Question

I try to build an application which uses pthreads and __m128 SSE type. According to GCC manual, default stack alignment is 16 bytes. In order to use __m128, the requirement is the 16-byte alignment.

My target CPU supports SSE. I use a GCC compiler which doesn’t support runtime stack realignment (e.g. -mstackrealign). I cannot use any other GCC compiler version.

My test application looks like:

#include <xmmintrin.h>
#include <pthread.h>
void *f(void *x){
   __m128 y;
   ...
}
int main(void){
  pthread_t p;
  pthread_create(&p, NULL, f, NULL);
}

The application generates an exception and exits. After a simple debugging (printf “%p”, &y), I found that the variable y is not 16-byte aligned.

My question is: how can I realign the stack properly (16-byte) without using any GCC flags and attributes (they don’t help)? Should I use GCC inline Assembler within this thread function f()?

Answer 1

I have solved this problem.
Here is my solution:

void another_function(){
   __m128 y;
   ...
}
void *f(void *x){
asm("pushl    %esp");
asm("subl    $16,%esp");
asm("andl    $-0x10,%esp");
another_function();
asm("popl %esp");
}

First, we increase the stack by 16 bytes. Second, we make least-significant nibble equal 0x0. We preserve the stack pointer using push/pop operands. We call another function, which has all its own local variables 16-byte aligned. All nested functions will also have their local variables 16-byte aligned.

And It works!