学步园

          闲着没事测试下if-else的执行效率

测试环境：Mac pro i7 2.3Ghz 。。。编译器gcc 4.9，代码没有进行优化-O0；

测试代码：c代码1：

int main(){
	int n=100000000;
	int b=-1;
	int a=0;
	while(n-->0){
		if(b<0) b--;else b--;
		if(b<0) b--;else b--;
		if(b<0) b--;else b--;
		if(b<0) b--;else b--;
		if(b<0) b--;else b--;
		if(b<0) b--;else b--;
		if(b<0) b--;else b--;
		if(b<0) b--;else b--;
		if(b<0) b--;else b--;
		if(b<0) b--;else b--;
		
	
	}
	return 0;

}

汇编代码1：

_main:
LFB0:
	pushl	%ebp
LCFI0:
	movl	%esp, %ebp
LCFI1:
	subl	$16, %esp
	movl	$100000000, -4(%ebp)
	movl	$-1, -8(%ebp)
	movl	$0, -12(%ebp)
	jmp	L2
L22:
	cmpl	$0, -8(%ebp)
	jns	L3
	subl	$1, -8(%ebp)
	jmp	L4
L3:
	subl	$1, -8(%ebp)
L4:
	cmpl	$0, -8(%ebp)
	jns	L5
	subl	$1, -8(%ebp)
	jmp	L6
L5:
	subl	$1, -8(%ebp)
L6:
	cmpl	$0, -8(%ebp)
	jns	L7
	subl	$1, -8(%ebp)
	jmp	L8
L7:
	subl	$1, -8(%ebp)
L8:
	cmpl	$0, -8(%ebp)
	jns	L9
	subl	$1, -8(%ebp)
	jmp	L10
L9:
	subl	$1, -8(%ebp)
L10:
	cmpl	$0, -8(%ebp)
	jns	L11
	subl	$1, -8(%ebp)
	jmp	L12
L11:
	subl	$1, -8(%ebp)
L12:
	cmpl	$0, -8(%ebp)
	jns	L13
	subl	$1, -8(%ebp)
	jmp	L14
L13:
	subl	$1, -8(%ebp)
L14:
	cmpl	$0, -8(%ebp)
	jns	L15
	subl	$1, -8(%ebp)
	jmp	L16
L15:
	subl	$1, -8(%ebp)
L16:
	cmpl	$0, -8(%ebp)
	jns	L17
	subl	$1, -8(%ebp)
	jmp	L18
L17:
	subl	$1, -8(%ebp)
L18:
	cmpl	$0, -8(%ebp)
	jns	L19
	subl	$1, -8(%ebp)
	jmp	L20
L19:
	subl	$1, -8(%ebp)
L20:
	cmpl	$0, -8(%ebp)
	jns	L21
	subl	$1, -8(%ebp)
	jmp	L2
L21:
	subl	$1, -8(%ebp)
L2:
	movl	-4(%ebp), %eax
	leal	-1(%eax), %edx
	movl	%edx, -4(%ebp)
	testl	%eax, %eax
	jg	L22
	movl	$0, %eax
	leave

c代码2：

int main(){
	int n=100000000;
	int b=-1;
	int a=0;
	while(n-->0){
		if(b>0) b--;else b--;
		if(b>0) b--;else b--;
		if(b>0) b--;else b--;
		if(b>0) b--;else b--;
		if(b>0) b--;else b--;
		if(b>0) b--;else b--;
		if(b>0) b--;else b--;
		if(b>0) b--;else b--;
		if(b>0) b--;else b--;
		if(b>0) b--;else b--;
	
	}
	return 0;

}

汇编代码2：

_main:
LFB0:
	pushl	%ebp
LCFI0:
	movl	%esp, %ebp
LCFI1:
	subl	$16, %esp
	movl	$100000000, -4(%ebp)
	movl	$-1, -8(%ebp)
	movl	$0, -12(%ebp)
	jmp	L2
L22:
	cmpl	$0, -8(%ebp)
	jle	L3
	subl	$1, -8(%ebp)
	jmp	L4
L3:
	subl	$1, -8(%ebp)
L4:
	cmpl	$0, -8(%ebp)
	jle	L5
	subl	$1, -8(%ebp)
	jmp	L6
L5:
	subl	$1, -8(%ebp)
L6:
	cmpl	$0, -8(%ebp)
	jle	L7
	subl	$1, -8(%ebp)
	jmp	L8
L7:
	subl	$1, -8(%ebp)
L8:
	cmpl	$0, -8(%ebp)
	jle	L9
	subl	$1, -8(%ebp)
	jmp	L10
L9:
	subl	$1, -8(%ebp)
L10:
	cmpl	$0, -8(%ebp)
	jle	L11
	subl	$1, -8(%ebp)
	jmp	L12
L11:
	subl	$1, -8(%ebp)
L12:
	cmpl	$0, -8(%ebp)
	jle	L13
	subl	$1, -8(%ebp)
	jmp	L14
L13:
	subl	$1, -8(%ebp)
L14:
	cmpl	$0, -8(%ebp)
	jle	L15
	subl	$1, -8(%ebp)
	jmp	L16
L15:
	subl	$1, -8(%ebp)
L16:
	cmpl	$0, -8(%ebp)
	jle	L17
	subl	$1, -8(%ebp)
	jmp	L18
L17:
	subl	$1, -8(%ebp)
L18:
	cmpl	$0, -8(%ebp)
	jle	L19
	subl	$1, -8(%ebp)
	jmp	L20
L19:
	subl	$1, -8(%ebp)
L20:
	cmpl	$0, -8(%ebp)
	jle	L21
	subl	$1, -8(%ebp)
	jmp	L2
L21:
	subl	$1, -8(%ebp)
L2:
	movl	-4(%ebp), %eax
	leal	-1(%eax), %edx
	movl	%edx, -4(%ebp)
	testl	%eax, %eax
	jg	L22
	movl	$0, %eax
	leave

执行结果：

看来else执行的效率高一些。。。。