lra: Stop eh_return data regs being incorrectly marked live [PR92989]

lra_assign has an assert to make sure that no pseudo is allocated
to a conflicting hard register.  It used to be restricted to
!flag_ipa_ra, but in g:a1e6ee38e708ef2bdef4 I'd enabled it for
flag_ipa_ra too.  It then tripped a few times while building
libstdc++ for mips-mti-linux.

Previous patches fixed one of the problems: registers clobbered
by the taking of an exception were being treated as live at the
beginning of the EH receiver, and this got propagated to predecessor
blocks.  But it turns out that there was a second problem: eh_return
data registers were also being marked live in the same way.

These registers are defined by the unwinder and so in reality they
are live on entry to the EH receiver.  But definitions can only happen
in blocks, not on edges, so for liveness purposes we use artificial
definitions at the start of the EH receiver.  process_bb_lives should
therefore model the effect of a definition, not a plain use.

2020-04-06  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR rtl-optimization/92989
	* lra-lives.c (process_bb_lives): Do not treat eh_return data
	registers as being live at the beginning of the EH receiver.

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index e10fb251c16a5054df5dcb4335b35692fcbec085..258e9dbc9568b0c5e5b12824b233e70e23ff2e17 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2020-04-06  Richard Sandiford  <richard.sandiford@arm.com>
+
+       PR rtl-optimization/92989
+       * lra-lives.c (process_bb_lives): Do not treat eh_return data
+       registers as being live at the beginning of the EH receiver.
+
 2020-04-05 Zachary Spytz  <zspytz@gmail.com>
 
        * extend.texi: Add free to list of ISO C90 functions that

diff --git a/gcc/lra-lives.c b/gcc/lra-lives.c
index c8780779a1105a773b2c1b3803c90ec34dad2ba4..917c617903f29d4390e87437ad1dfd2755caeb33 100644 (file)
--- a/gcc/lra-lives.c
+++ b/gcc/lra-lives.c
@@ -1014,13 +1014,19 @@ process_bb_lives (basic_block bb, int &curr_point, bool dead_insn_p)
     }
 
   if (bb_has_eh_pred (bb))
+    /* Any pseudos that are currently live conflict with the eh_return
+       data registers.  For liveness purposes, these registers are set
+       by artificial definitions at the start of the BB, so are not
+       actually live on entry.  */
     for (j = 0; ; ++j)
       {
        unsigned int regno = EH_RETURN_DATA_REGNO (j);
 
        if (regno == INVALID_REGNUM)
          break;
+
        make_hard_regno_live (regno);
+       make_hard_regno_dead (regno);
       }
 
   /* Pseudos can't go in stack regs at the start of a basic block that