PATCH: fix infinite loop in CSE

[Sandra Loosemore <sandra at codesourcery dot com>]

The test case in the attached patch gets stuck in an infinite loop in 
find_comparison_args in CSE when compiled for MIPS at -O2.  This bug has 
been present at least as far back as GCC 4.5 and probably much earlier 
than that.

The problem is that the inner loop over equivalences in the hash table 
is finding something that rewrites to exactly the same expression that 
we've already got in the outer loop, and there is no test for circular 
rewrites.

This patch fixes the specific problem in the test case by skipping over 
equivalences that would rewrite to exactly the same expression as on the 
current iteration.  But, it's not clear that there can't also be cycles 
of length > 1.  I don't see much point in getting fancy here (I assume 
that if this were a common problem it would have been reported and fixed 
long before now) so I just added a simple limit on the number of 
iterations to be sure the outer loop always terminates.

I regression-tested this in a GCC 4.5-based build for mips-linux-gnu and 
also did a full bootstrap and regression test for i686-pc-linux-gnu on 
mainline head.  OK to check in on mainline?

-Sandra


2011-12-12  Sandra Loosemore  <sandra@codesourcery.com>
	    Tom de Vries <tom@codesourcery.com>

	gcc/
	* cse.c (find_comparison_args): Add cap on number of iterations
	to avoid infinite loop in case of cycles.  Avoid obvious case
	that would result in a circular rewrite.

	gcc/testsuite/
	* gcc.dg/cse-bug.c: New testcase.

Index: gcc/cse.c
===================================================================
--- gcc/cse.c	(revision 181994)
+++ gcc/cse.c	(working copy)
@@ -2967,12 +2967,14 @@ find_comparison_args (enum rtx_code code
 		      enum machine_mode *pmode1, enum machine_mode *pmode2)
 {
   rtx arg1, arg2;
+  int iter = 0;
+  int maxiter = 16;
 
   arg1 = *parg1, arg2 = *parg2;
 
   /* If ARG2 is const0_rtx, see what ARG1 is equivalent to.  */
 
-  while (arg2 == CONST0_RTX (GET_MODE (arg1)))
+  while (arg2 == CONST0_RTX (GET_MODE (arg1)) && iter++ < maxiter)
     {
       /* Set nonzero when we find something of interest.  */
       rtx x = 0;
@@ -3055,6 +3057,10 @@ find_comparison_args (enum rtx_code code
 	  if (! exp_equiv_p (p->exp, p->exp, 1, false))
 	    continue;
 
+	  /* If it's the same comparison we're already looking at, skip it.  */
+	  if (COMPARISON_P (p->exp) && XEXP (p->exp, 0) == arg1)
+	    continue;
+
 	  if (GET_CODE (p->exp) == COMPARE
 	      /* Another possibility is that this machine has a compare insn
 		 that includes the comparison code.  In that case, ARG1 would
Index: gcc/testsuite/gcc.dg/cse-bug.c
===================================================================
--- gcc/testsuite/gcc.dg/cse-bug.c	(revision 0)
+++ gcc/testsuite/gcc.dg/cse-bug.c	(revision 0)
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+/* { dg-timeout 10 } */
+
+/* This test used to get stuck in an infinite loop in find_comparison_args
+   when compiling for MIPS.  */
+
+__attribute__ ((__noreturn__)) extern void fail (void);
+
+char x;
+
+void foo (const unsigned char y)
+{
+   ((void) (__builtin_expect((!! y == y), 1) ? 0 : (fail (), 0)));
+   x = ! y;
+}