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Date:   Wed, 27 Dec 2017 09:31:07 +0100
From:   Waldemar Brodkorb <wbx@openadk.org>
To:     James Hogan <jhogan@kernel.org>
Cc:     Ralf Baechle <ralf@linux-mips.org>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
        "Maciej W . Rozycki" <macro@mips.com>,
        Matthew Fortune <matthew.fortune@mips.com>,
        Florian Fainelli <florian@openwrt.org>,
        Waldemar Brodkorb <wbx@openadk.org>, linux-mips@linux-mips.org
Subject: Re: [PATCH] MIPS: Implement __multi3 for GCC7 MIPS64r6 builds
Message-ID: <20171227083107.GI27558@waldemar-brodkorb.de>
References: <20171206085034.3869dc9d@windsurf.lan>
 <20171207072046.31125-1-jhogan@kernel.org>
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Hi James,
James Hogan wrote,

> GCC7 is a bit too eager to generate suboptimal __multi3 calls (128bit
> multiply with 128bit result) for MIPS64r6 builds, even in code which
> doesn't explicitly use 128bit types, such as the following:
> 
> unsigned long func(unsigned long a, unsigned long b)
> {
> 	return a > (~0UL) / b;
> }
> 
> Which GCC rearanges to:
> 
> return (unsigned __int128)a * (unsigned __int128)b > 0xffffffff;
> 
> Therefore implement __multi3, but only for MIPS64r6 with GCC7 as under
> normal circumstances we wouldn't expect any calls to __multi3 to be
> generated from kernel code.

I tested the patch and it works fine for me. I can build a mips64r6
kernel and run the uClibc-ng testsuite inside qemu system emulation.

It works for me with 4.9.x and 4.14.x kernels.

Thanks
 Waldemar