memcpy2.s

memcpy函数优化代码,使用汇编实现,可提高memcpy的实现性能.

#define CPU_ARM

#ifdef CPU_ARM 
    .section .icode,"ax",%progbits 
#else 
    .section .icode,"ax",@progbits 
#endif 


    .align 2 
     
     
different_aligns: 
    /* We must do byte copies */ 
    stmfd r13!, {r0, lr } 
byte_loop: 
    ldrb r3, [r1], #1 
    strb r3, [r0], #1 
    subs r2, r2, #1 
    bne byte_loop 
    ldmfd r13!, {r0, pc } 



        .global memcpy2 
        .type memcpy2, %function 
memcpy2: 
@void* memcpy2( void* dst, const void* src, size_t len ) 


@@ Register usage: 
@@ r0: dst 
@@ r1: src 
@@ r2: len 
@@ 
@@ r3: various bitmasks, load and store for different_aligns loop 
@@ r4: counter for multi loop, not used for different_aligns 
@@ r5-r8: load and store, not used for different_aligns 


    cmp r2, #0 
    moveq pc, lr @ just return if caller wants to copy zero bytes 
     
    cmp r2, #8 
    bls different_aligns 
     
    /*check for src alignment*/ 
    eor r3, r0, r1 @ r3 = dest | src 
    tst r3, #3 @ test for same alignment 
    bne different_aligns @ jump if align( r1 ) != align( ro ) 


    /* else, they have the same same alignment */ 
    stmfd r13!, {r0, r4-r8, lr } @ save regs 
    ands r3, r0, #3 @ find out what that alignment is 
    beq multi @ iff align( r1 ) == align( r0 ) == 0 skip to multi 
     
    /* otherwise, move up to three bytes to get to a word alignment 
       if align = 1, we need to move forward 3 bytes to get to a word boundry 
       if align = 2, we need to move forward 2 bytes to get to a word boundry 
       if align = 3, we need to move forward 1 byte to get to a word boundry 
    */ 
    cmp r3, #2 @ "subtract" 2 from either 1, 2, or 3 
    ldrneb r4, [r1], #1 @ one aligned or three aligned ( 1 != 2 || 3 !- 2 ), ((byte*) src)++ 
    strneb r4, [r0], #1 @ one aligned or three aligned ( 1 != 2 || 3 !- 2 ), store to *dst++ 
    @ldrlsh r4, [r1], #2 @ one or two ( 1 <= 2 || 2 <= 2 ) aligned, ((halfword*) src)++ 
    @strlsh r4, [r0], #2 @ one or two ( 1 <= 2 || 2 <= 2 ) aligned, store to *dst++ 
     
    ldrlsb r4, [r1], #1 @ one aligned or two aligned, ((byte*) src)++ 
    strlsb r4, [r0], #1 @ one aligned or two aligned, store to *dst++ 
    ldrlsb r4, [r1], #1 @ one aligned or two aligned, ((byte*) src)++ 
    strlsb r4, [r0], #1 @ one aligned or two aligned, store to *dst++ 
     
    sub r3, r3, #4 
    add r2, r2, r3 @ length -= bytes written 


multi: 
    /* once we get here, we're word aligned */ 
     
    /* 
    bytes = length 
    words = byte / 4, rem = byte moves 
    instr = quadword = words / 4, rem = partial instructions 
    loop = instr / 4, rem = jump to instr 
    
    010101010 
    llliiwwbb 
     
    21 
    52631 
    684268421 
    */ 
  
    ands r3, r2, #48 @ #32 | #16, r3 = partial loop count << 3 
    mov r4, r2, LSR #6 @ r4 = loop count 
     
    /* Now, like Duff's device, jump into the loop to perform the extra instructions */ 
    /* Replace later with direct adjustment of PC */ 
    beq loop_test 
    cmp r3, #32 @r3 = 16 or 32 or 48, corresponding to 1 or 2 or 3 
    bhi loop3 
    beq loop2 
    blo loop1 
     
loop: 
    ldmia r1!, { r5-r8 } @load four registers 
    stmia r0!, { r5-r8 } @store four registers 
loop3: 
    ldmia r1!, { r5-r8 } @load four registers 
    stmia r0!, { r5-r8 } @store four registers 
loop2: 
    ldmia r1!, { r5-r8 } @load four registers 
    stmia r0!, { r5-r8 } @store four registers 
loop1: 
    ldmia r1!, { r5-r8 } @load four registers 
    stmia r0!, { r5-r8 } @store four registers 
     
loop_test: 
    cmp r4, #0 
    subne r4, r4, #1 
    bne loop 
     
    /* Now do the extra words */ 
    ands r3, r2, #12 @ #8 | #4, r3 = extra words << 2 
    beq extra_bytes 
    cmp r3, #8 
    ldrne r5, [r1], #4 @ if r5 !=8 then it's 4 or 12, so load and store 
    strne r5, [r0], #4 
    ldmhsia r1!, {r5-r6} @ if r5 >= 8, it's 8 or 12, so load and store 2 
    stmhsia r0!, {r5-r6} 
     
    /* Now do the extra bytes */ 
extra_bytes: 
    tst r2, #2 @ any extra bytes? 
    beq clean_up 
    cmp r3, #2 @ subtract 2 from either 1, 2, or 3 
    ldrneb r5, [r1], #1 @ load and store one byte iff r3 != 2 (i.e, r3 == 1 || r3 == 3) 
    strneb r5, [r0], #1 @ 1 or 3 
    ldrhsb r5, [r1], #1 @ load and store a byte iff r3 >= 2 
    strhsb r5, [r0], #1 @ 2 or 3 
    ldrhsb r5, [r1], #1 @ load and store a byte iff r3 > 2 
    strhsb r5, [r0], #1 @ 2 or 3 
     
clean_up: 
    ldmfd r13!, {r0, r4-r8, pc } @r0 is the retval, must equal original dst 


end: 
    .size different_aligns, .end-memcpy2 
        .align 2 
     
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