This patch cures PR79286 by restoring the REG_DEAD note test used
prior to r235660, but modified to only exclude insns that may trap.
I'd like to allow combine/move without a REG_DEAD note in loops
because insns in loops often lack such notes, and I recall seeing
quite a few cases at the time I wrote r235660 where loops benefited
from allowing the combine/move to happen.
I've been battling hardware instability on my x86_64 box all day, so
hopefully this finally passes bootstrap and regression testing
overnight. OK to apply assuming no regressions?
PR rtl-optimization/79286
* ira.c (combine_and_move_insns): Don't combine or move when
use_insn does not have a REG_DEAD note and def_insn may trap.
testsuite/
* gcc.c-torture/execute/pr79286.c: New.
diff --git a/gcc/ira.c b/gcc/ira.c
index 96b4b62..cdde775 100644
--- a/gcc/ira.c
+++ b/gcc/ira.c
@@ -3682,6 +3682,14 @@ combine_and_move_insns (void)
gcc_assert (DF_REG_DEF_COUNT (regno) == 1 && DF_REF_INSN_INFO (def));
rtx_insn *def_insn = DF_REF_INSN (def);
+ /* Even though we know this reg is set exactly once and used
+ exactly once, check that the reg dies in the use insn or that
+ the def insn can't trap. This is to exclude degenerate cases
+ in loops where the use occurs before the def. See PR79286. */
+ if (!find_reg_note (use_insn, REG_DEAD, regno_reg_rtx[regno])
+ && may_trap_p (PATTERN (def_insn)))
+ continue;
+
/* We may not move instructions that can throw, since that
changes basic block boundaries and we are not prepared to
adjust the CFG to match. */
diff --git a/gcc/testsuite/gcc.c-torture/execute/pr79286.c
b/gcc/testsuite/gcc.c-torture/execute/pr79286.c
new file mode 100644
index 0000000..e6d0e93
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr79286.c
@@ -0,0 +1,15 @@
+int a = 0, c = 0;
+static int d[][8] = {};
+
+int main ()
+{
+ int e;
+ for (int b = 0; b < 4; b++)
+ {
+ __builtin_printf ("%d\n", b, e);
+ while (a && c++)
+ e = d[300000000000000000][0];
+ }
+
+ return 0;
+}
--
Alan Modra
Australia Development Lab, IBM
