Verifier for sub-variables vs mem-ssa

[Diego Novillo <dnovillo at redhat dot com>]

With some local changes I've done in the mem-ssa branch I'm getting a verification
error in execute/20050206-1.c:

foo ()
{
  short unsigned int u[1];

  # SFT.0_9 = VDEF <.MEM_8(D)>
  u[0] = 1;

  # SFT.0_10 = VDEF <SFT.0_9>
  u[0] = 0;

  # u_11 = VDEF <.MEM_8(D)>
  u[1] = 1;

  # u_12 = VDEF <u_11>
  u[2] = 2;

  [ ... ]
}

The problem here is that since 'u' is an array of one element, we only ever 
create an SFT for u[0], which is reasonable:

Variable: u, UID 1523, short unsigned int[1], sub-vars: { SFT.0 }


The problem starts when the operand scanner in mem-ssa determines that since neither 
u[1] nor u[2] have corresponding SFTs, they get to use 'u' directly.  This is different 
from mainline.  In mainline we use SFT.0 for all of them:

foo ()
{
  short unsigned int u[1];

  #   SFT.0_2 = V_MUST_DEF <SFT.0_1>;
  u[0] = 1;

  #   SFT.0_3 = V_MUST_DEF <SFT.0_2>;
  u[0] = 0;

  #   SFT.0_4 = V_MAY_DEF <SFT.0_3>;
  u[1] = 1;

  #   SFT.0_5 = V_MAY_DEF <SFT.0_4>;
  u[2] = 2;

  [ ... ]
}

Which does not trigger the validation error, but I believe it's less precise 
than what is computed in mem-ssa.  Certainly u[1] and u[2] cannot affect u[0].
Nor can they affect each other, but this is undefined code, so it's not such a
big deal.

The verifier is being tricked because it uses a fairly simple test:

  if (is_virtual && var_ann (SSA_NAME_VAR (ssa_name))
      && get_subvars_for_var (SSA_NAME_VAR (ssa_name)) != NULL)
    {
      error ("found real variable when subvariables should have appeared");
    ....

I'm meaning to change the verifier to check if the statement is really
referencing a sub-variable, but first I wanted to check if the idea
makes sense.


Thanks.