[PATCH v2 0/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Conor Dooley]

On Wed, Jun 21, 2023 at 05:42:08PM +0100, Conor Dooley wrote:
> On Wed, Jun 21, 2023 at 07:53:59AM -0700, Palmer Dabbelt wrote:
> > On Tue, 20 Jun 2023 17:13:17 PDT (-0700), Palmer Dabbelt wrote:
> > > On Tue, 20 Jun 2023 14:08:33 PDT (-0700), Palmer Dabbelt wrote:
> > >> On Tue, 20 Jun 2023 13:47:07 PDT (-0700), ndesaulniers at google.com wrote:
> > >>> On Tue, Jun 20, 2023 at 4:41 PM Palmer Dabbelt <palmer at dabbelt.com> wrote:
> > >>>>
> > >>>> On Tue, 20 Jun 2023 13:32:32 PDT (-0700), ndesaulniers at google.com wrote:
> > >>>> > On Tue, Jun 20, 2023 at 4:13 PM Conor Dooley <conor at kernel.org> wrote:
> > >>>> >>
> > >>>> >> On Tue, Jun 20, 2023 at 04:05:55PM -0400, Nick Desaulniers wrote:
> > >>>> >> > On Mon, Jun 19, 2023 at 6:06 PM Palmer Dabbelt <palmer at dabbelt.com> wrote:
> > >>>> >> > > On Thu, 15 Jun 2023 06:54:33 PDT (-0700), Palmer Dabbelt wrote:
> > >>>> >> > > > On Wed, 14 Jun 2023 09:25:49 PDT (-0700), jszhang at kernel.org wrote:
> > >>>> >> > > >> On Wed, Jun 14, 2023 at 07:49:17AM -0700, Palmer Dabbelt wrote:
> > >>>> >> > > >>> On Tue, 23 May 2023 09:54:58 PDT (-0700), jszhang at kernel.org wrote:
> > >>>> >>
> > >>>> >> > > >> Commit 3b90b09af5be ("riscv: Fix orphan section warnings caused by
> > >>>> >> > > >> kernel/pi") touches vmlinux.lds.S, so to make the merge easy, this
> > >>>> >> > > >> series is based on 6.4-rc2.
> > >>>> >> > > >
> > >>>> >> > > > Thanks.
> > >>>> >> > >
> > >>>> >> > > Sorry to be so slow here, but I think this is causing LLD to hang on
> > >>>> >> > > allmodconfig.  I'm still getting to the bottom of it, there's a few
> > >>>> >> > > other things I have in flight still.
> > >>>> >> >
> > >>>> >> > Confirmed with v3 on mainline (linux-next is pretty red at the moment).
> > >>>> >> > https://lore.kernel.org/linux-riscv/20230517082936.37563-1-falcon@tinylab.org/
> > >>>> >>
> > >>>> >> Just FYI Nick, there's been some concurrent work here from different
> > >>>> >> people working on the same thing & the v3 you linked (from Zhangjin) was
> > >>>> >> superseded by this v2 (from Jisheng).
> > >>>> >
> > >>>> > Ah! I've been testing the deprecated patch set, sorry I just looked on
> > >>>> > lore for "dead code" on riscv-linux and grabbed the first thread,
> > >>>> > without noticing the difference in authors or new version numbers for
> > >>>> > distinct series. ok, nevermind my noise.  I'll follow up with the
> > >>>> > correct patch set, sorry!
> > >>>>
> > >>>> Ya, I hadn't even noticed the v3 because I pretty much only look at
> > >>>> patchwork these days.  Like we talked about in IRC, I'm going to go test
> > >>>> the merge of this one and see what's up -- I've got it staged at
> > >>>> <https://git.kernel.org/pub/scm/linux/kernel/git/palmer/linux.git/commit/?h=for-next&id=1bd2963b21758a773206a1cb67c93e7a8ae8a195>,
> > >>>> though that won't be a stable hash if it's actually broken...
> > >>>
> > >>> Ok, https://lore.kernel.org/linux-riscv/20230523165502.2592-1-jszhang@kernel.org/
> > >>> built for me.  If you're seeing a hang, please let me know what
> > >>> version of LLD you're using and I'll build that tag from source to see
> > >>> if I can reproduce, then bisect if so.
> > >>>
> > >>> $ ARCH=riscv LLVM=1 /usr/bin/time -v make -j128 allmodconfig vmlinux
> > >>> ...
> > >>>         Elapsed (wall clock) time (h:mm:ss or m:ss): 2:35.68
> > >>> ...
> > >>>
> > >>> Tested-by: Nick Desaulniers <ndesaulniers at google.com> # build
> > >>
> > >> OK, it triggered enough of a rebuild that it might take a bit for
> > >> anything to filter out.
> > >
> > > I'm on LLVM 16.0.2
> > >
> > >     $ git describe
> > >     llvmorg-16.0.2
> > >     $ git log | head -n1
> > >     commit 18ddebe1a1a9bde349441631365f0472e9693520
> > >
> > > that seems to hang for me -- or at least run for an hour without
> > > completing, so I assume it's hung.  I'm not wed to 16.0.2, it just
> > > happens to be the last time I bumped the toolchain.  I'm moving to
> > > 16.0.5 to see if that changes anything.
> > 
> > That also takes at least an hour to link.  I tried running on LLVM trunk 
> > from last night
> > 
> >     $ git log | head -n1
> >     commit 5e9173c43a9b97c8614e36d6f754317f731e71e9
> > 
> > and that completed.  Just as a curiosity I tried to re-spin it to see 
> > how long it takes, and it's been running for 23 minutes so far.
> 
> After some misdirection through stupid user error, I have also
> reproduced this for an LLVM=1 build w/ llvmorg-16.0.0
> 
> > 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..

Probably as expected, swapping out LLD for LD linked normally & using
gcc-13.1 + LLD hit the same problems with linking.

Cheers,
Conor.
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