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authorAlan Modra <amodra@gmail.com>2013-08-17 18:40:11 +0930
committerAlan Modra <amodra@gmail.com>2013-10-04 10:39:32 +0930
commitdb9b4570c5dc550074140ac1d1677077fba29a26 (patch)
treec6469b8ce6b7ec28cc9f7c27484e67d351ce3349 /sysdeps/powerpc/powerpc64/strlen.S
parentf7c399cff5bd04ee9dc117fb6b0f39597dc047c6 (diff)
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PowerPC LE strlen
http://sourceware.org/ml/libc-alpha/2013-08/msg00097.html This is the first of nine patches adding little-endian support to the existing optimised string and memory functions. I did spend some time with a power7 simulator looking at cycle by cycle behaviour for memchr, but most of these patches have not been run on cpu simulators to check that we are going as fast as possible. I'm sure PowerPC can do better. However, the little-endian support mostly leaves main loops unchanged, so I'm banking on previous authors having done a good job on big-endian.. As with most code you stare at long enough, I found some improvements for big-endian too. Little-endian support for strlen. Like most of the string functions, I leave the main word or multiple-word loops substantially unchanged, just needing to modify the tail. Removing the branch in the power7 functions is just a tidy. .align produces a branch anyway. Modifying regs in the non-power7 functions is to suit the new little-endian tail. * sysdeps/powerpc/powerpc64/power7/strlen.S (strlen): Add little-endian support. Don't branch over align. * sysdeps/powerpc/powerpc32/power7/strlen.S: Likewise. * sysdeps/powerpc/powerpc64/strlen.S (strlen): Add little-endian support. Rearrange tmp reg use to suit. Comment. * sysdeps/powerpc/powerpc32/strlen.S: Likewise.
Diffstat (limited to 'sysdeps/powerpc/powerpc64/strlen.S')
-rw-r--r--sysdeps/powerpc/powerpc64/strlen.S75
1 files changed, 55 insertions, 20 deletions
diff --git a/sysdeps/powerpc/powerpc64/strlen.S b/sysdeps/powerpc/powerpc64/strlen.S
index 0f9b5eea9f..4ed1ba3ad1 100644
--- a/sysdeps/powerpc/powerpc64/strlen.S
+++ b/sysdeps/powerpc/powerpc64/strlen.S
@@ -29,7 +29,12 @@
1 is subtracted you get a value in the range 0x00-0x7f, none of which
have their high bit set. The expression here is
(x + 0xfefefeff) & ~(x | 0x7f7f7f7f), which gives 0x00000000 when
- there were no 0x00 bytes in the word.
+ there were no 0x00 bytes in the word. You get 0x80 in bytes that
+ match, but possibly false 0x80 matches in the next more significant
+ byte to a true match due to carries. For little-endian this is
+ of no consequence since the least significant match is the one
+ we're interested in, but big-endian needs method 2 to find which
+ byte matches.
2) Given a word 'x', we can test to see _which_ byte was zero by
calculating ~(((x & 0x7f7f7f7f) + 0x7f7f7f7f) | x | 0x7f7f7f7f).
@@ -62,7 +67,7 @@
Answer:
1) Added a Data Cache Block Touch early to prefetch the first 128
byte cache line. Adding dcbt instructions to the loop would not be
- effective since most strings will be shorter than the cache line.*/
+ effective since most strings will be shorter than the cache line. */
/* Some notes on register usage: Under the SVR4 ABI, we can use registers
0 and 3 through 12 (so long as we don't call any procedures) without
@@ -78,7 +83,7 @@
ENTRY (strlen)
CALL_MCOUNT 1
-#define rTMP1 r0
+#define rTMP4 r0
#define rRTN r3 /* incoming STR arg, outgoing result */
#define rSTR r4 /* current string position */
#define rPADN r5 /* number of padding bits we prepend to the
@@ -88,9 +93,9 @@ ENTRY (strlen)
#define rWORD1 r8 /* current string doubleword */
#define rWORD2 r9 /* next string doubleword */
#define rMASK r9 /* mask for first string doubleword */
-#define rTMP2 r10
-#define rTMP3 r11
-#define rTMP4 r12
+#define rTMP1 r10
+#define rTMP2 r11
+#define rTMP3 r12
dcbt 0,rRTN
clrrdi rSTR, rRTN, 3
@@ -100,30 +105,36 @@ ENTRY (strlen)
addi r7F7F, r7F7F, 0x7f7f
li rMASK, -1
insrdi r7F7F, r7F7F, 32, 0
-/* That's the setup done, now do the first pair of doublewords.
