[Bug rtl-optimization/64895] RA picks the wrong register for -fipa-ra

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64895

--- Comment #6 from vries at gcc dot gnu.org ---
>From PR64342 comment 7:

Register allocation seems to progress similarly, up until this message in
reload, which seems to be directly related to the r216154 patch:
...
    Spill r86 after risky transformations
  Reassigning non-reload pseudos
           Assign 3 to r86 (freq=3000)
...

I've looked a bit in more detail at this message.

fipa-ra manages to allocate hardreg 1 (dx) to the call-crossing liverange as
before. But setup_live_pseudos_and_spill_after_risky_transforms looks at the
conflict regs, and finds that lra_reg_info[regno].conflict_hard_regs contains
dx. Hence, it spills the liverange into an available reg, which happens to be
hardreg 3 (bx).

So, the risky transformations and fipa-ra have in common that they allocate
registers from the conflict_hard_regs set. In the case of -fipa-ra, we don't
want setup_live_pseudos_and_spill_after_risky_transforms to undo the -fipa-ra
allocation, but currently there's no way to distinguish between the two.

Using this patch, we make sure the conflict registers don't include dx for this
testcase, and the test-cases passes again:
...
diff --git a/gcc/lra-lives.c b/gcc/lra-lives.c
index 9dfffb6..0231057 100644
--- a/gcc/lra-lives.c
+++ b/gcc/lra-lives.c
@@ -636,8 +636,11 @@ check_pseudos_live_through_calls (int regno)
   if (! sparseset_bit_p (pseudos_live_through_calls, regno))
     return;
   sparseset_clear_bit (pseudos_live_through_calls, regno);
+
   IOR_HARD_REG_SET (lra_reg_info[regno].conflict_hard_regs,
-                   call_used_reg_set);
+                   !hard_reg_set_empty_p
(lra_reg_info[regno].actual_call_used_reg_set)
+                   ? lra_reg_info[regno].actual_call_used_reg_set
+                   : call_used_reg_set);

   for (hr = 0; hr < FIRST_PSEUDO_REGISTER; hr++)
     if (HARD_REGNO_CALL_PART_CLOBBERED (hr, PSEUDO_REGNO_MODE (regno)))
...