c語言if-else的效率比較
阿新 • • 發佈：2019-02-08
閒著沒事測試下if-else的執行效率
測試環境：Mac pro i7 2.3Ghz 。。。編譯器gcc 4.9，程式碼沒有進行優化-O0；
測試程式碼：c程式碼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; }
彙編程式碼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程式碼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; }
彙編程式碼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
看來else執行的效率高一些。。。。