- We make an exception and use method (2) on the first two doublewords,
- to reduce overhead. */
+/* We use method (2) on the first two doublewords, because rFEFE isn't
+ required which reduces setup overhead. Also gives a faster return
+ for small strings on big-endian due to needing to recalculate with
+ method (2) anyway. */
+#ifdef __LITTLE_ENDIAN__
+ sld rMASK, rMASK, rPADN
+#else
srd rMASK, rMASK, rPADN
+#endif
and rTMP1, r7F7F, rWORD1
or rTMP2, r7F7F, rWORD1
lis rFEFE, -0x101
add rTMP1, rTMP1, r7F7F
addi rFEFE, rFEFE, -0x101
- nor rTMP1, rTMP2, rTMP1
- and. rWORD1, rTMP1, rMASK
+ nor rTMP3, rTMP2, rTMP1
+ and. rTMP3, rTMP3, rMASK
mtcrf 0x01, rRTN
bne L(done0)
- sldi rTMP1, rFEFE, 32
- add rFEFE, rFEFE, rTMP1
+ sldi rTMP1, rFEFE, 32
+ add rFEFE, rFEFE, rTMP1
/* Are we now aligned to a doubleword boundary? */
bt 28, L(loop)
/* Handle second doubleword of pair. */
+/* Perhaps use method (1) here for little-endian, saving one instruction? */
ldu rWORD1, 8(rSTR)
and rTMP1, r7F7F, rWORD1
or rTMP2, r7F7F, rWORD1
add rTMP1, rTMP1, r7F7F
- nor. rWORD1, rTMP2, rTMP1
+ nor. rTMP3, rTMP2, rTMP1
bne L(done0)
/* The loop. */
@@ -137,28 +148,52 @@ L(loop):
add rTMP3, rFEFE, rWORD2
nor rTMP4, r7F7F, rWORD2
bne L(done1)
- and. rTMP1, rTMP3, rTMP4
+ and. rTMP3, rTMP3, rTMP4
beq L(loop)
+#ifndef __LITTLE_ENDIAN__
and rTMP1, r7F7F, rWORD2
add rTMP1, rTMP1, r7F7F
- andc rWORD1, rTMP4, rTMP1
+ andc rTMP3, rTMP4, rTMP1
b L(done0)
L(done1):
and rTMP1, r7F7F, rWORD1
subi rSTR, rSTR, 8
add rTMP1, rTMP1, r7F7F
- andc rWORD1, rTMP2, rTMP1
+ andc rTMP3, rTMP2, rTMP1
/* When we get to here, rSTR points to the first doubleword in the string that
- contains a zero byte, and the most significant set bit in rWORD1 is in that
- byte. */
+ contains a zero byte, and rTMP3 has 0x80 for bytes that are zero, and 0x00
+ otherwise. */
L(done0):
- cntlzd rTMP3, rWORD1
+ cntlzd rTMP3, rTMP3
subf rTMP1, rRTN, rSTR
srdi rTMP3, rTMP3, 3
add rRTN, rTMP1, rTMP3
blr
+#else
+
+L(done0):
+ addi rTMP1, rTMP3, -1 /* Form a mask from trailing zeros. */
+ andc rTMP1, rTMP1, rTMP3
+ cntlzd rTMP1, rTMP1 /* Count bits not in the mask. */
+ subf rTMP3, rRTN, rSTR
+ subfic rTMP1, rTMP1, 64-7
+ srdi rTMP1, rTMP1, 3
+ add rRTN, rTMP1, rTMP3
+ blr
+
+L(done1):
+ addi rTMP3, rTMP1, -1
+ andc rTMP3, rTMP3, rTMP1
+ cntlzd rTMP3, rTMP3
+ subf rTMP1, rRTN, rSTR
+ subfic rTMP3, rTMP3, 64-7-64
+ sradi rTMP3, rTMP3, 3
+ add rRTN, rTMP1, rTMP3
+ blr
+#endif
+
END (strlen)
libc_hidden_builtin_def (strlen)