about summary refs log tree commit diff
path: root/REORG.TODO/sysdeps/i386/strrchr.S
diff options
context:
space:
mode:
Diffstat (limited to 'REORG.TODO/sysdeps/i386/strrchr.S')
-rw-r--r--REORG.TODO/sysdeps/i386/strrchr.S334
1 files changed, 334 insertions, 0 deletions
diff --git a/REORG.TODO/sysdeps/i386/strrchr.S b/REORG.TODO/sysdeps/i386/strrchr.S
new file mode 100644
index 0000000000..95b304dc0b
--- /dev/null
+++ b/REORG.TODO/sysdeps/i386/strrchr.S
@@ -0,0 +1,334 @@
+/* strrchr (str, ch) -- Return pointer to last occurrence of CH in STR.
+   For Intel 80x86, x>=3.
+   Copyright (C) 1994-2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>
+   Some optimisations by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au>
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <sysdep.h>
+#include "asm-syntax.h"
+
+#define PARMS	4+8	/* space for 2 saved regs */
+#define RTN	PARMS
+#define STR	RTN
+#define CHR	STR+4
+
+	.text
+ENTRY (strrchr)
+
+	pushl %edi		/* Save callee-safe registers used here.  */
+	cfi_adjust_cfa_offset (4)
+	cfi_rel_offset (edi, 0)
+	pushl %esi
+	cfi_adjust_cfa_offset (4)
+
+	xorl %eax, %eax
+	movl STR(%esp), %esi
+	cfi_rel_offset (esi, 0)
+	movl CHR(%esp), %ecx
+
+	/* At the moment %ecx contains C.  What we need for the
+	   algorithm is C in all bytes of the dword.  Avoid
+	   operations on 16 bit words because these require an
+	   prefix byte (and one more cycle).  */
+	movb %cl, %ch		/* now it is 0|0|c|c */
+	movl %ecx, %edx
+	shll $16, %ecx		/* now it is c|c|0|0 */
+	movw %dx, %cx		/* and finally c|c|c|c */
+
+	/* Before we start with the main loop we process single bytes
+	   until the source pointer is aligned.  This has two reasons:
+	   1. aligned 32-bit memory access is faster
+	   and (more important)
+	   2. we process in the main loop 32 bit in one step although
+	      we don't know the end of the string.  But accessing at
+	      4-byte alignment guarantees that we never access illegal
+	      memory if this would not also be done by the trivial
+	      implementation (this is because all processor inherent
+	      boundaries are multiples of 4.  */
+
+	testl $3, %esi		/* correctly aligned ? */
+	jz L(19)		/* yes => begin loop */
+	movb (%esi), %dl	/* load byte in question (we need it twice) */
+	cmpb %dl, %cl		/* compare byte */
+	jne L(11)			/* target found => return */
+	movl %esi, %eax		/* remember pointer as possible result */
+L(11):	orb %dl, %dl		/* is NUL? */
+	jz L(2)			/* yes => return NULL */
+	incl %esi		/* increment pointer */
+
+	testl $3, %esi		/* correctly aligned ? */
+	jz L(19)		/* yes => begin loop */
+	movb (%esi), %dl	/* load byte in question (we need it twice) */
+	cmpb %dl, %cl		/* compare byte */
+	jne L(12)			/* target found => return */
+	movl %esi, %eax		/* remember pointer as result */
+L(12):	orb %dl, %dl		/* is NUL? */
+	jz L(2)			/* yes => return NULL */
+	incl %esi		/* increment pointer */
+
+	testl $3, %esi		/* correctly aligned ? */
+	jz L(19)		/* yes => begin loop */
+	movb (%esi), %dl	/* load byte in question (we need it twice) */
+	cmpb %dl, %cl		/* compare byte */
+	jne L(13)			/* target found => return */
+	movl %esi, %eax		/* remember pointer as result */
+L(13):	orb %dl, %dl		/* is NUL? */
+	jz L(2)			/* yes => return NULL */
+	incl %esi		/* increment pointer */
+
+	/* No we have reached alignment.  */
+	jmp L(19)		/* begin loop */
+
+      /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
+	 change any of the hole bits of LONGWORD.
