[PR115565] cse: Don't use a valid regno for non-register in comparison_qty

Use INT_MIN rather than -1 in `comparison_qty' where a comparison is not
with a register, because the value of -1 is actually a valid reference
to register 0 in the case where it has not been assigned a quantity.

Using -1 makes `REG_QTY (REGNO (folded_arg1)) == ent->comparison_qty'
comparison in `fold_rtx' to incorrectly trigger in rare circumstances
and return true for a memory reference, making CSE consider a comparison
operation to evaluate to a constant expression and consequently make the
resulting code incorrectly execute or fail to execute conditional
blocks.

This has caused a miscompilation of rwlock.c from LinuxThreads for the
`alpha-linux-gnu' target, where `rwlock->__rw_writer != thread_self ()'
expression (where `thread_self' returns the thread pointer via a PALcode
call) has been decided to be always true (with `ent->comparison_qty'
using -1 for a reference to to `rwlock->__rw_writer', while register 0
holding the thread pointer retrieved by `thread_self') and code for the
false case has been optimized away where it mustn't have, causing
program lockups.

The issue has been observed as a regression from commit 08a692679fb8
("Undefined cse.c behaviour causes 3.4 regression on HPUX"),
<https://gcc.gnu.org/ml/gcc-patches/2004-10/msg02027.html>, and up to
commit 932ad4d9b550 ("Make CSE path following use the CFG"),
<https://gcc.gnu.org/ml/gcc-patches/2006-12/msg00431.html>, where CSE
has been restructured sufficiently for the issue not to trigger with the
original reproducer anymore.  However the original bug remains and can
trigger, because `comparison_qty' will still be assigned -1 for a memory
reference and the `reg_qty' member of a `cse_reg_info_table' entry will
still be assigned -1 for register 0 where the entry has not been
assigned a quantity, e.g. at initialization.

Use INT_MIN then as noted above, so that the value remains negative, for
consistency with the REGNO_QTY_VALID_P macro (even though not used on
`comparison_qty'), and then so that it should not ever match a valid
negated register number, fixing the regression with commit 08a692679fb8.

	gcc/
	PR rtl-optimization/115565
	* cse.cc (record_jump_cond): Use INT_MIN rather than -1 for
	`comparison_qty' if !REG_P.

(cherry picked from commit 69bc5fb97dc3fada81869e00fa65d39f7def6acf)

diff --git a/gcc/cse.cc b/gcc/cse.cc
index ca53974810ed6f07fd93e7299d7f2611f5187aea..32e8ea79980d5ec0680b4a7782188c73de68fea7 100644 (file)
--- a/gcc/cse.cc
+++ b/gcc/cse.cc
@@ -239,7 +239,7 @@ static int next_qty;
    the constant being compared against, or zero if the comparison
    is not against a constant.  `comparison_qty' holds the quantity
    being compared against when the result is known.  If the comparison
-   is not with a register, `comparison_qty' is -1.  */
+   is not with a register, `comparison_qty' is INT_MIN.  */
 
 struct qty_table_elem
 {
@@ -4068,7 +4068,7 @@ record_jump_cond (enum rtx_code code, machine_mode mode, rtx op0,
       else
        {
          ent->comparison_const = op1;
-         ent->comparison_qty = -1;
+         ent->comparison_qty = INT_MIN;
        }
 
       return;