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SubjectRe: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
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On 2024/3/9 16:20, Arnd Bergmann wrote:
> On Sat, Mar 9, 2024, at 07:14, liuyuntao (F) wrote:
>> On 2024/3/8 21:15, Arnd Bergmann wrote:
>>> On Thu, Mar 7, 2024, at 16:12, Yuntao Liu wrote:
>>
>> Thanks for the tests, CONFIG_LD_DEAD_CODE_DATA_ELIMINATION and
>> CONFIG_TRIM_UNUSED_KSYMS do indeed result in a significant improvement.
>> I found that arm32 still doesn't support CONFIG_LTO_CLANG. I've done
>> some work on it, but without success. I'd like to learn more about the
>> CONFIG_LTO_CLANG patch. Do you have any relevant links?
>
> I did not try to get it to boot and gave up when I did not see
> any size improvement. I think there were previous attempts to
> do it elsewhere, which I did not try to find.
>

I tested this patch, the size improvement was only about one
ten-thousandth, and the compilation time had increased by about a quarter,
and the kernel did not boot.

Strangely, LTO has actually increased the compilation time
significantly, which seems contrary to its purpose.

+ +trim +dce +trim+dce
no lto 5995384 5858720 5841024 5299032
lto 5990040 5854544 5839992 5289576
shrink 8.9‱ 7.1‱ 1.7‱ 17.8‱


+ +trim +dce +trim+dce
no lto 34.616 33.03 36.093 32.211
lto 46.881 45.324 47.247 43.246
increase 26.20% 27.10% 23.60% 25.50%



> The patch below makes it build, but it still requires disabling
> CONFIG_THUMB2_KERNEL, which totally defeats the purpose of shrinking
> the kernel as it adds some 40% size overhead in the vmlinux.
> There are probably also runtime bugs that get introduced by this.
>
> Arnd
> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index de78ceb821df..7ebfda4839e8 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -2,6 +2,8 @@
> config ARM
> bool
> default y
> + select ARCH_SUPPORTS_LTO_CLANG
> + select ARCH_SUPPORTS_LTO_CLANG_THIN
> select ARCH_32BIT_OFF_T
> select ARCH_CORRECT_STACKTRACE_ON_KRETPROBE if HAVE_KRETPROBES && FRAME_POINTER && !ARM_UNWIND
> select ARCH_HAS_BINFMT_FLAT
> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> index 726ecabcef09..f2ddce451ab9 100644
> --- a/arch/arm/boot/compressed/Makefile
> +++ b/arch/arm/boot/compressed/Makefile
> @@ -9,6 +9,8 @@ OBJS =
>
> HEAD = head.o
> OBJS += misc.o decompress.o
> +CFLAGS_REMOVE_misc.o += $(CC_FLAGS_LTO)
> +CFLAGS_REMOVE_decompress.o += $(CC_FLAGS_LTO)

Wow, I've encountered this issue before and didn't think to solve it in
this way. You really have a thorough understanding of these parameters.
On a side note, if CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled, but
only a few rodata sections are removed and no functions are eliminated,
are there any compiler or linker options that can control this behavior?
thanks.

> ifeq ($(CONFIG_DEBUG_UNCOMPRESS),y)
> OBJS += debug.o
> AFLAGS_head.o += -DDEBUG
> diff --git a/arch/arm/mm/flush.c b/arch/arm/mm/flush.c
> index d19d140a10c7..aee9e13023a8 100644
> --- a/arch/arm/mm/flush.c
> +++ b/arch/arm/mm/flush.c
> @@ -38,15 +38,14 @@ EXPORT_SYMBOL(arm_heavy_mb);
> static void flush_pfn_alias(unsigned long pfn, unsigned long vaddr)
> {
> unsigned long to = FLUSH_ALIAS_START + (CACHE_COLOUR(vaddr) << PAGE_SHIFT);
> - const int zero = 0;
>
> set_top_pte(to, pfn_pte(pfn, PAGE_KERNEL));
>
> - asm( "mcrr p15, 0, %1, %0, c14\n"
> - " mcr p15, 0, %2, c7, c10, 4"
> + asm("mcrr p15, 0, %1, %0, c14"
> :
> - : "r" (to), "r" (to + PAGE_SIZE - 1), "r" (zero)
> + : "r" (to), "r" (to + PAGE_SIZE - 1)
> : "cc");
> + dsb();
> }
>
> static void flush_icache_alias(unsigned long pfn, unsigned long vaddr, unsigned long len)
> @@ -68,11 +67,11 @@ void flush_cache_mm(struct mm_struct *mm)
> }
>
> if (cache_is_vipt_aliasing()) {
> - asm( "mcr p15, 0, %0, c7, c14, 0\n"
> - " mcr p15, 0, %0, c7, c10, 4"
> + asm("mcr p15, 0, %0, c7, c14, 0"
> :
> : "r" (0)
> : "cc");
> + dsb();
> }
> }
>
> @@ -84,11 +83,11 @@ void flush_cache_range(struct vm_area_struct *vma, unsigned long start, unsigned
> }
>
> if (cache_is_vipt_aliasing()) {
> - asm( "mcr p15, 0, %0, c7, c14, 0\n"
> - " mcr p15, 0, %0, c7, c10, 4"
> + asm("mcr p15, 0, %0, c7, c14, 0"
> :
> : "r" (0)
> : "cc");
> + dsb();
> }
>
> if (vma->vm_flags & VM_EXEC)

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