Created
March 5, 2011 20:02
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what code gcc generates for `x/2` and `x*.5`
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*.o |
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/* | |
http://stackoverflow.com/questions/895574/what-are-some-good-code-optimization-methods/1354688#1354688 | |
USAGE: | |
$ gcc -O3 -msse3 -fomit-frame-pointer -c *.c && objdump -Mintel -S *.o | |
*/ | |
int posx(unsigned int screen_width, unsigned int window_width) { | |
return (screen_width / 2) - (window_width / 2); | |
} | |
int posx05(unsigned int screen_width, unsigned int window_width) { | |
return (screen_width * 0.5) - (window_width * 0.5); | |
} |
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gcc -O3 -msse3 -fomit-frame-pointer -c *.c && objdump -Mintel -S *.o | |
posx.o: file format elf64-x86-64 | |
Disassembly of section .text: | |
0000000000000000 <posx>: | |
0: 89 f8 mov eax,edi | |
2: d1 ee shr esi,1 | |
4: d1 e8 shr eax,1 | |
6: 29 f0 sub eax,esi | |
8: c3 ret | |
9: eb 05 jmp 10 <posx05> | |
b: 90 nop | |
c: 90 nop | |
d: 90 nop | |
e: 90 nop | |
f: 90 nop | |
0000000000000010 <posx05>: | |
10: 89 ff mov edi,edi | |
12: 89 f6 mov esi,esi | |
14: f2 48 0f 2a c7 cvtsi2sd xmm0,rdi | |
19: f2 48 0f 2a ce cvtsi2sd xmm1,rsi | |
1e: f2 0f 59 05 00 00 00 mulsd xmm0,QWORD PTR [rip+0x0] # 26 <posx05+0x16> | |
25: 00 | |
26: f2 0f 59 0d 00 00 00 mulsd xmm1,QWORD PTR [rip+0x0] # 2e <posx05+0x1e> | |
2d: 00 | |
2e: f2 0f 58 c1 addsd xmm0,xmm1 | |
32: f2 0f 2c c0 cvttsd2si eax,xmm0 | |
36: c3 ret |
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