reordering of trapping operations and volatile

Martin Uecker ma.uecker@gmail.com
Sat Jan 8 08:32:24 GMT 2022


Hi Richard,

I have a question regarding reodering of volatile
accesses and trapping operations. My initial
assumption (and  hope) was that compilers take
care to avoid creating traps that are incorrectly
ordered relative to observable behavior.

I had trouble finding examples, and my cursory
glace at the code seemed to confirm that GCC
carefully avoids this.  But then someone showed
me this example, where this can happen in GCC:


volatile int x;

int foo(int a, int b, _Bool store_to_x)
{
  if (!store_to_x)
    return a / b;
  x = b;
  return a / b;
}


https://godbolt.org/z/vq3r8vjxr

In this example a division is hoisted 
before the volatile store. (the division
by zero which could trap is UB, of course).

As Martin Sebor pointed out this is done
as part of redundancy elimination 
in tree-ssa-pre.c and that this might
simply be an oversight (and could then be
fixed with a small change).

Could you clarify whether such reordering
is intentional and could be exploited in
general also in other optimizations or
confirm that this is an oversight that
affects only this specific case?

If this is intentional, are there examples
where this is important for optimization?


Martin








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