strncpy.s   [plain text]

 * Copyright (c) 2002 Apple Computer, Inc. All rights reserved.
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * and read it before using this
 * file.
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * Please see the License for the specific language governing rights and
 * limitations under the License.
#include <mach/ppc/asm.h>

#include <machine/cpu_capabilities.h>

/* We use mode-independent "g" opcodes such as "srgi".  These expand
 * into word operations when targeting __ppc__, and into doubleword
 * operations when targeting __ppc64__.
#include <architecture/ppc/mode_independent_asm.h>

// *****************
// * S T R N C P Y *
// *****************
// char*	strncpy(const char *dst, const char *src, size_t len);
// We optimize the move by doing it word parallel.  This introduces
// a complication: if we blindly did word load/stores until finding
// a 0, we might get a spurious page fault by touching bytes past it.
// To avoid this, we never do a "lwz" that crosses a page boundary,
// or store unnecessary bytes.
// The test for 0s relies on the following inobvious but very efficient
// word-parallel test:
//		x =  dataWord + 0xFEFEFEFF
//		y = ~dataWord & 0x80808080
//		if (x & y) == 0 then no zero found
// The test maps any non-zero byte to zero, and any zero byte to 0x80,
// with one exception: 0x01 bytes preceeding the first zero are also
// mapped to 0x80.
// This algorithm is doubleword parallel in 64-bit mode.

        .globl EXT(strncpy)

        .align 	5
LEXT(strncpy)                       // char* strncpy(const char *dst, const char *src, size_t len));
        andi.	r0,r4,GPR_BYTES-1   // is source aligned?
#if defined(__ppc__)
        lis		r6,hi16(0xFEFEFEFF)	// start to generate 32-bit magic constants
        lis		r7,hi16(0x80808080)
        ori		r6,r6,lo16(0xFEFEFEFF)
        ori		r7,r7,lo16(0x80808080)
        ld		r6,_COMM_PAGE_MAGIC_FE(0)	// get 0xFEFEFEFE FEFEFEFF from commpage
        ld		r7,_COMM_PAGE_MAGIC_80(0)	// get 0x80808080 80808080 from commpage
        mr		r9,r3				// use r9 for dest ptr (must return r3 intact)
        add		r2,r3,r5			// remember where end of buffer is
        beq		Laligned			// source is aligned
        subfic	r0,r0,GPR_BYTES     // r0 <- #bytes to align source
// Copy min(r0,r5) bytes, until 0-byte.
//		r0 = #bytes we propose to copy (NOTE: must be >0)
//		r2 = ptr to 1st byte not in buffer
//		r4 = source ptr (unaligned)
//		r5 = length remaining in buffer (may be 0)
//		r6 = 0xFEFEFEFF
//		r7 = 0x80808080
//		r9 = dest ptr (unaligned)

        cmpgi	r5,0				// buffer empty? (note: length is unsigned)
        beqlr--						// buffer full but 0 not found
        lbz		r8,0(r4)			// r8 <- next source byte
        subic.	r0,r0,1				// decrement count of bytes to move
        addi	r4,r4,1
        subi	r5,r5,1				// decrement buffer length remaining
        cmpwi	cr1,r8,0			// 0-byte?
        stb		r8,0(r9)			// pack into dest
        addi	r9,r9,1
        beq		cr1,L0found			// byte was 0
        bne		Lbyteloop			// r0!=0, source not yet aligned
// Source is aligned.  Loop over words or doublewords until end of buffer.  Note that
// we have aligned the source, rather than the dest, in order to avoid spurious
// page faults.
//		r2 = ptr to 1st byte not in buffer
//		r4 = source ptr (aligned)
//		r5 = length remaining in buffer
//		r6 = 0xFEFEFEFF
//		r7 = 0x80808080
//		r9 = dest ptr (unaligned)

        srgi.	r8,r5,LOG2_GPR_BYTES// get #words or doublewords in buffer
        addi	r0,r5,1				// if none, compare rest of buffer
        beq--	Lbyteloop			// r8==0, no words
        mtctr	r8					// set up word loop count
        rlwinm	r5,r5,0,GPR_BYTES-1 // mask buffer length down to leftover bytes
        b		Lwordloop
// Move a word or a doubleword at a time, until one of two conditions:
//		- a zero byte is found
//		- end of buffer
// At this point, registers are as follows:
//		r2 = ptr to 1st byte not in buffer
//		r4 = source ptr (aligned)
//		r5 = leftover bytes in buffer (0..GPR_BYTES-1)
//		r6 = 0xFEFEFEFF
//		r7 = 0x80808080
//		r9 = dest ptr (unaligned)
//     ctr = whole words or doublewords left in buffer

        .align	5					// align inner loop, which is 8 words long
        lg		r8,0(r4)			// r8 <- next 4 or 8 source bytes
        addi	r9,r9,GPR_BYTES     // bump dest addr while we wait for data
        addi	r4,r4,GPR_BYTES
        add		r10,r8,r6			// r10 <-  word + 0xFEFEFEFF
        andc	r12,r7,r8			// r12 <- ~word & 0x80808080
        stg		r8,-GPR_BYTES(r9)   // pack word or doubleword into destination
        and.	r11,r10,r12			// r11 <- nonzero iff word has a 0-byte
        bdnzt	eq,Lwordloop		// loop if ctr!=0 and cr0_eq
        addi	r0,r5,1				// if no 0-byte found...
        beq--	Lbyteloop			// ...fill rest of buffer a byte at a time

// Found a 0-byte, point to following byte with r9.
        slgi	r0,r8,7				// move 0x01 false hit bits to 0x80 position
        andc	r11,r11,r0			// mask out false hits
        cntlzg	r0,r11				// find the 0-byte (r0 = 0,8,16, or 24)
        srwi	r0,r0,3				// now r0 = 0, 1, 2, or 3 (0..7 if 64-bit)
        subfic	r0,r0,GPR_BYTES-1   // now r0 = 3, 2, 1, or 0
        sub		r9,r9,r0			// now r9 points one past the 0-byte
// Zero rest of buffer, if any.  We use the commpage bzero() routine.
//		r2 = ptr to 1st byte not in buffer
//		r9 = ptr to 1st byte to zero
// NB: commpage bzero() preserves r10-r12 by contract.

        mflr    r12                 // save return
        mr      r11,r3              // save original dest ptr
        sub		r4,r2,r9			// #bytes to zero (ie, rest of buffer)
        mr      r3,r9               // point to 1st byte to zero
        bla     _COMM_PAGE_BZERO
        mtlr    r12                 // restore our return
        mr      r3,r11              // restore ptr to original dest