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carloscn
carloscn
09_ARMv8_内嵌汇编（内联汇编）Inline assembly
09_ARMv8_内嵌汇编（内联汇编）Inline assembly
内联汇编并非ARCH64专门的使用方法，而是GNU编译器通用的做法。目的有二，其一，对于时间敏感的函数使用内联汇编减少执行开销；其二，C语言无法访问架构级的特殊指令，比如内存屏障功能。
1. 基本用法
1.1 基础内联汇编格式
Define:asm ("asm instruction")
【示例】：
asm(icicalluis) 调用一条高速缓存维护指令。
1.2 扩展内联汇编代码
-
Define:
asm qualifier-asm (AssemblerInstruction) -
Instruction:
- 格式：指令部分：输出部分：输入部分：损坏部分
- 编译器不会解析，按照字符串处理
-
限定词：
volatile,inline- volatile：无特殊情况下的限定词
- inline：asm的语句视为代码最小可能性
【示例1】：
static inline unsigned long array_index_mask_nospec(unsigned long idx, unsigned long sz)
{
unsigned long mask;
asm volatile(
" cmp %1, %2\n" // 指令部第一行
" sbc %0, xzr, xzr\n" // 指令部第二行，使用\n换行
: "=r" (mask) // 输出部分，指定只写属性的变量mask(%0)
: "r" (idx), "Ir" (sz) // 输入部分，执行只读部分的变量idx(%1), sz(%2)
: "cc"); // 损坏部分，跳出汇编
csdb();
return mask;
}
【示例2】：
static inline unsigned long arch_local_irq_save(void)
{
unsigned long flags;
asm volatile(
" mrs %0, daif\n"
" msr daifset, #2\n"
: "=r" (flag)
:
: "memory"
);
}
1.2.1 修饰符[^1]
| C | Describe |
|---|---|
| = | Means that this operand is written to by this instruction: the previous value is discarded and replaced by new data. （只能写） |
| + | Means that this operand is both read and written by the instruction. （读写） |
| & | Means (in a particular alternative) that this operand is an earlyclobber operand, which is written before the instruction is finished using the input operands. Therefore, this operand may not lie in a register that is read by the instruction or as part of any memory address.（输入参数的指令执行完成之后才能写入） |
1.2.2 约束符[^2]
| C | Describe |
|---|---|
| p | An operand that is a valid memory address is allowed. This is for “load address” and “push address” instructions. （内存地址） |
| m | A memory operand is allowed, with any kind of address that the machine supports in general. Note that the letter used for the general memory constraint can be re-defined by a back end using the TARGET_MEM_CONSTRAINT macro.（内存变量） |
| o | A memory operand is allowed, but only if the address is offsettable. This means that adding a small integer (actually, the width in bytes of the operand, as determined by its machine mode) may be added to the address and the result is also a valid memory address.（内存地址，基地址寻址） |
| r | A register operand is allowed provided that it is in a general register. 通用寄存器 |
| i | An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only at assembly time or later. 立即数 |
| V | A memory operand that is not offsettable. In other words, anything that would fit the ‘m’ constraint but not the ‘o’ constraint. 内存变量，不允许偏移的内存操作数 |
| n | An immediate integer operand with a known numeric value is allowed. Many systems cannot support assembly-time constants for operands less than a word wide. Constraints for these operands should use ‘n’ rather than ‘i’.离结束 |
除了通用的约束符之外，还有AARCH64特有的约束符[^3]
| C | Describe |
|---|---|
| k | The stack pointer register (SP) |
| w | Floating point register, Advanced SIMD vector register or SVE vector register |
| x | Like w, but restricted to registers 0 to 15 inclusive. |
| y | Like w, but restricted to registers 0 to 7 inclusive. |