+
+	 1) Is this safe?  Will it catch all the zero bytes?
+	 Suppose there is a byte with all zeros.  Any carry bits
+	 propagating from its left will fall into the hole at its
+	 least significant bit and stop.  Since there will be no
+	 carry from its most significant bit, the LSB of the
+	 byte to the left will be unchanged, and the zero will be
+	 detected.
+
+	 2) Is this worthwhile?  Will it ignore everything except
+	 zero bytes?  Suppose every byte of LONGWORD has a bit set
+	 somewhere.  There will be a carry into bit 8.	If bit 8
+	 is set, this will carry into bit 16.  If bit 8 is clear,
+	 one of bits 9-15 must be set, so there will be a carry
+	 into bit 16.  Similarly, there will be a carry into bit
+	 24.  If one of bits 24-31 is set, there will be a carry
+	 into bit 32 (=carry flag), so all of the hole bits will
+	 be changed.
+
+	 3) But wait!  Aren't we looking for C, not zero?
+	 Good point.  So what we do is XOR LONGWORD with a longword,
+	 each of whose bytes is C.  This turns each byte that is C
+	 into a zero.  */
+
+	/* Each round the main loop processes 16 bytes.  */
+
+	/* Jump to here when the character is detected.  We chose this
+	   way around because the character one is looking for is not
+	   as frequent as the rest and taking a conditional jump is more
+	   expensive than ignoring it.
+
+	   Some more words to the code below: it might not be obvious why
+	   we decrement the source pointer here.  In the loop the pointer
+	   is not pre-incremented and so it still points before the word
+	   we are looking at.  But you should take a look at the instruction
+	   which gets executed before we get into the loop: `addl $16, %esi'.
+	   This makes the following subs into adds.  */
+
+	/* These fill bytes make the main loop be correctly aligned.
+	   We cannot use align because it is not the following instruction
+	   which should be aligned.  */
+	.byte 0, 0
+#ifndef	PROF
+	/* Profiling adds some code and so changes the alignment.  */
+	.byte 0
+#endif
+
+L(4):	subl $4, %esi		/* adjust pointer */
+L(41):	subl $4, %esi
+L(42):	subl $4, %esi
+L(43):	testl $0xff000000, %edx	/* is highest byte == C? */
+	jnz L(33)		/* no => try other bytes */
+	leal 15(%esi), %eax	/* store address as result */
+	jmp L(1)		/* and start loop again */
+
+L(3):	subl $4, %esi		/* adjust pointer */
+L(31):	subl $4, %esi
+L(32):	subl $4, %esi
+L(33):	testl $0xff0000, %edx	/* is C in third byte? */
+	jnz L(51)		/* no => try other bytes */
+	leal 14(%esi), %eax	/* store address as result */
+	jmp L(1)		/* and start loop again */
+
+L(51):
+	/* At this point we know that the byte is in one of the lower bytes.
