How Can C Inline Assembly with the Base Pointer Register Avoid Function Call Overhead?

Avoiding Function Call Overhead with Base Pointer Register in C Inline Assembly

To utilize the base pointer register (%rbp) within C inline assembly, follow this corrected code snippet:

void Foo(int &x) {
    int tmp;
    long tmplong;
    asm volatile(
        "lea -16 + %[mem1], %%rbp\n\t"
        "imul , %%rbp, %q[reg1]\n\t"  // Register allocated to tmplong
        "add %k[reg1], %k[reg1]\n\t"    // Register allocated to tmp
        "movl , %[mem1]\n\t" // Store value in memory pointed to by tmp
        : [mem1] "=&m"(tmp), [reg1] "=r"(tmplong)
        :
        : "%rbp"
    );
    x = 5;
}

Explanation:

• Instead of modifying the red zone below %RSP, we use a local variable tmp as a buffer for inline ASM.
• Scratch memory is provided in the red zone, accessed through %[mem1], and references %rbp (base pointer register) to calculate the correct address.
• The "=m" constraint (memory operand) ensures the compiler saves tmp to the memory pointed to by %[mem1].
• Registers are allocated by the compiler for %[reg1].
• The ": %rbp" clobber list informs the compiler that %rbp is modified inside the inline assembly, enabling its restoration.

Additional Notes:

• Keep inline assembly simple, primarily for instructions that the compiler cannot emit. Use constraints instead of explicit mov instructions.
• Prefer writing entire functions in assembly than using inline assembly for function prologues and epilogues.
• Consider using other inline assembly resources such as the inline-assembly tag wiki and the GNU C Manual.

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