Fork of alistair23 Linux kernel for reMarkable from https://github.com/alistair23/linux
1891ef21d9
fls() and fls64() are using __builtin_ctz() and _builtin_ctzll().
On powerpc, those builtins trivially use ctlzw and ctlzd power
instructions.
Allthough those instructions provide the expected result with
input argument 0, __builtin_ctz() and __builtin_ctzll() are
documented as undefined for value 0.
The easiest fix would be to use fls() and fls64() functions
defined in include/asm-generic/bitops/builtin-fls.h and
include/asm-generic/bitops/fls64.h, but GCC output is not optimal:
00000388 <testfls>:
388: 2c 03 00 00 cmpwi r3,0
38c: 41 82 00 10 beq 39c <testfls+0x14>
390: 7c 63 00 34 cntlzw r3,r3
394: 20 63 00 20 subfic r3,r3,32
398: 4e 80 00 20 blr
39c: 38 60 00 00 li r3,0
3a0: 4e 80 00 20 blr
000003b0 <testfls64>:
3b0: 2c 03 00 00 cmpwi r3,0
3b4: 40 82 00 1c bne 3d0 <testfls64+0x20>
3b8: 2f 84 00 00 cmpwi cr7,r4,0
3bc: 38 60 00 00 li r3,0
3c0: 4d 9e 00 20 beqlr cr7
3c4: 7c 83 00 34 cntlzw r3,r4
3c8: 20 63 00 20 subfic r3,r3,32
3cc: 4e 80 00 20 blr
3d0: 7c 63 00 34 cntlzw r3,r3
3d4: 20 63 00 40 subfic r3,r3,64
3d8: 4e 80 00 20 blr
When the input of fls(x) is a constant, just check x for nullity and
return either 0 or __builtin_clz(x). Otherwise, use cntlzw instruction
directly.
For fls64() on PPC64, do the same but with __builtin_clzll() and
cntlzd instruction. On PPC32, lets take the generic fls64() which
will use our fls(). The result is as expected:
00000388 <testfls>:
388: 7c 63 00 34 cntlzw r3,r3
38c: 20 63 00 20 subfic r3,r3,32
390: 4e 80 00 20 blr
000003a0 <testfls64>:
3a0: 2c 03 00 00 cmpwi r3,0
3a4: 40 82 00 10 bne 3b4 <testfls64+0x14>
3a8: 7c 83 00 34 cntlzw r3,r4
3ac: 20 63 00 20 subfic r3,r3,32
3b0: 4e 80 00 20 blr
3b4: 7c 63 00 34 cntlzw r3,r3
3b8: 20 63 00 40 subfic r3,r3,64
3bc: 4e 80 00 20 blr
Fixes:
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Documentation | ||
LICENSES | ||
arch | ||
block | ||
certs | ||
crypto | ||
drivers | ||
fs | ||
include | ||
init | ||
ipc | ||
kernel | ||
lib | ||
mm | ||
net | ||
samples | ||
scripts | ||
security | ||
sound | ||
tools | ||
usr | ||
virt | ||
.clang-format | ||
.cocciconfig | ||
.get_maintainer.ignore | ||
.gitattributes | ||
.gitignore | ||
.mailmap | ||
COPYING | ||
CREDITS | ||
Kbuild | ||
Kconfig | ||
MAINTAINERS | ||
Makefile | ||
README |
README
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.