作业还是memcpy？设置数组成员变量的首选方法是什么？

Question

对于这个例子,我正在使用objective-c,但欢迎来自更广泛的C/C++社区的答案.

@interface BSWidget : NSObject {
    float tre[3];
}
@property(assign) float* tre;

.

- (void)assignToTre:(float*)triplet {
    tre[0] = triplet[0];
    tre[1] = triplet[1];
    tre[2] = triplet[2];
}

.

- (void)copyToTre:(float*)triplet {
    memcpy(tre, triplet, sizeof(tre) );
}

所以在这两种方法之间,并考虑到这些setter函数通常只能处理2,3或4的维数......

对于这种情况,最有效的方法是什么？

gcc一般会将这些减少到相同的基本操作吗？

谢谢.

Answer 1

快速测试似乎表明编译器在优化时会将memcpy调用替换为执行赋值的指令.

反编译以下代码,当未经优化编译并使用-O2时,表明在优化的情况下testMemcpy函数不包含对memcpy的调用.

struct test src = { .a=1, .b='x' };

void testMemcpy(void)
{
  struct test *dest = malloc(sizeof(struct test));
  memcpy(dest, &src, sizeof(struct test));
}

void testAssign(void)
{
  struct test *dest = malloc(sizeof(struct test));
  *dest = src;
}

未经优化的testMemcpy,具有预期的memcpy调用

(gdb) disassemble testMemcpy 
Dump of assembler code for function testMemcpy:
   0x08048414 <+0>: push   %ebp
   0x08048415 <+1>: mov    %esp,%ebp
   0x08048417 <+3>: sub    $0x28,%esp
   0x0804841a <+6>: movl   $0x8,(%esp)
   0x08048421 <+13>:    call   0x8048350 <malloc@plt>
   0x08048426 <+18>:    mov    %eax,-0xc(%ebp)
   0x08048429 <+21>:    movl   $0x8,0x8(%esp)
   0x08048431 <+29>:    movl   $0x804a018,0x4(%esp)
   0x08048439 <+37>:    mov    -0xc(%ebp),%eax
   0x0804843c <+40>:    mov    %eax,(%esp)
   0x0804843f <+43>:    call   0x8048340 <memcpy@plt>
   0x08048444 <+48>:    leave  
   0x08048445 <+49>:    ret 

优化testAssign

(gdb) disassemble testAssign 
Dump of assembler code for function testAssign:
   0x080483f0 <+0>: push   %ebp
   0x080483f1 <+1>: mov    %esp,%ebp
   0x080483f3 <+3>: sub    $0x18,%esp
   0x080483f6 <+6>: movl   $0x8,(%esp)
   0x080483fd <+13>:    call   0x804831c <malloc@plt>
   0x08048402 <+18>:    mov    0x804a014,%edx
   0x08048408 <+24>:    mov    0x804a018,%ecx
   0x0804840e <+30>:    mov    %edx,(%eax)
   0x08048410 <+32>:    mov    %ecx,0x4(%eax)
   0x08048413 <+35>:    leave  
   0x08048414 <+36>:    ret   

优化的testMemcpy不包含memcpy调用

(gdb) disassemble testMemcpy 
Dump of assembler code for function testMemcpy:
   0x08048420 <+0>: push   %ebp
   0x08048421 <+1>: mov    %esp,%ebp
   0x08048423 <+3>: sub    $0x18,%esp
   0x08048426 <+6>: movl   $0x8,(%esp)
   0x0804842d <+13>:    call   0x804831c <malloc@plt>
   0x08048432 <+18>:    mov    0x804a014,%edx
   0x08048438 <+24>:    mov    0x804a018,%ecx
   0x0804843e <+30>:    mov    %edx,(%eax)
   0x08048440 <+32>:    mov    %ecx,0x4(%eax)
   0x08048443 <+35>:    leave  
   0x08048444 <+36>:    ret