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With the fls() functions as defined in arch/powerpc/include/asm/bitops.h
GCC will not optimise the code in case of constant parameter.
This patch replaces __fls() by the builtin function, and modifies
fls() and fls64() to use builtins instead of inline assembly
For non constant calls, the generated code is doing the same:
int testfls(unsigned int x)
{
return fls(x);
}
unsigned long test__fls(unsigned long x)
{
return __fls(x);
}
int testfls64(__u64 x)
{
return fls64(x);
}
On PPC32, before the patch:
00000064 <testfls>:
64: 7c 63 00 34 cntlzw r3,r3
68: 20 63 00 20 subfic r3,r3,32
6c: 4e 80 00 20 blr
00000070 <test__fls>:
70: 7c 63 00 34 cntlzw r3,r3
74: 20 63 00 1f subfic r3,r3,31
78: 4e 80 00 20 blr
0000007c <testfls64>:
7c: 2c 03 00 00 cmpwi r3,0
80: 40 82 00 10 bne 90 <testfls64+0x14>
84: 7c 83 00 34 cntlzw r3,r4
88: 20 63 00 20 subfic r3,r3,32
8c: 4e 80 00 20 blr
90: 7c 63 00 34 cntlzw r3,r3
94: 20 63 00 40 subfic r3,r3,64
98: 4e 80 00 20 blr
On PPC32, after the patch:
00000054 <testfls>:
54: 7c 63 00 34 cntlzw r3,r3
58: 20 63 00 20 subfic r3,r3,32
5c: 4e 80 00 20 blr
00000060 <test__fls>:
60: 7c 63 00 34 cntlzw r3,r3
64: 20 63 00 1f subfic r3,r3,31
68: 4e 80 00 20 blr
0000006c <testfls64>:
6c: 2c 03 00 00 cmpwi r3,0
70: 41 82 00 10 beq 80 <testfls64+0x14>
74: 7c 63 00 34 cntlzw r3,r3
78: 20 63 00 40 subfic r3,r3,64
7c: 4e 80 00 20 blr
80: 7c 83 00 34 cntlzw r3,r4
84: 20 63 00 40 subfic r3,r3,32
88: 4e 80 00 20 blr
On PPC64, before the patch:
00000000000000a0 <.testfls>:
a0: 7c 63 00 34 cntlzw r3,r3
a4: 20 63 00 20 subfic r3,r3,32
a8: 7c 63 07 b4 extsw r3,r3
ac: 4e 80 00 20 blr
00000000000000b0 <.test__fls>:
b0: 7c 63 00 74 cntlzd r3,r3
b4: 20 63 00 3f subfic r3,r3,63
b8: 7c 63 07 b4 extsw r3,r3
bc: 4e 80 00 20 blr
00000000000000c0 <.testfls64>:
c0: 7c 63 00 74 cntlzd r3,r3
c4: 20 63 00 40 subfic r3,r3,64
c8: 7c 63 07 b4 extsw r3,r3
cc: 4e 80 00 20 blr
On PPC64, after the patch:
0000000000000090 <.testfls>:
90: 7c 63 00 34 cntlzw r3,r3
94: 20 63 00 20 subfic r3,r3,32
98: 7c 63 07 b4 extsw r3,r3
9c: 4e 80 00 20 blr
00000000000000a0 <.test__fls>:
a0: 7c 63 00 74 cntlzd r3,r3
a4: 20 63 00 3f subfic r3,r3,63
a8: 4e 80 00 20 blr
ac: 60 00 00 00 nop
00000000000000b0 <.testfls64>:
b0: 7c 63 00 74 cntlzd r3,r3
b4: 20 63 00 40 subfic r3,r3,64
b8: 7c 63 07 b4 extsw r3,r3
bc: 4e 80 00 20 blr
Those builtins have been in GCC since at least 3.4.6 (see
https://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Other-Builtins.html )
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.