问题描述
考虑下面的汇编代码循环: #include< iostream>
$ b $ #define ADD_LOOP(i,n,v)\
asm volatile(\
movw%1,%% cx;\
movq%2,%% rax;\
movq $ 0,%% rbx;\
为:;\
addq %% rax,%% rbx ;\
decw %% cx;\
jnz for;\
movq %% rbx,%0;\
: =x(n)\
:%cx,%rax,%rbx\
);
int main(){
uint16_t iter(10000);
uint64_t num(5);
uint64_t val;
ADD_LOOP(iter,num,val)
std :: cout<< val<<的std :: ENDL;
返回0;
$ b 有可能从内部调用C函数(或它的机器代码输出)如上所述的循环?
例如:
#include < wmmintrin.h>
int main(){
__m128i x,y;
for(int i = 0; i x = __builtin_ia32_aesenc128(x,y);
}
返回0;
谢谢
解决方案没有。内建函数不是可以用 call
调用的真实函数。它们在C / C ++中总是内联。
例如,如果您希望得到 popcnt -mpopcnt
的目标,或者针对不支持 popcnt 指令,你运气不好。您必须亲自 #ifdef
,并使用 popcnt
或其他指令序列。
您正在讨论的函数 __ builtin_ia32_aesenc128
仅仅是,您可以如果使用asm编写,直接使用。
如果您编写asm而不是使用C ++内在函数(如 #include< immintrin.h>
表现出色,您需要查看来编写更高效的asm(例如使用%ecx
作为循环计数器,而不是%cx
。您使用16位局部寄存器没有任何好处)。
您也可以编写更有效的内联汇总约束,例如 movq %% rbx,%0
是浪费指令。您可以在整个过程中使用%0
而不是显式%rbx
。如果内联asm以mov指令开始或结束于复制到输出/输入操作数的输入/输出操作,通常情况下你做错了。让编译器为你分配寄存器。请参阅标记wiki的问题。
更好的是,。具有内在函数的代码通常可以编译x86。请参阅: #include< immintrin.h> ;
并使用 __ m128i _mm_aesenc_si128(__m128i a,__m128i RoundKey)
。 (在gcc中,它只是 __ builtin_ia32_aesenc128
的包装器,但它使您的代码可以移植到其他x86编译器中。)
Considering the following assembly code loop:
#include <iostream>
#define ADD_LOOP(i, n, v) \
asm volatile ( \
"movw %1, %%cx ;" \
"movq %2, %%rax ;" \
"movq $0, %%rbx ;" \
"for: ;" \
"addq %%rax, %%rbx ;" \
"decw %%cx ;" \
"jnz for ;" \
"movq %%rbx, %0 ;" \
: "=x"(v) \
: "n"(i), "x"(n) \
: "%cx", "%rax", "%rbx" \
);
int main() {
uint16_t iter(10000);
uint64_t num(5);
uint64_t val;
ADD_LOOP(iter, num, val)
std::cout << val << std::endl;
return 0;
}
Is possible to call a C function (or it's machine code output) from within a loop as specified above?
for example:
#include <wmmintrin.h>
int main() {
__m128i x, y;
for(int i = 0; i < 10; i++) {
x = __builtin_ia32_aesenc128(x, y);
}
return 0;
}
Thanks
解决方案 No. Builtin functions aren't real functions that you can call with call
. They always inline when used in C / C++.
For example, if you want int __builtin_popcount (unsigned int x)
to get either a popcnt
instruction for targets with -mpopcnt
, or a byte-wise lookup table for targets that don't support the popcnt
instruction, you are out of luck. You will have to #ifdef
yourself and use popcnt
or an alternative sequence of instructions.
The function you're talking about, __builtin_ia32_aesenc128
is just a wrapper for the aesenc
assembly instruction which you can just use directly if writing in asm.
If you're writing asm instead of using C++ intrinsics (like #include <immintrin.h>
for performance, you need to have a look at http://agner.org/optimize/ to write more efficient asm (e.g. use %ecx
as a loop counter, not %cx
. You're gaining nothing from using a 16-bit partial register).
You could also write more efficient inline-asm constraints, e.g. the movq %%rbx, %0
is a waste of an instruction. You could have used %0
the whole time instead of an explict %rbx
. If your inline asm starts or ends with a mov instruction to copy to/from an output/input operand, usually you're doing it wrong. Let the compiler allocate registers for you. See the inline-assembly tag wiki.
Or better, https://gcc.gnu.org/wiki/DontUseInlineAsm. Code with intrinsics typically compiles well for x86. See Intel's intrinsics guide: #include <immintrin.h>
and use __m128i _mm_aesenc_si128 (__m128i a, __m128i RoundKey)
. (In gcc that's just a wrapper for __builtin_ia32_aesenc128
, but it makes your code portable to other x86 compilers.)
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