是否有可能从C ++汇编中调用内置函数 [英] Is it possible to call a built in function from assembly in C++
问题描述
#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 <10; 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 int __builtin_popcount（unsigned int x） 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 aesenc 汇编指令，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 http ：//agner.org/optimize/ 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 inline-assembly 标记wiki的问题。
更好的是， https：// gcc。 gnu.org/wiki/DontUseInlineAsm 。具有内在函数的代码通常可以编译x86。请参阅英特尔内部指南： #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
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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