Question about tree-switch-conversion.c

[Ian Bolton]

I am in the process of fixing PR44328
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44328)

The problem is that gen_inbound_check in tree-switch-conversion.c subtracts
info.range_min from info.index_expr, which can cause the MIN and MAX values
for info.index_expr to become invalid.

For example:


typedef enum {
  FIRST = 0,
  SECOND,
  THIRD,
  FOURTH
} ExampleEnum;

int dummy (const ExampleEnum e)
{
  int mode = 0;
  switch (e)
  {
    case SECOND: mode = 20; break;
    case THIRD: mode = 30; break;
    case FOURTH: mode = 40; break;
  }
  return mode;
}


tree-switch-conversion would like to create a lookup table for this, so
that SECOND maps to entry 0, THIRD maps to entry 1 and FOURTH maps to
entry 2.  It achieves this by subtracting SECOND from index_expr.  The
problem is that after the subtraction, the type of the result can have a
value outside the range 0-3.

Later, when tree-vrp.c sees the inbound check as being <= 2 with a possible
range for the type as 0-3, it converts the <=2 into a != 3, which is
totally wrong.  If e==FIRST, then we can end up looking for entry 255 in
the lookup table!

I think the solution is to update the type of the result of the subtraction
to show that it is no longer in the range 0-3, but I have had trouble
implementing this.  The attached patch (based off 4.5 branch) shows my
current approach, but I ran into LTO issues:

lto1: internal compiler error: in lto_get_pickled_tree, at lto-streamer-in.c

I am guessing this is because the debug info for the type does not match
the new range I have set for it.

Is there a *right* way to update the range such that LTO doesn't get
unhappy?  (Maybe a cast with fold_convert_loc would be right?)

pr44328.gcc4.5.fix.patch
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