我有以下代碼不同的編譯選項:內在__lzcnt64返回不同的值與
#include <stdint.h>
#include <stdio.h>
#include <x86intrin.h>
long long lzcnt(long long l)
{
return __lzcnt64(l);
}
int main(int argc, char** argv)
{
printf("%lld\n", lzcnt(atoll(argv[1])));
return 0;
}
用不同的編譯器和參數執行我得到(組件示出):
鏘
$ clang -Wall src/test.c -D__LZCNT__ && ./a.out 2047
53
0000000000400560 <lzcnt>:
400560: 55 push %rbp
400561: 48 89 e5 mov %rsp,%rbp
400564: 48 89 7d f0 mov %rdi,-0x10(%rbp)
400568: 48 8b 7d f0 mov -0x10(%rbp),%rdi
40056c: 48 89 7d f8 mov %rdi,-0x8(%rbp)
400570: 48 8b 7d f8 mov -0x8(%rbp),%rdi
400574: 48 0f bd ff bsr %rdi,%rdi
400578: 48 83 f7 3f xor $0x3f,%rdi
40057c: 89 f8 mov %edi,%eax
40057e: 48 63 c0 movslq %eax,%rax
400581: 5d pop %rbp
400582: c3 retq
400583: 66 66 66 66 2e 0f 1f data32 data32 data32 nopw %cs:0x0(%rax,%rax,1)
40058a: 84 00 00 00 00 00
GCC without -mlzcnt
$ gcc -Wall src/test.c -D__LZCNT__ && ./a.out 2047
53
0000000000400580 <lzcnt>:
400580: 55 push %rbp
400581: 48 89 e5 mov %rsp,%rbp
400584: 48 89 7d e8 mov %rdi,-0x18(%rbp)
400588: 48 8b 45 e8 mov -0x18(%rbp),%rax
40058c: 48 89 45 f8 mov %rax,-0x8(%rbp)
400590: 48 0f bd 45 f8 bsr -0x8(%rbp),%rax
400595: 48 83 f0 3f xor $0x3f,%rax
400599: 48 98 cltq
40059b: 5d pop %rbp
40059c: c3 retq
GCC與-mlzcnt
$ gcc -Wall src/test.c -D__LZCNT__ -mlzcnt && ./a.out 2047
10
0000000000400580 <lzcnt>:
400580: 55 push %rbp
400581: 48 89 e5 mov %rsp,%rbp
400584: 48 89 7d e8 mov %rdi,-0x18(%rbp)
400588: 48 8b 45 e8 mov -0x18(%rbp),%rax
40058c: 48 89 45 f8 mov %rax,-0x8(%rbp)
400590: f3 48 0f bd 45 f8 lzcnt -0x8(%rbp),%rax
400596: 48 98 cltq
400598: 5d pop %rbp
400599: c3 retq
G ++不-mlzcnt
$ g++ -Wall src/test.c -D__LZCNT__ && ./a.out 2047
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.8.2/include/immintrin.h:64:0,
from /usr/lib/gcc/x86_64-redhat-linux/4.8.2/include/x86intrin.h:62,
from src/test.c:3:
/usr/lib/gcc/x86_64-redhat-linux/4.8.2/include/lzcntintrin.h: In function ‘short unsigned int __lzcnt16(short unsigned int)’:
/usr/lib/gcc/x86_64-redhat-linux/4.8.2/include/lzcntintrin.h:38:29: error: ‘__builtin_clzs’ was not declared in this scope
return __builtin_clzs (__X);
G ++與-mlzcnt
$ g++ -Wall src/test.c -D__LZCNT__ -mlzcnt && ./a.out 2047
10
0000000000400640 <_Z5lzcntx>:
400640: 55 push %rbp
400641: 48 89 e5 mov %rsp,%rbp
400644: 48 89 7d e8 mov %rdi,-0x18(%rbp)
400648: 48 8b 45 e8 mov -0x18(%rbp),%rax
40064c: 48 89 45 f8 mov %rax,-0x8(%rbp)
400650: f3 48 0f bd 45 f8 lzcnt -0x8(%rbp),%rax
400656: 48 98 cltq
400658: 5d pop %rbp
400659: c3 retq
的區別是很清楚使用-mlzcnt的,但是我實際上在C++中工作,沒有這個選項,它不能在g ++上編譯(clang ++很好)。它看起來像使用-mlzcnt時,結果是63-(沒有-mlzct的結果)。是否有關於gcc的-mlzcnt選項的任何文檔(我查看了信息文件,但找不到任何內容)?它是否做更多選擇lzcnt指令?
您是否反彙編程序以查看它們是否使用了預期的指令?你確定兩個平臺都有*指令嗎? – unwind
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