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openrisc is the only architecture using the linux/unaligned/*memmove infrastructure. There is a comment saying that this version is more efficient, but this was added in 2011 before the openrisc gcc port was merged upstream. I checked a couple of files to see what the actual difference is with the mainline gcc (9.4 and 11.1), and found that the generic header seems to produce better code now, regardless of the gcc version. Specifically, the be_memmove leads to allocating a stack slot and copying the data one byte at a time, then reading the whole word from the stack: 00000000 <test_get_unaligned_memmove>: 0: 9c 21 ff f4 l.addi r1,r1,-12 4: d4 01 10 04 l.sw 4(r1),r2 8: 8e 63 00 00 l.lbz r19,0(r3) c: 9c 41 00 0c l.addi r2,r1,12 10: 8e 23 00 01 l.lbz r17,1(r3) 14: db e2 9f f4 l.sb -12(r2),r19 18: db e2 8f f5 l.sb -11(r2),r17 1c: 8e 63 00 02 l.lbz r19,2(r3) 20: 8e 23 00 03 l.lbz r17,3(r3) 24: d4 01 48 08 l.sw 8(r1),r9 28: db e2 9f f6 l.sb -10(r2),r19 2c: db e2 8f f7 l.sb -9(r2),r17 30: 85 62 ff f4 l.lwz r11,-12(r2) 34: 85 21 00 08 l.lwz r9,8(r1) 38: 84 41 00 04 l.lwz r2,4(r1) 3c: 44 00 48 00 l.jr r9 40: 9c 21 00 0c l.addi r1,r1,12 while the be_struct version reads each byte into a register and does a shift to the right position: 00000000 <test_get_unaligned_struct>: 0: 9c 21 ff f8 l.addi r1,r1,-8 4: 8e 63 00 00 l.lbz r19,0(r3) 8: aa 20 00 18 l.ori r17,r0,0x18 c: e2 73 88 08 l.sll r19,r19,r17 10: 8d 63 00 01 l.lbz r11,1(r3) 14: aa 20 00 10 l.ori r17,r0,0x10 18: e1 6b 88 08 l.sll r11,r11,r17 1c: e1 6b 98 04 l.or r11,r11,r19 20: 8e 23 00 02 l.lbz r17,2(r3) 24: aa 60 00 08 l.ori r19,r0,0x8 28: e2 31 98 08 l.sll r17,r17,r19 2c: d4 01 10 00 l.sw 0(r1),r2 30: d4 01 48 04 l.sw 4(r1),r9 34: 9c 41 00 08 l.addi r2,r1,8 38: e2 31 58 04 l.or r17,r17,r11 3c: 8d 63 00 03 l.lbz r11,3(r3) 40: e1 6b 88 04 l.or r11,r11,r17 44: 84 41 00 00 l.lwz r2,0(r1) 48: 85 21 00 04 l.lwz r9,4(r1) 4c: 44 00 48 00 l.jr r9 50: 9c 21 00 08 l.addi r1,r1,8 According to Stafford Horne, the new version should in fact perform better. In the trivial example, the struct version is a few instructions longer, but building a whole kernel shows an overall reduction in code size, presumably because it now has to manage fewer stack slots: text data bss dec hex filename 4792010 181480 82324 5055814 4d2546 vmlinux-unaligned-memmove 4790642 181480 82324 5054446 4d1fee vmlinux-unaligned-struct Remove the memmove version completely and let openrisc use the same code as everyone else, as a simplification. Signed-off-by: Arnd Bergmann <arnd@arndb.de> Acked-by: Stafford Horne <shorne@gmail.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.