On Mon, Jan 18, 2016 at 10:06:44AM +0100, Eric Botcazou wrote:
> > The following testcase is miscompiled on i686-linux at -O3.
> > The bug is in DSE record_store, which for group_id < 0 uses mem_addr
> > set to result of get_addr (base->val_rtx) (plus optional offset),
> > which is fine for canon_true_dependence with other MEMs in that function,
> > but we also store that address in store_info. The problem is if later on
> > e.g. some read uses the same e.g. hard register as get_addr returned, but
> > that register contains at that later point a different value.
> > canon_true_dependence then happily returns the read does not alias the
> > store, although it might.
> > The fix is to store the VALUE (plus optional offset) into
> > store_info->mem_addr instead, then at some later insn when get_addr is
> > called on it it will either return the same register or expression (if it
> > has not changed), or some different one otherwise.
>
> I presume that the origin of the bug is:
>
> /* get_addr can only handle VALUE but cannot handle expr like:
> VALUE + OFFSET, so call get_addr to get original addr for
> mem_addr before plus_constant. */
> mem_addr = get_addr (mem_addr);
> if (offset)
> mem_addr = plus_constant (get_address_mode (mem), mem_addr, offset);
>
> both in record_store and check_mem_read_rtx, so I wonder if we shouldn't bite
> the bullet and try enhancing get_addr since it's a mainline-only regression.
So, do you suggest to tweak get_addr like the patch below, and remove the
mem_addr = get_addr (mem_addr);
line above and the comment?
--- gcc/alias.c.jj 2016-01-14 17:01:09.316932111 +0100
+++ gcc/alias.c 2016-01-18 10:30:46.780994699 +0100
@@ -2203,7 +2203,23 @@ get_addr (rtx x)
struct elt_loc_list *l;
if (GET_CODE (x) != VALUE)
- return x;
+ {
+ if ((GET_CODE (x) == PLUS || GET_CODE (x) == MINUS)
+ && GET_CODE (XEXP (x, 0)) == VALUE
+ && CONST_SCALAR_INT_P (XEXP (x, 1)))
+ {
+ rtx op0 = get_addr (XEXP (x, 0));
+ if (op0 != XEXP (x, 0))
+ {
+ if (GET_CODE (x) == PLUS
+ && GET_CODE (XEXP (x, 1)) == CONST_INT)
+ return plus_constant (GET_MODE (x), op0, INTVAL (XEXP (x, 1)));
+ return simplify_gen_binary (GET_CODE (x), GET_MODE (x),
+ op0, XEXP (x, 1));
+ }
+ }
+ return x;
+ }
v = CSELIB_VAL_PTR (x);
if (v)
{
Jakub
