_MARCH_ZEC12_FEATURES.  clang support is
pure guesswork.

Signed-off-by: George Spelvin <lkml at sdf.org>
---
 arch/s390/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index ed554b09eb3f..43e6dc677f7d 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -372,6 +372,7 @@ endchoice
 
 config 64BIT
 	def_bool y
+	select ARCH_SUPPORTS_INT128 if GCC_VERSION >= 70000 && HAVE_MARCH_ZEC12_FEATURES || CC_IS_CLANG
 
 config COMPAT
 	def_bool y
-- 
2.20.1