------- Additional Comments From aoliva at gcc dot gnu dot org 2005-03-10
20:38 -------
Subject: Re: [PR middle-end/18628] do not fold to label load from tablejump to
reg
On Mar 10, 2005, Richard Henderson <[EMAIL PROTECTED]> wrote:
> On Wed, Mar 09, 2005 at 07:26:37AM -0300, Alexandre Oliva wrote:
>> + /* If it's not a REG, the REG_EQUAL note is inappropriate. */
>> + if (REG_P (SET_DEST (set)))
>> + set_unique_reg_note (insn, REG_EQUAL, label);
> I don't think this is a good idea at all. This is just
> asking for reload to recreate a reference to the deleted label.
Here's a patch with that bit removed, along with the change in
cse_init that it required. Ok?
Index: gcc/ChangeLog
from Alexandre Oliva <[EMAIL PROTECTED]>
PR middle-end/18628
* cse.c (fold_rtx_mem): Don't fold a load from a jumptable into a
register.
Index: gcc/cse.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/cse.c,v
retrieving revision 1.349
diff -u -p -r1.349 cse.c
--- gcc/cse.c 8 Mar 2005 13:56:56 -0000 1.349
+++ gcc/cse.c 10 Mar 2005 20:36:36 -0000
@@ -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)
Index: gcc/testsuite/ChangeLog
from Alexandre Oliva <[EMAIL PROTECTED]>
* gcc.dg/pr18628.c: New.
Index: gcc/testsuite/gcc.dg/pr18628.c
===================================================================
RCS file: gcc/testsuite/gcc.dg/pr18628.c
diff -N gcc/testsuite/gcc.dg/pr18628.c
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ gcc/testsuite/gcc.dg/pr18628.c 10 Mar 2005 20:36:52 -0000
@@ -0,0 +1,31 @@
+/* { dg-do link } */
+/* { dg-options "-O2" } */
+
+/* PR middle-end/18628 exposed a problem in which cse folded a load
+ from a jump table into the label that was the target of the branch.
+ Unfortunately, the indirect jump was moved to a different basic
+ block, and the LABEL_REF copied to the register wasn't enough to
+ keep the cfg from optimizing the otherwise-unused label away. So
+ we ended up with a dangling reference to the label. */
+
+int i;
+
+int main()
+{
+ for (;;)
+ {
+ switch (i)
+ {
+ case 0:
+ case 1:
+ return 1;
+
+ case 2:
+ case 3:
+ return 0;
+
+ case 5:
+ --i;
+ }
+ }
+}
--
Alexandre Oliva http://www.ic.unicamp.br/~oliva/
Red Hat Compiler Engineer [EMAIL PROTECTED], gcc.gnu.org}
Free Software Evangelist [EMAIL PROTECTED], gnu.org}
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18628
