How to access C variable for inline assembly manipulation?

by

In GNU C inline asm, with x86 AT&T syntax:
(But https://gcc.gnu.org/wiki/DontUseInlineAsm if you can avoid it).

// this example doesn't really need volatile: the result is the same every time
asm volatile("movl $0, %[some]"
    : [some] "=r" (x)
);

after this, x contains 0.

Note that you should generally avoid mov as the first or last instruction of an asm statement. Don’t copy from %[some] to a hard-coded register like %%eax, just use %[some] as a register, letting the compiler do register allocation.

See https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html and https://stackoverflow.com/tags/inline-assembly/info for more docs and guides.

Not all compilers support GNU syntax.
For example, for MSVC you do this:

__asm mov x, 0 and x will have the value of 0 after this statement.

Please specify the compiler you would want to use.

Also note, doing this will restrict your program to compile with only a specific compiler-assembler combination, and will be targeted only towards a particular architecture.

In most cases, you’ll get as good or better results from using pure C and intrinsics, not inline asm.

More Related Contents:

  1. Assembly code fsqrt and fmul instructions
  2. How can I indicate that the memory *pointed* to by an inline ASM argument may be used?
  3. What is the difference between ‘asm’, ‘__asm’ and ‘__asm__’?
  4. Can I use Intel syntax of x86 assembly with GCC?
  5. When to use earlyclobber constraint in extended GCC inline assembly?
  6. Looping over arrays with inline assembly
  7. Reading a register value into a C variable [duplicate]
  8. Why we need Clobbered registers list in Inline Assembly?
  9. How to tell GCC to generate 16-bit code for real mode
  10. Using condition flags as GNU C inline asm outputs
  11. Using GCC to produce readable assembly?
  12. Why does GCC allocate more space than necessary on the stack, beyond what’s needed for alignment?
  13. Stack allocation, padding, and alignment
  14. How does GCC implement variable-length arrays?
  15. What does it mean to align the stack?
  16. Labels in GCC inline assembly
  17. Efficient integer compare function
  18. Why is the value of EDX overwritten when making call to printf?
  19. In GNU C inline asm, what are the size-override modifiers for xmm/ymm/zmm for a single operand?
  20. Why can’t local variable be used in GNU C basic inline asm statements?
  21. Why does gcc use movl instead of push to pass function args?
  22. x86_64 : is stack frame pointer almost useless?
  23. execute binary machine code from C
  24. calling assembly function from c
  25. The difference between asm, asm volatile and clobbering memory
  26. Why do compilers insist on using a callee-saved register here?
  27. Why is gcc allowed to speculatively load from a struct?
  28. Produce loops without cmp instruction in GCC
  29. How to get the size of a C function from inside a C program or with inline assembly?
  30. How do I compile the asm generated by GCC?

Leave a Comment