On Thu, 19 Jan 2017, Denys Vlasenko wrote:
> A lot of asm-optimized routines in arch/x86/crypto/ keep its
> constants in .data. This is wrong, they should be on .rodata.
>
> Mnay of these constants are the same in different modules.
> For example, 128-bit shuffle mask 0x000102030405060708090A0B0C0D0E0F
> exists in at least half a dozen places.
>
> There is a way to let linker merge them and use just one copy.
> The rules are as follows: mergeable objects of different sizes
> should not share sections. You can't put them all in one .rodata
> section, they will lose "mergeability".
>
> GCC puts its mergeable constants in ".rodata.cstSIZE" sections,
> or ".rodata.cstSIZE.<object_name>" if -fdata-sections is used.
> This patch does the same:
>
> .section .rodata.cst16.SHUF_MASK, "aM", @progbits, 16
>
> It is important that all data in such section consists of
> 16-byte elements, not larger ones, and there are no implicit
> use of one element from another.
>
> When this is not the case, use non-mergeable section:
>
> .section .rodata[.VAR_NAME], "a", @progbits
>
> This reduces .data by ~15 kbytes:
>
> text data bss dec hex filename
> 11097415 2705840 2630712 16433967 fac32f vmlinux-prev.o
> 11112095 2690672 2630712 16433479 fac147 vmlinux.o
And at the same time it increases text size by ~15k. The overall change in
total size is 488 byte reduction. Weird.
Thanks,
tglx
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