LRA: Don't use 0 as initialization for sp_offset

[Michael Matz]

this is part of making m68k work with LRA.  See PR116374.
m68k has the property that sometimes the elimation offset
between %sp and %argptr is zero.  During setting up elimination
infrastructure it's changes between sp_offset and previous_offset
that feed into insns_with_changed_offsets that ultimately will
setup looking at the instructions so marked.

But the initial values for sp_offset and previous_offset are
also zero.  So if the targets INITIAL_ELIMINATION_OFFSET (called
in update_reg_eliminate) is zero then nothing changes, the
instructions in question don't get into the list to consider and
the sp_offset tracking goes wrong.

Solve this by initializing those member with -1 instead of zero.
An initial offset of that value seems very unlikely, as it's
in word-sized increments.  This then also reveals a problem in
eliminate_regs_in_insn where it always uses sp_offset-previous_offset
as offset adjustment, even in the first_p pass.  That was harmless
when previous_offset was uninitialized as zero.  But all the other
code uses a different idiom of checking for first_p (or rather
update_p which is !replace_p&&!first_p), and using sp_offset directly.
So use that as well in eliminate_regs_in_insn.

        PR target/116374
        * lra-eliminations.cc (init_elim_table): Use -1 as initializer.
        (update_reg_eliminate): Accept -1 as not-yet-used marker.
        (eliminate_regs_in_insn): Use previous_sp_offset only when
        not first_p.
---
 gcc/lra-eliminations.cc | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
---

Regstrapped on x86-64-linux.  Okay?

diff --git a/gcc/lra-eliminations.cc b/gcc/lra-eliminations.cc
index 5bed259cffe..96772f2904a 100644
--- a/gcc/lra-eliminations.cc
+++ b/gcc/lra-eliminations.cc
@@ -969,7 +969,8 @@ eliminate_regs_in_insn (rtx_insn *insn, bool replace_p, 
bool first_p,
          if (! replace_p)
            {
              if (known_eq (update_sp_offset, 0))
-               offset += (ep->offset - ep->previous_offset);
+               offset += (!first_p
+                          ? ep->offset - ep->previous_offset : ep->offset);
              if (ep->to_rtx == stack_pointer_rtx)
                {
                  if (first_p)
@@ -1212,7 +1213,7 @@ update_reg_eliminate (bitmap insns_with_changed_offsets)
              if (lra_dump_file != NULL)
                fprintf (lra_dump_file, "    Using elimination %d to %d now\n",
                         ep1->from, ep1->to);
-             lra_assert (known_eq (ep1->previous_offset, 0));
+             lra_assert (known_eq (ep1->previous_offset, -1));
              ep1->previous_offset = ep->offset;
            }
          else
@@ -1283,7 +1284,7 @@ init_elim_table (void)
   for (ep = reg_eliminate, ep1 = reg_eliminate_1;
        ep < &reg_eliminate[NUM_ELIMINABLE_REGS]; ep++, ep1++)
     {
-      ep->offset = ep->previous_offset = 0;
+      ep->offset = ep->previous_offset = -1;
       ep->from = ep1->from;
       ep->to = ep1->to;
       value_p = (targetm.can_eliminate (ep->from, ep->to)
-- 
2.39.1