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Date:   Wed, 21 Jun 2023 11:19:31 -0700 (PDT)
Subject:     Re: [PATCH v2 0/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
In-Reply-To: <87wmzwn1po.fsf@all.your.base.are.belong.to.us>
CC:     Conor Dooley <conor@kernel.org>, jszhang@kernel.org,
        llvm@lists.linux.dev, Paul Walmsley <paul.walmsley@sifive.com>,
        aou@eecs.berkeley.edu, Arnd Bergmann <arnd@arndb.de>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org
From:   Palmer Dabbelt <palmer@dabbelt.com>
To:     bjorn@kernel.org, ndesaulniers@google.com, nathan@kernel.org
Message-ID: <mhng-1d790a82-44ad-4b9c-bfe4-6303f09b0705@palmer-ri-x1c9a>
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On Wed, 21 Jun 2023 10:51:15 PDT (-0700), bjorn@kernel.org wrote:
> Conor Dooley <conor@kernel.org> writes:
>
> [...]
>
>>> So I'm no longer actually sure there's a hang, just something slow.  
>>> That's even more of a grey area, but I think it's sane to call a 1-hour 
>>> link time a regression -- unless it's expected that this is just very 
>>> slow to link?
>>
>> I dunno, if it was only a thing for allyesconfig, then whatever - but
>> it's gonna significantly increase build times for any large kernels if LLD
>> is this much slower than LD. Regression in my book.
>>
>> I'm gonna go and experiment with mixed toolchain builds, I'll report
>> back..
>
> I took palmer/for-next (1bd2963b2175 ("Merge patch series "riscv: enable
> HAVE_LD_DEAD_CODE_DATA_ELIMINATION"")) for a tuxmake build with llvm-16:
>
>   | ~/src/tuxmake/run -v --wrapper ccache --target-arch riscv \
>   |     --toolchain=llvm-16 --runtime docker --directory . -k \
>   |     allyesconfig
>
> Took forever, but passed after 2.5h.

Thanks.  I just re-ran mine 17/trunk LLD under time (rather that just 
checking top sometimes), it's at 1.5h but even that seems quite long.

I guess this is sort of up to the LLVM folks: if it's expected that DCE 
takes a very long time to link then I'm not opposed to allowing it, but 
if this is probably a bug in LLD then it seems best to turn it off until 
we sort things out over there.

I think maybe Nick or Nathan is the best bet to know?

> CONFIG_CC_VERSION_TEXT="Debian clang version 16.0.6 (++20230610113307+7cbf1a259152-1~exp1~20230610233402.106)"
>
>
> Björn