Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Arnd Bergmann]

On Mon, Mar 11, 2024, at 10:14, Geert Uytterhoeven wrote:
> On Sat, Mar 9, 2024 at 2:24 PM liuyuntao (F) <liuyuntao12@xxxxxxxxxx> wrote:
>> On 2024/3/9 16:20, Arnd Bergmann wrote:
>>
>> 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.
>
> The purpose of LTO is to reduce code size. Doing so requires more
> processing, hence the total build time increases.

I think llvm treats it purely as a performance optimization of
the resulting binary, allowing cross-unit inlining and constant
folding, but I don't think it actually tries or succeeds to make
the output smaller. I do remember seeing size improvements with
LTO using gcc in the past, but this never made it into the
mainline kernel. The last time someone tried to add it was 2022[1],
not sure why there was no follow-up.

     Arnd

[1] https://lore.kernel.org/lkml/20221114114344.18650-1-jirislaby@kernelorg/