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PowerPC: Add comments to optimized strncpy

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* sysdeps/powerpc/powerpc64/power8/strncpy.S: Added comments to some
	assembly instructions.
This commit is contained in:
Gabriel F. T. Gomes 2015-09-15 10:51:08 -03:00 committed by Tulio Magno Quites Machado Filho
parent 850713336e
commit b0f81637d5
2 changed files with 43 additions and 2 deletions
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5
ChangeLog
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@ -1,3 +1,8 @@
2015-10-01 Gabriel F. T. Gomes <gftg@linux.vnet.ibm.com>
* sysdeps/powerpc/powerpc64/power8/strncpy.S: Added comments to some
assembly instructions.
2015-10-01 Gabriel F. T. Gomes <gftg@linux.vnet.ibm.com>
* sysdeps/powerpc/powerpc64/power8/strncpy.S: Remove or add register

40
sysdeps/powerpc/powerpc64/power8/strncpy.S
View file

@ -82,44 +82,80 @@ EALIGN (FUNC_NAME, 4, 0)
L(short_path):
mr r9,r3
L(short_path_1):
/* Return if there are no more bytes to be written. */
cmpdi cr7,r5,0
beq cr7,L(short_path_loop_end_1)
L(short_path_2):
/* Copy one char from src (r4) and write it to dest (r9). If it is the
end-of-string, start the null padding. Continue, otherwise. */
lbz r10,0(r4)
cmpdi cr7,r10,0
stb r10,0(r9)
beq cr7,L(zero_pad_start_1)
/* If there are no more bytes to be written, return. */
cmpdi cr0,r5,1
addi r8,r9,1
addi r6,r5,-1
beq cr0,L(short_path_loop_end_0)
/* Copy another char from src (r4) to dest (r9). Check again if it is
the end-of-string. If so, start the null padding. */
lbz r10,1(r4)
cmpdi cr7,r10,0
stb r10,1(r9)
beq cr7,L(zero_pad_start_prepare_1)
/* Eagerly decrement r5 by 3, which is the number of bytes already
written, plus one write that will be performed later on. */
addi r10,r5,-3
b L(short_path_loop_1)
.align 4
L(short_path_loop):
/* At this point, the induction variable, r5, as well as the pointers
to dest and src (r9 and r4, respectivelly) have been updated.
Note: The registers r7 and r10 are induction variables derived from
r5. They are used to determine if the total number of writes has
been reached at every other write.
Copy one char from src (r4) and write it to dest (r9). If it is the
end-of-string, start the null padding. Continue, otherwise. */
lbz r8,0(r4)
addi r7,r10,-2
cmpdi cr5,r8,0
stb r8,0(r9)
beq cr5,L(zero_pad_start_1)
beq cr7,L(short_path_loop_end_0)
/* Copy another char from src (r4) to dest (r9). Check again if it is
the end-of-string. If so, start the null padding. */
lbz r8,1(r4)
cmpdi cr7,r8,0
stb r8,1(r9)
beq cr7,L(zero_pad_start)
mr r10,r7
L(short_path_loop_1):
/* This block is reached after two chars have been already written to
dest. Nevertheless, r5 (the induction variable), r9 (the pointer to
dest), and r4 (the pointer to src) have not yet been updated.
At this point:
r5 holds the count of bytes yet to be written plus 2.
r9 points to the last two chars that were already written to dest.
r4 points to the last two chars that were already copied from src.
The algorithm continues by decrementing r5, the induction variable,
so that it reflects the last two writes. The pointers to dest (r9)
and to src (r4) are increment by two, for the same reason.
Note: Register r10 is another induction variable, derived from r5,
which determines if the total number of writes has been reached. */
addic. r5,r5,-2
addi r9,r9,2
cmpdi cr7,r10,0
cmpdi cr7,r10,0 /* Eagerly check if the next write is the last. */
addi r4,r4,2
addi r6,r9,1
bne cr0,L(short_path_loop)
bne cr0,L(short_path_loop) /* Check if the total number of writes
has been reached at every other
write. */
#ifdef USE_AS_STPNCPY
mr r3,r9
b L(short_path_loop_end)