[cfe-dev] Speculative load optimization

[Kreitzer, David L via cfe-dev]

This llvm patch, https://reviews.llvm.org/D37289, attempts to do an optimization
that involves speculating loads. The patch itself needs some work regardless,
but we are questioning the legality of the optimization, which is currently
performed by both gcc and icc. The desired transformation looks like this:

    typedef struct S {
      char padding[4088];
      struct S *p1;
      struct S *p2;
    } S;

    struct S* f1(struct S *s, int x)
    {
      S *r;
      if (x)
        r = s->p1;
      else
        r = s->p2;
      return r;
    }

TO

    struct S* f1(struct S *s, int x)
    {
      return (x) ? s->p1 : s->p2;
    }

The fundamental question seems to be whether loading one member of struct S
makes it valid to load other members of the same struct. Both gcc & icc
seem to think so and make a distinction between this case and a similar case
where one field is accessed through an lvalue of a different type:

    typedef struct T {
      char padding[4088];
      struct S *p1;
    } T;

    struct S* f2(struct S *s, int x)
    {
      S *r;
      if (x)
        r = ((T*)s)->p1;
      else
        r = s->p2;
      return r;
    }

Neither compiler will transform this case.

Any insight on the validity of this optimization in relation to the C/C++
standards would be appreciated.

Thanks,
Dave Kreitzer
Intel Compilers