[PATCH, PR55315] Don't assume a nonzero address plus a const is a nonzero address

[Tom de Vries]

Richard,

This patch fixes PR 55315.

When compiling for the mips target with -O2, function f is folded to 0, while
the address of data is not known compile-time:
...
int data[4096];

int
f (void)
{
  return (((unsigned int) &data[0]) == 0xdeadbea0U);
}
....

What happens is that expand turns the comparison into the equivalent of:
...
  (((unsigned int) &data[0]) + (~0xdeadbea0U + 1)) == 0
...
and then nonzero_address_p triggers here during cse:
...
    case PLUS:
      if (CONST_INT_P (XEXP (x, 1)))
        return nonzero_address_p (XEXP (x, 0));
...
and '(((unsigned int) &data[0]) + (~0xdeadbea0U + 1))' is considered a nonzero
address, and the comparison evaluates to 0.

This example is related to PR 29519, in fact you mention the code pattern expand
generates in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29519#c5.
But the patch for that PR does not address this example.

This patch prevents nonzero_address_p from assuming that a nonzero address plus
a const is a nonzero address.

Build and reg-tested on mips.

OK for trunk?

Thanks,
- Tom

2012-11-17  Tom de Vries  <t...@codesourcery.com>

        PR rtl-optimization/55315

        * rtlanal.c (nonzero_address_p): Don't assume a nonzero address plus a
        const is a nonzero address.

        * gcc.target/mips/pr55315.c: New test.

Index: gcc/rtlanal.c
===================================================================
--- gcc/rtlanal.c (revision 193478)
+++ gcc/rtlanal.c (working copy)
@@ -392,10 +392,8 @@ nonzero_address_p (const_rtx x)
       return nonzero_address_p (XEXP (x, 0));
 
     case PLUS:
-      if (CONST_INT_P (XEXP (x, 1)))
-        return nonzero_address_p (XEXP (x, 0));
       /* Handle PIC references.  */
-      else if (XEXP (x, 0) == pic_offset_table_rtx
+      if (XEXP (x, 0) == pic_offset_table_rtx
 	       && CONSTANT_P (XEXP (x, 1)))
 	return true;
       return false;
Index: gcc/testsuite/gcc.target/mips/pr55315.c
===================================================================
--- /dev/null (new file)
+++ gcc/testsuite/gcc.target/mips/pr55315.c (revision 0)
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+
+int data[4096];
+
+int
+f (void)
+{
+  return (((unsigned int) &data[0]) == 0xdeadbea0U);
+}
+
+/* { dg-final { scan-assembler-not "\tmove\t\\\$2,\\\$0" } } */