Why Does My Inline Assembly Using the Base Pointer Register (%rbp) Cause a Seg Fault in C ?

Using the Base Pointer Register in C Inline ASM

Question:

I am attempting to use the base pointer register (%rbp) within inline assembly. However, when I access the variable after the inline asm, the program seg faults. The code snippet below illustrates the issue:

void Foo(int &x) {
    asm volatile ("pushq %%rbp;"
                  "movq %%rsp, %%rbp;"
                  "subq , %%rsp;"
                  "movl , -12(%%rbp);"
                  "movq %%rbp, %%rsp;"
                  "popq %%rbp;"
                  : : : );
    x = 5;
}

Problem Diagnosis:

The code seg faults because it overwrites a value stored in the "red zone" below RSP, which GCC uses to store a value.

Solution:

To avoid this issue, allocate scratch space for your inline asm using an "=m" output operand or explicitly skip over the red zone using the sub $-128, %rsp instruction. Alternatively, avoid using scratch space in the first place and let the compiler allocate and save registers for you.

Best Practices for Inline ASM:

• Keep inline ASM as concise as possible, ideally only including the instructions the compiler cannot generate.
• Use input/output constraints to specify the flow of data between ASM and C code.
• Avoid using inline ASM for large code blocks. Consider writing assembly functions instead.

Additional Tips:

• When using inline ASM, be aware of the "red zone" area below the stack pointer.
• Input constraints can be used to specify registers for inputs, reducing the need for explicit register allocation.
• Declare a "cc" (condition code) clobber if your code modifies the flags.

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