[PATCH] middle-end/100786 - constant folding from incompatible alias

[Richard Biener <rguenther@suse.de>]

The following avoids us ICEing doing constant folding from variables
with aliases of different types.  The formerly used fold_convert
wasn't entirely correct even for the cases it handled and using
a VIEW_CONVERT_EXPR avoids the ICE.  Reading from a larger alias
will cause unfolded constants to appear but appearantly we handle
that just "fine".

  b.0_1 = VIEW_CONVERT_EXPR<double>(1);

There is no obvious spot and way to just disable the constant folding
so I've not attempted to do that.

Boostrapped on x86_64-unknown-linux-gnu, testing in progress.

2022-01-20  Richard Biener  <rguenther@suse.de>

	PR middle-end/100786
	* gimple-fold.cc (fold_stmt_1): Use a VIEW_CONVERT_EXPR on type
	mismatches.

	* gcc.dg/torture/pr100786.c: New testcase.
---
 gcc/gimple-fold.cc                      | 2 +-
 gcc/testsuite/gcc.dg/torture/pr100786.c | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/torture/pr100786.c

diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc
index 08d3cc214ff..a4f1fdabc18 100644
--- a/gcc/gimple-fold.cc
+++ b/gcc/gimple-fold.cc
@@ -6264,7 +6264,7 @@ fold_stmt_1 (gimple_stmt_iterator *gsi, bool inplace, tree (*valueize) (tree))
 	if (new_rhs
 	    && !useless_type_conversion_p (TREE_TYPE (lhs),
 					   TREE_TYPE (new_rhs)))
-	  new_rhs = fold_convert (TREE_TYPE (lhs), new_rhs);
+	  new_rhs = fold_build1 (VIEW_CONVERT_EXPR, TREE_TYPE (lhs), new_rhs);
 	if (new_rhs
 	    && (!inplace
 		|| get_gimple_rhs_num_ops (TREE_CODE (new_rhs)) < old_num_ops))
diff --git a/gcc/testsuite/gcc.dg/torture/pr100786.c b/gcc/testsuite/gcc.dg/torture/pr100786.c
new file mode 100644
index 00000000000..42f4e485593
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr100786.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+
+const double a = 0;
+extern int b __attribute__((alias("a")));
+void inc() { b++; }
+
+const int a2 = 0;
+extern double b2 __attribute__((alias("a2")));
+void inc2() { b2+=1; }
-- 
2.31.1