[PATCH] [C FE] Fold trivial exprs that refer to const vars

[Patrick Palka]

There is a minor inconsistency in the folding behavior within the C
frontend.  The C frontend does not currently fold the expression "x",
where x is a const int, yet the FE does fold the expression "x + 0".

This happens because decl_constant_value is called in c_fully_fold only
while recursing over the operands of the expression being folded, i.e.
there is no top-level call to decl_constant_value to handle the case
where the expression being folded happens to be a singular expression
such as "x", as opposed to "x + 5" (where x is a const variable).

To fix this inconsistency, this patch calls decl_constant_value in
c_fully fold after folding the given expression.

Bootstrap + regtest in progress on x86_64-pc-linux-gnu, OK to commit if
testing succeeds?

gcc/c/ChangeLog:

	* c-fold.c (c_fully_fold): Call
	decl_constant_value_for_optimization after folding
	the given expression.
	* c-typeck.c (digest_init): Remove redundant call to
	decl_constant_value_for_optimization.

gcc/testsuite/ChangeLog:

	* gcc.dg/fold-const-1.c: New test.
---
 gcc/c/c-fold.c                      |  1 +
 gcc/c/c-typeck.c                    |  1 -
 gcc/testsuite/gcc.dg/fold-const-1.c | 23 +++++++++++++++++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/fold-const-1.c

diff --git a/gcc/c/c-fold.c b/gcc/c/c-fold.c
index c554e17..ab0b37f 100644
--- a/gcc/c/c-fold.c
+++ b/gcc/c/c-fold.c
@@ -88,6 +88,7 @@ c_fully_fold (tree expr, bool in_init, bool *maybe_const)
     }
   ret = c_fully_fold_internal (expr, in_init, maybe_const,
 			       &maybe_const_itself, false);
+  ret = decl_constant_value_for_optimization (ret);
   if (eptype)
     ret = fold_convert_loc (loc, eptype, ret);
   *maybe_const &= maybe_const_itself;
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index b691072..4886fc2 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -6791,7 +6791,6 @@ digest_init (location_t init_loc, tree type, tree init, tree origtype,
       inside_init = TREE_OPERAND (inside_init, 0);
     }
   inside_init = c_fully_fold (inside_init, require_constant, &maybe_const);
-  inside_init = decl_constant_value_for_optimization (inside_init);
 
   /* Initialization of an array of chars from a string constant
      optionally enclosed in braces.  */
diff --git a/gcc/testsuite/gcc.dg/fold-const-1.c b/gcc/testsuite/gcc.dg/fold-const-1.c
new file mode 100644
index 0000000..9f043b8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-const-1.c
@@ -0,0 +1,23 @@
+/* { dg-options "-O -fdump-tree-gimple -fdump-tree-ccp1" } */
+
+extern void dummy (const void *);
+
+int
+f1 (void)
+{
+  const int x = 7;
+  dummy (&x);
+  return x;
+}
+
+void foo (int);
+
+void
+f2 (void)
+{
+  const int x = 7;
+  foo (x);
+}
+
+/* { dg-final { scan-tree-dump "foo \\(7\\);" "gimple" } } */
+/* { dg-final { scan-tree-dump-times "return 7;" 2 "ccp1" } } */
-- 
2.6.3.517.g5b116b4.dirty