Re: [PATCH -next] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

From: "liuyuntao (F)" <liuyuntao12@huawei.com>
To: Arnd Bergmann <arnd@arndb.de>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>
Cc: Russell King <linux@armlinux.org.uk>, Andrew Davis <afd@ti.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Jonathan Corbet <corbet@lwn.net>, Mike Rapoport <rppt@kernel.org>,
	Eric DeVolder <eric.devolder@oracle.com>,
	Rob Herring <robh@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Walleij <linus.walleij@linaro.org>
Subject: Re: [PATCH -next] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
Date: Thu, 22 Feb 2024 19:24:30 +0800	[thread overview]
Message-ID: <4e9396ca-460b-49ca-818e-73f0a8997b15@huawei.com> (raw)
In-Reply-To: <30b01c65-12f2-4ee0-81d5-c7a2da2c36b4@app.fastmail.com>


On 2024/2/21 23:51, Arnd Bergmann wrote:
> I have given your patch some build testing with random
> configurations in my build setup and it seems to work
> fine with gcc/binutils, but unfortunately I came across
> a link failure using clang/lld:
> 
> ld.lld: error: ./arch/arm/kernel/vmlinux.lds:35: ( expected, but got }
>>>>   __vectors_lma = .; OVERLAY 0xffff0000 : AT(__vectors_lma) { .vectors { KEEP(*(.vectors)) } .vectors.bhb.loop8 { KEEP(*(.vectors.bhb.loop8)) } .vectors.bhb.bpiall { KEEP(*(.vectors.bhb.bpiall)) } } __vectors_start = LOADADDR(.vectors); __vectors_end = LOADADDR(.vectors) + SIZEOF(.vectors); __vectors_bhb_loop8_start = LOADADDR(.vectors.bhb.loop8); __vectors_bhb_loop8_end = LOADADDR(.vectors.bhb.loop8) + SIZEOF(.vectors.bhb.loop8); __vectors_bhb_bpiall_start = LOADADDR(.vectors.bhb.bpiall); __vectors_bhb_bpiall_end = LOADADDR(.vectors.bhb.bpiall) + SIZEOF(.vectors.bhb.bpiall); . = __vectors_lma + SIZEOF(.vectors) + SIZEOF(.vectors.bhb.loop8) + SIZEOF(.vectors.bhb.bpiall); __stubs_lma = .; .stubs ADDR(.vectors) + 0x1000 : AT(__stubs_lma) { *(.stubs) } __stubs_start = LOADADDR(.stubs); __stubs_end = LOADADDR(.stubs) + SIZEOF(.stubs); . = __stubs_lma + SIZEOF(.stubs); PROVIDE(vector_fiq_offset = vector_fiq - ADDR(.vectors));
>>>>                                                                                            ^
> 
> I don't immediately see what the problem is here, I hope you
> can address it before you send a v2.
> 
>       Arnd

I reproduced this issue with make LLVM=1 ARCH=arm -j320 > ../make.txt.
Based on the position of the caret, I speculate that the issue arises from
lld's inability to recognize the KEEP() command preceding the OUTPUT
section in the OVERLAY command.
 >
The full syntax of the OVERLAY command is as follows: OVERLAY [start] : 
[NOCROSSREFS] [AT ( ldaddr )] { secname1 { output-section-command 
output-section-command … } [:phdr…] [=fill] secname2 { 
output-section-command output-section-command … } [:phdr…] [=fill] … } 
[>region] [:phdr…] [=fill] [,]
 >
I attempted to modify KEEP(*(.vectors)) to KEEP(*(.vectors))(.vectors),
and received the following error message:
ld.lld: error: ./arch/arm/kernel/vmlinux.lds:36: ( expected, but got } > 
__vectors_lma = .; OVERLAY 0xffff0000 : AT(__vectors_lma) { .vectors { 
KEEP(*(.vectors))(.vectors) } .vectors.bhb.loop8 { 
KEEP(*(.vectors.bhb.loop8)) } .vectors.bhb.bpiall { 
KEEP(*(.vectors.bhb.bpiall)) } } __vectors_start = LOADADDR(.vectors); 
__vectors_end = LOADADDR(.vectors) + SIZEOF(.vectors); 
__vectors_bhb_loop8_start = LOADADDR(.vectors.bhb.loop8); 
__vectors_bhb_loop8_end = LOADADDR(.vectors.bhb.loop8) + 
SIZEOF(.vectors.bhb.loop8); __vectors_bhb_bpiall_start = 
LOADADDR(.vectors.bhb.bpiall); __vectors_bhb_bpiall_end = 
LOADADDR(.vectors.bhb.bpiall) + SIZEOF(.vectors.bhb.bpiall); . = 
__vectors_lma + SIZEOF(.vectors) + SIZEOF(.vectors.bhb.loop8) + 
SIZEOF(.vectors.bhb.bpiall); __stubs_lma = .; .stubs ADDR(.vectors) + 
0x1000 : AT(__stubs_lma) { *(.stubs) } __stubs_start = LOADADDR(.stubs); 
__stubs_end = LOADADDR(.stubs) + SIZEOF(.stubs); . = __stubs_lma + 
SIZEOF(.stubs); PROVIDE(vector_fiq_offset = vector_fiq - 
ADDR(.vectors)); > ^
The position of the caret has been moved below the right brace
of { KEEP(*(.vectors.bhb.loop8)) }, indicating that lld is treating
the entire `KEEP(*(.vectors))` as a file name. This could potentially be 
a bug in lld. Perhaps we can temporarily
enable the DCE option only when option LD_IS_LLD is disabled,
like risc-v:
`select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD`.