+	   We make a guess and correct it if necessary.  This reduces the
+	   number of necessary jumps.  */
+	leal 12(%esi), %eax	/* guess address of lowest byte as result */
+	testb %dh, %dh		/* is guess correct? */
+	jnz L(1)		/* yes => start loop */
+	leal 13(%esi), %eax	/* correct guess to second byte */
+
+L(1):	addl $16, %esi		/* increment pointer for full round */
+
+L(19):	movl (%esi), %edx	/* get word (= 4 bytes) in question */
+	movl $0xfefefeff, %edi	/* magic value */
+	addl %edx, %edi		/* add the magic value to the word.  We get
+				   carry bits reported for each byte which
+				   is *not* 0 */
+
+	/* According to the algorithm we had to reverse the effect of the
+	   XOR first and then test the overflow bits.  But because the
+	   following XOR would destroy the carry flag and it would (in a
+	   representation with more than 32 bits) not alter then last
+	   overflow, we can now test this condition.  If no carry is signaled
+	   no overflow must have occurred in the last byte => it was 0.	*/
+
+	jnc L(20)			/* found NUL => check last word */
+
+	/* We are only interested in carry bits that change due to the
+	   previous add, so remove original bits */
+	xorl %edx, %edi		/* (word+magic)^word */
+
+	/* Now test for the other three overflow bits.  */
+	orl $0xfefefeff, %edi	/* set all non-carry bits */
+	incl %edi		/* add 1: if one carry bit was *not* set
+				   the addition will not result in 0.  */
+
+	/* If at least one byte of the word is C we don't get 0 in %edi.  */
+	jnz L(20)			/* found NUL => check last word */
+
+	/* Now we made sure the dword does not contain the character we are
+	   looking for.  But because we deal with strings we have to check
+	   for the end of string before testing the next dword.  */
+
+	xorl %ecx, %edx		/* XOR with word c|c|c|c => bytes of str == c
+				   are now 0 */
+	movl $0xfefefeff, %edi	/* magic value */
+	addl %edx, %edi		/* add the magic value to the word.  We get
+				   carry bits reported for each byte which
+				   is *not* 0 */
+	jnc L(4)		/* highest byte is C => examine dword */
+	xorl %edx, %edi		/* ((word^charmask)+magic)^(word^charmask) */
+	orl $0xfefefeff, %edi	/* set all non-carry bits */
+	incl %edi		/* add 1: if one carry bit was *not* set
+				   the addition will not result in 0.  */
+	jnz L(3)		/* C is detected in the word => examine it */
+
+	movl 4(%esi), %edx	/* get word (= 4 bytes) in question */
+	movl $0xfefefeff, %edi	/* magic value */
+	addl %edx, %edi		/* add the magic value to the word.  We get
+				   carry bits reported for each byte which
+				   is *not* 0 */
+	jnc L(21)		/* found NUL => check last word */
+	xorl %edx, %edi		/* (word+magic)^word */
+	orl $0xfefefeff, %edi	/* set all non-carry bits */
+	incl %edi		/* add 1: if one carry bit was *not* set
+				   the addition will not result in 0.  */
+	jnz L(21)		/* found NUL => check last word */
+	xorl %ecx, %edx		/* XOR with word c|c|c|c => bytes of str == c
+				   are now 0 */
+	movl $0xfefefeff, %edi	/* magic value */
+	addl %edx, %edi		/* add the magic value to the word.  We get
+				   carry bits reported for each byte which
+				   is *not* 0 */
+	jnc L(41)		/* highest byte is C => examine dword */
+	xorl %edx, %edi		/* ((word^charmask)+magic)^(word^charmask) */
+	orl $0xfefefeff, %edi	/* set all non-carry bits */
+	incl %edi		/* add 1: if one carry bit was *not* set
+				   the addition will not result in 0.  */
+	jnz L(31)		/* C is detected in the word => examine it */
+
+	movl 8(%esi), %edx	/* get word (= 4 bytes) in question */
+	movl $0xfefefeff, %edi	/* magic value */
