Re: [PATCH v2 0/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

Nick Desaulniers
2023-06-23T17:18:10Z

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On Thu, Jun 22, 2023 at 11:18:03PM +0000, Nathan Chancellor wrote:
> If you wanted to restrict it to just LD_IS_BFD in arch/riscv/Kconfig,
> that would be fine with me too.
>
> select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if LD_IS_BFD

Hi Jisheng, would you mind sending a v3 with the attached patch applied
on top / at the end of your series?

>
> Nick said he would work on a report for the LLVM side, so as long as
> this issue is handled in some way to avoid regressing LLD builds until
> it is resolved, I don't think there is anything else for the kernel to
> do. We like to have breadcrumbs via issue links, not sure if the report
> will be internal to Google or on LLVM's issue tracker though;
> regardless, we will have to touch this block to add a version check
> later, at which point we can add a link to the fix in LLD.

https://github.com/ClangBuiltLinux/linux/issues/1881

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From 3e5e010958ee41b9fb408cfade8fb017c2fe7169 Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers@google.com>
Date: Fri, 23 Jun 2023 10:06:17 -0700
Subject: [PATCH] riscv: disable HAVE_LD_DEAD_CODE_DATA_ELIMINATION for LLD

Linking allyesconfig with ld.lld-17 with CONFIG_DEAD_CODE_ELIMINATION=y
takes hours. Assuming this is a performance regression that can be
fixed, tentatively disable this for now so that allyesconfig builds
don't start timing out. If and when there's a fix to ld.lld, this can
be converted to a version check instead so that users of older but still
supported versions of ld.lld don't hurt themselves by enabling
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y.

Link: https://github.com/ClangBuiltLinux/linux/issues/1881
Reported-by: Palmer Dabbelt <palmer@dabbelt.com>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Hi Jisheng, would you mind sending a v3 with this patch on top/at the
end of your patch series?

arch/riscv/Kconfig | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 8effe5bb7788..0573991e9b78 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -116,7 +116,8 @@ config RISCV
select HAVE_KPROBES if !XIP_KERNEL
select HAVE_KPROBES_ON_FTRACE if !XIP_KERNEL
select HAVE_KRETPROBES if !XIP_KERNEL
- select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
+ # https://github.com/ClangBuiltLinux/linux/issues/1881
+ select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD
select HAVE_MOVE_PMD
select HAVE_MOVE_PUD
select HAVE_PCI
--
2.41.0.162.gfafddb0af9-goog

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