Messages in this thread | | | Date | Mon, 11 Mar 2024 12:41:48 +0100 | From | "Arnd Bergmann" <> | Subject | Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION |
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On Mon, Mar 11, 2024, at 10:14, Geert Uytterhoeven wrote: > On Sat, Mar 9, 2024 at 2:24 PM liuyuntao (F) <liuyuntao12@huawei.com> 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/
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