| Upl | One of the low eight SVE predicate registers (P0 to P7) |
| Upa | Any of the SVE predicate registers (P0 to P15) |
| I | Integer constant that is valid as an immediate operand in an ADD instruction |
| J | Integer constant that is valid as an immediate operand in a SUB instruction (once negated) |
| K | Integer constant that can be used with a 32-bit logical instruction |
| L | Integer constant that can be used with a 32-bit logical instruction |
| M | Integer constant that is valid as an immediate operand in a 32-bit MOV pseudo instruction. The MOV may be assembled to one of several different machine instructions depending on the value |
| N | Integer constant that is valid as an immediate operand in a 64-bit MOV pseudo instruction |
| S | An absolute symbolic address or a label reference |
| Y | Floating point constant zero |
| Z | Integer constant zero |
| Ush | The high part (bits 12 and upwards) of the pc-relative address of a symbol within 4GB of the instruction |
| Q | A memory address which uses a single base register with no offset |
| Ump | A memory address suitable for a load/store pair instruction in SI, DI, SF and DF modes |
【示例3】：分析atomic_add函数内嵌汇编
void my_atomic_add(unsigned long val, void *p)
{
unsigned long tmp;
int result;
asm volatile (
" 1: ldxr %0, [%2]\n"
" add %0, %0, %3\n"
" stxr %w1, %0, [%2]\n"
" cbnz %w1, 1b\m"
: "+r" (tmp), "+r" (result), "+Q" (*(unsigned long *)p)
: "r" (val)
: "cc", "memory"
);
}
内联汇编还支持助记符：
int add(int i, int j) {
int ret = 0;
asm volatile (
" add %w[result], %w[input_i], %w[input_j]\n"
: [result] "=r" (res)
: [input_i] "r" (i), [input_j] "r" (j)
:
);
}
实验：实现memcpy，使用内联汇编的方式。
void test_memcpy(void)
{
unsigned long src_addr = 0x80000, dest_addr = 0x200000;
unsigned long sz = 32;
asm volatile (
" mov x6, %x[_src_addr]\n"
" mov x7, %x[_dest_addr]\n"
" add x8, x6, %x[_sz]\n"
" 1: ldr x9, [x6], #8\n"
" str x9, [x7], #8\n"
" cmp x6, x8\n"
" b.cc 1b\n"
:
: [_src_addr] "r" (src_addr), [_dest_addr] "r" (dest_addr), [_sz] "r" (sz)
: "cc", "memory"
);
}
以下需要注意：
- GDB不能单步调试内联汇编。建议使用纯汇编单独编写调试之后移植到C语言内部。
- 内联汇编的参数属性是易错点。
- 输出部和输入部的修饰符不能用错，否则程序会跑飞。比如参数寄存器x0，指定在输入部，汇编内部对参数做了修改比如用了add指令，那么就跑飞了。
再做一个例子memset
void test_memset(void)
{
unsigned long addr = 0x80000;
unsigned long sz = 16;
unsigned long i = 0;
asm volatile (
" mov x4, #0\n"
" 1: stp %x[_count], %x[_count], [%x[_addr]], #16\n"
" add %x[_sz], %x[_sz], #16\n"
" cmp %x[_sz], %x[_addr]\n"
" bne 1b\n"
: [_addr] "+r" (addr), [_sz] "+r" (sz), [_count] "+r" (i)
:
: "memory"
);
}
2. 宏函数
内联汇编也可以和C语言的宏联系到一起，使用##字符串拼接的方式，在内联汇编里面使用""双引号来引用宏参数。
#define MY_OPS(ops, asm_ops) \
static inline void my_asm_##ops(unsigned long mask, void *p) { \
unsigned long tmp; \
asm volatile ( \
" ldr %1, [%0]\n" \
" "#asm_ops" %1, %1, %2\n" \
" str %1, [%0]\n" \
: "+r" (p), "+r" (tmp) \
: "r" (mask) \
: "memory" \
); \
}
MY_OPS(or, orr)
MY_OPS(and, and)
MY_OPS(andnot, bic)
最后宏展开为几个函数：
- my_asm_or
- my_asm_and
- my_asm_andnot
这三个函数只是在内联汇编里面 "#asm_ops"位置不同。
Ref
[^1]:GCC - 6.47.3.3 Constraint Modifier Characters [^2]:GCC - 6.47.3.1 Simple Constraints [^3]:GCC - 6.47.3.4 Constraints for Particular Machines
