[PATCH] Fix part of PR87314, folding of &decl != "foo"

[Richard Biener]

This fixes $subject and also "foo" != "bar" folding which was
somehow missing.  It fixes only parts of the PR since the PR
is about PTA tracking string constants.

It might help PR88775 but unless I can confirm that this is
just queued for GCC10.

You might notice I'm treating string merging possibilities
conservatively (defer to runtime).

Bootstrapped and tested on x86_64-unknown-linux-gnu.

Richard.

>From 8f14bac370b8334a42f985027394e9f3fdf9e2f1 Mon Sep 17 00:00:00 2001
From: Richard Guenther <rguent...@suse.de>
Date: Thu, 10 Jan 2019 10:24:20 +0100
Subject: [PATCH] fix-pr87314-1

2019-01-10  Richard Biener  <rguent...@suse.de>

        PR middle-end/87314
        * match.pd (cmp (convert1?@2 addr@0) (convert2? addr@1)):
        Handle STRING_CST vs DECL or STRING_CST.

        * gcc.dg/pr87314-1.c: New testcase.

diff --git a/gcc/match.pd b/gcc/match.pd
index 60b12f94f9e..95fa4e4a4dd 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3896,6 +3896,9 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
                    || TREE_CODE (base1) == SSA_NAME
                    || TREE_CODE (base1) == STRING_CST))
          equal = (base0 == base1);
+       HOST_WIDE_INT ioff0 = -1, ioff1 = -1;
+       off0.is_constant (&ioff0);
+       off1.is_constant (&ioff1);
      }
      (if (equal == 1
          && (cmp == EQ_EXPR || cmp == NE_EXPR
@@ -3919,10 +3922,23 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
        (if (cmp == GT_EXPR && (known_gt (off0, off1) || known_le (off0, off1)))
        { constant_boolean_node (known_gt (off0, off1), type); }))
       (if (equal == 0
-          && DECL_P (base0) && DECL_P (base1)
-          /* If we compare this as integers require equal offset.  */
-          && (!INTEGRAL_TYPE_P (TREE_TYPE (@2))
-              || known_eq (off0, off1)))
+          && ((DECL_P (base0) && DECL_P (base1)
+               /* If we compare this as integers require equal offset.  */
+               && (!INTEGRAL_TYPE_P (TREE_TYPE (@2))
+                   || known_eq (off0, off1)))
+              || (DECL_P (base0) && TREE_CODE (base1) == STRING_CST)
+              || (TREE_CODE (base0) == STRING_CST && DECL_P (base1))
+              || (TREE_CODE (base0) == STRING_CST
+                  && TREE_CODE (base1) == STRING_CST
+                  && ioff0 >= 0 && ioff1 >= 0
+                  && ioff0 < TREE_STRING_LENGTH (base0)
+                  && ioff1 < TREE_STRING_LENGTH (base1)
+                  /* This is a too conservative test that the STRING_CSTs
+                     will not end up being string-merged.  */
+                  && strncmp (TREE_STRING_POINTER (base0) + ioff0,
+                              TREE_STRING_POINTER (base1) + ioff1,
+                              MIN (TREE_STRING_LENGTH (base0) - ioff0,
+                                   TREE_STRING_LENGTH (base1) - ioff1)) != 0)))
        (switch
        (if (cmp == EQ_EXPR)
         { constant_boolean_node (false, type); })
diff --git a/gcc/testsuite/gcc.dg/pr87314-1.c b/gcc/testsuite/gcc.dg/pr87314-1.c
new file mode 100644
index 00000000000..4dc85c8eee6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr87314-1.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-original" } */
+
+int f(){ int a; return &a==(void *)"hello"; }
+int g(){ return "bye"=="hello"; }
+int h() { return "bye"=="hellobye"+5; }
+
+/* { dg-final { scan-tree-dump-times "hello" 1 "original" } } */
+/* The test in h() should be retained because the result depends on
+   string merging.  */
+/* { dg-final { scan-assembler "hello" } } */