[C PATCH] Kill bogus warnings on _Atomics (PR c/60195)

[Marek Polacek]

This PR is about bogus warnings on _Atomics.  E.g., for _Atomic int,
the warning "right-hand operand of comma has no effect" makes no sense.  One
problem was that when making COMPOUND_EXPR out of atomic variable, we create
artificial variable via create_tmp_var, but we shouldn't warn on those.  So
I've set TREE_NO_WARNING on such variables.
Another issue is that we warned that a variable is unused, even when it's
actually used in __atomic_load; mark_exp_read fixed it.

Also I noticed a typo, so fixed (s/val/old/).

Regtested/bootstrapped on x86_64-linux, ok for 5.0?  (Or 4.9 instead?)

2014-02-18  Marek Polacek  <polacek@redhat.com>

	PR c/60195
c/
	* c-typeck.c (convert_lvalue_to_rvalue): Set TREE_NO_WARNING on tmp.
	Call mark_exp_read on exp.value.
	(build_atomic_assign): Set TREE_NO_WARNING on val and old.  Set
	TREE_ADDRESSABLE on old instead of val.
	(emit_side_effect_warnings): Warn only if RHS has !TREE_NO_WARNING.
testsuite/
	* gcc.dg/pr60195.c: New test.

diff --git gcc/c/c-typeck.c gcc/c/c-typeck.c
index da6a6fc..2b54290 100644
--- gcc/c/c-typeck.c
+++ gcc/c/c-typeck.c
@@ -2009,6 +2009,7 @@ convert_lvalue_to_rvalue (location_t loc, struct c_expr exp,
       tmp = create_tmp_var (nonatomic_type, NULL);
       tmp_addr = build_unary_op (loc, ADDR_EXPR, tmp, 0);
       TREE_ADDRESSABLE (tmp) = 1;
+      TREE_NO_WARNING (tmp) = 1;
 
       /* Issue __atomic_load (&expr, &tmp, SEQ_CST);  */
       fndecl = builtin_decl_explicit (BUILT_IN_ATOMIC_LOAD);
@@ -2017,6 +2018,9 @@ convert_lvalue_to_rvalue (location_t loc, struct c_expr exp,
       params->quick_push (seq_cst);
       func_call = build_function_call_vec (loc, vNULL, fndecl, params, NULL);
 
+      /* EXPR is always read.  */
+      mark_exp_read (exp.value);
+
       /* Return tmp which contains the value loaded.  */
       exp.value = build2 (COMPOUND_EXPR, nonatomic_type, func_call, tmp);
     }
@@ -3615,6 +3619,7 @@ build_atomic_assign (location_t loc, tree lhs, enum tree_code modifycode,
   nonatomic_rhs_type = build_qualified_type (rhs_type, TYPE_UNQUALIFIED);
   val = create_tmp_var (nonatomic_rhs_type, NULL);
   TREE_ADDRESSABLE (val) = 1;
+  TREE_NO_WARNING (val) = 1;
   rhs = build2 (MODIFY_EXPR, nonatomic_rhs_type, val, rhs);
   SET_EXPR_LOCATION (rhs, loc);
   add_stmt (rhs);
@@ -3643,7 +3648,8 @@ build_atomic_assign (location_t loc, tree lhs, enum tree_code modifycode,
   /* Create the variables and labels required for the op= form.  */
   old = create_tmp_var (nonatomic_lhs_type, NULL);
   old_addr = build_unary_op (loc, ADDR_EXPR, old, 0);
-  TREE_ADDRESSABLE (val) = 1;
+  TREE_ADDRESSABLE (old) = 1;
+  TREE_NO_WARNING (old) = 1;
 
   newval = create_tmp_var (nonatomic_lhs_type, NULL);
   newval_addr = build_unary_op (loc, ADDR_EXPR, newval, 0);
@@ -9661,6 +9667,7 @@ emit_side_effect_warnings (location_t loc, tree expr)
       if (!TREE_SIDE_EFFECTS (r)
 	  && !VOID_TYPE_P (TREE_TYPE (r))
 	  && !CONVERT_EXPR_P (r)
+	  && !TREE_NO_WARNING (r)
 	  && !TREE_NO_WARNING (expr))
 	warning_at (cloc, OPT_Wunused_value,
 		    "right-hand operand of comma expression has no effect");
diff --git gcc/testsuite/gcc.dg/pr60195.c gcc/testsuite/gcc.dg/pr60195.c
index e69de29..0a50a30 100644
--- gcc/testsuite/gcc.dg/pr60195.c
+++ gcc/testsuite/gcc.dg/pr60195.c
@@ -0,0 +1,56 @@
+/* PR c/60195 */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -Wpedantic -Wall" } */
+
+typedef _Atomic int atomic_int;
+
+atomic_int
+fn1 (void)
+{
+  atomic_int y = 0;
+  return y;
+}
+
+atomic_int
+fn2 (void)
+{
+  atomic_int y = 0;
+  y;
+  return y;
+}
+
+atomic_int
+fn3 (void)
+{
+  atomic_int y = 0;
+  y++;
+  return y;
+}
+
+void
+fn4 (void)
+{
+  atomic_int y;
+  y = 0;
+  (void) y;
+}
+
+void
+fn5 (void)
+{
+  atomic_int y = 0; /* { dg-warning "unused variable" } */
+}
+
+void
+fn6 (void)
+{
+  atomic_int y;  /* { dg-warning "set but not used" } */
+  y = 0;
+}
+
+void
+fn7 (void)
+{
+  atomic_int y = 0;
+  y++;
+}

	Marek