re PR rtl-optimization/18628 (miscompilation of switch statement in loop)

gcc/ChangeLog:
PR middle-end/18628
* cse.c (fold_rtx_mem): Don't fold a load from a jumptable into a
register.
gcc/testsuite/ChangeLog:
* gcc.dg/pr18628.c: New.

From-SVN: r96445

1 files changed, 24 insertions, 2 deletions

diff --git a/gcc/cse.c b/gcc/cse.c
index d7f3027..fd5e21a 100644
--- a/gcc/cse.c
+++ b/gcc/cse.c
@@ -3515,8 +3515,30 @@ fold_rtx_mem (rtx x, rtx insn)
 	    if (offset >= 0
 		&& (offset / GET_MODE_SIZE (GET_MODE (table))
 		    < XVECLEN (table, 0)))
-	      return XVECEXP (table, 0,
-			      offset / GET_MODE_SIZE (GET_MODE (table)));
+	      {
+		rtx label = XVECEXP
+		  (table, 0, offset / GET_MODE_SIZE (GET_MODE (table)));
+		rtx set;
+
+		/* If we have an insn that loads the label from the
+		   jumptable into a reg, we don't want to set the reg
+		   to the label, because this may cause a reference to
+		   the label to remain after the label is removed in
+		   some very obscure cases (PR middle-end/18628).  */
+		if (!insn)
+		  return label;
+
+		set = single_set (insn);
+
+		if (! set || SET_SRC (set) != x)
+		  return x;
+
+		/* If it's a jump, it's safe to reference the label.  */
+		if (SET_DEST (set) == pc_rtx)
+		  return label;
+
+		return x;
+	      }
 	  }
 	if (table_insn && JUMP_P (table_insn)
 	    && GET_CODE (PATTERN (table_insn)) == ADDR_DIFF_VEC)