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.
解决方法
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