+	addl %edx, %edi		/* add the magic value to the word.  We get
+				   carry bits reported for each byte which
+				   is *not* 0 */
+	jnc L(22)		/* found NUL => check last word */
+	xorl %edx, %edi		/* (word+magic)^word */
+	orl $0xfefefeff, %edi	/* set all non-carry bits */
+	incl %edi		/* add 1: if one carry bit was *not* set
+				   the addition will not result in 0.  */
+	jnz L(22)		/* found NUL => check last word */
+	xorl %ecx, %edx		/* XOR with word c|c|c|c => bytes of str == c
+				   are now 0 */
+	movl $0xfefefeff, %edi	/* magic value */
+	addl %edx, %edi		/* add the magic value to the word.  We get
+				   carry bits reported for each byte which
+				   is *not* 0 */
+	jnc L(42)		/* highest byte is C => examine dword */
+	xorl %edx, %edi		/* ((word^charmask)+magic)^(word^charmask) */
+	orl $0xfefefeff, %edi	/* set all non-carry bits */
+	incl %edi		/* add 1: if one carry bit was *not* set
+				   the addition will not result in 0.  */
+	jnz L(32)		/* C is detected in the word => examine it */
+
+	movl 12(%esi), %edx	/* get word (= 4 bytes) in question */
+	movl $0xfefefeff, %edi	/* magic value */
+	addl %edx, %edi		/* add the magic value to the word.  We get
+				   carry bits reported for each byte which
+				   is *not* 0 */
+	jnc L(23)		/* found NUL => check last word */
+	xorl %edx, %edi		/* (word+magic)^word */
+	orl $0xfefefeff, %edi	/* set all non-carry bits */
+	incl %edi		/* add 1: if one carry bit was *not* set
+				   the addition will not result in 0.  */
+	jnz L(23)		/* found NUL => check last word */
+	xorl %ecx, %edx		/* XOR with word c|c|c|c => bytes of str == c
+				   are now 0 */
+	movl $0xfefefeff, %edi	/* magic value */
+	addl %edx, %edi		/* add the magic value to the word.  We get
+				   carry bits reported for each byte which
+				   is *not* 0 */
+	jnc L(43)		/* highest byte is C => examine dword */
+	xorl %edx, %edi		/* ((word^charmask)+magic)^(word^charmask) */
+	orl $0xfefefeff, %edi	/* set all non-carry bits */
+	incl %edi		/* add 1: if one carry bit was *not* set
+				   the addition will not result in 0.  */
+	jz L(1)			/* C is not detected => restart loop */
+	jmp L(33)		/* examine word */
+
+L(23):	addl $4, %esi		/* adjust pointer */
+L(22):	addl $4, %esi
+L(21):	addl $4, %esi
+
+	/* What remains to do is to test which byte the NUL char is and
+	   whether the searched character appears in one of the bytes
+	   before.  A special case is that the searched byte maybe NUL.
+	   In this case a pointer to the terminating NUL char has to be
+	   returned.  */
+
+L(20):	cmpb %cl, %dl		/* is first byte == C? */
+	jne L(24)		/* no => skip */
+	movl %esi, %eax		/* store address as result */
+L(24):	testb %dl, %dl		/* is first byte == NUL? */
+	jz L(2)			/* yes => return */
+
+	cmpb %cl, %dh		/* is second byte == C? */
+	jne L(25)		/* no => skip */
+	leal 1(%esi), %eax	/* store address as result */
+L(25):	testb %dh, %dh		/* is second byte == NUL? */
+	jz L(2)			/* yes => return */
+
+	shrl $16,%edx		/* make upper bytes accessible */
+	cmpb %cl, %dl		/* is third byte == C */
+	jne L(26)		/* no => skip */
+	leal 2(%esi), %eax	/* store address as result */
+L(26):	testb %dl, %dl		/* is third byte == NUL */
+	jz L(2)			/* yes => return */
+
+	cmpb %cl, %dh		/* is fourth byte == C */
+	jne L(2)		/* no => skip */
+	leal 3(%esi), %eax	/* store address as result */
+
+L(2):	popl %esi		/* restore saved register content */
+	cfi_adjust_cfa_offset (-4)
+	cfi_restore (esi)
+	popl %edi
+	cfi_adjust_cfa_offset (-4)
+	cfi_restore (edi)
+
+	ret
+END (strrchr)
+
+weak_alias (strrchr, rindex)
+libc_hidden_builtin_def (strrchr)