PR c/106264: Silence warnings from __builtin_modf et al.

This middle-end patch resolves PR c/106264 which is a spurious warning
regression caused by the tree-level expansion of modf, frexp and remquo
producing "expression has no-effect" when the built-in function's result
is ignored.  When these built-ins were first expanded at tree-level,
fold_builtin_n would blindly set TREE_NO_WARNING for all built-ins. Now
that we're more discerning, we should precisely call suppress_warning
selectively on those COMPOUND_EXPRs that need them.

2022-07-19  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	PR c/106264
	* builtins.cc (fold_builtin_frexp): Call suppress_warning on
	COMPOUND_EXPR to silence spurious warning if result isn't used.
	(fold_builtin_modf): Likewise.
	(do_mpfr_remquo): Likewise.

gcc/testsuite/ChangeLog
	PR c/106264
	* gcc.dg/pr106264.c: New test case.

2 files changed, 40 insertions, 6 deletions

diff --git a/gcc/builtins.cc b/gcc/builtins.cc
index 35b9197..91b9c9f 100644
--- a/gcc/builtins.cc
+++ b/gcc/builtins.cc
@@ -8625,7 +8625,7 @@ fold_builtin_frexp (location_t loc, tree arg0, tree arg1, tree rettype)
   if (TYPE_MAIN_VARIANT (TREE_TYPE (arg1)) == integer_type_node)
     {
       const REAL_VALUE_TYPE *const value = TREE_REAL_CST_PTR (arg0);
-      tree frac, exp;
+      tree frac, exp, res;
 
       switch (value->cl)
       {
@@ -8656,7 +8656,9 @@ fold_builtin_frexp (location_t loc, tree arg0, tree arg1, tree rettype)
       /* Create the COMPOUND_EXPR (*arg1 = trunc, frac). */
       arg1 = fold_build2_loc (loc, MODIFY_EXPR, rettype, arg1, exp);
       TREE_SIDE_EFFECTS (arg1) = 1;
-      return fold_build2_loc (loc, COMPOUND_EXPR, rettype, arg1, frac);
+      res = fold_build2_loc (loc, COMPOUND_EXPR, rettype, arg1, frac);
+      suppress_warning (res, OPT_Wunused_value);
+      return res;
     }
 
   return NULL_TREE;
@@ -8682,6 +8684,7 @@ fold_builtin_modf (location_t loc, tree arg0, tree arg1, tree rettype)
     {
       const REAL_VALUE_TYPE *const value = TREE_REAL_CST_PTR (arg0);
       REAL_VALUE_TYPE trunc, frac;
+      tree res;
 
       switch (value->cl)
       {
@@ -8711,8 +8714,10 @@ fold_builtin_modf (location_t loc, tree arg0, tree arg1, tree rettype)
       arg1 = fold_build2_loc (loc, MODIFY_EXPR, rettype, arg1,
 			  build_real (rettype, trunc));
       TREE_SIDE_EFFECTS (arg1) = 1;
-      return fold_build2_loc (loc, COMPOUND_EXPR, rettype, arg1,
-			  build_real (rettype, frac));
+      res = fold_build2_loc (loc, COMPOUND_EXPR, rettype, arg1,
+			     build_real (rettype, frac));
+      suppress_warning (res, OPT_Wunused_value);
+      return res;
     }
 
   return NULL_TREE;
@@ -10673,8 +10678,10 @@ do_mpfr_remquo (tree arg0, tree arg1, tree arg_quo)
 						  integer_quo));
 		  TREE_SIDE_EFFECTS (result_quo) = 1;
 		  /* Combine the quo assignment with the rem.  */
-		  result = non_lvalue (fold_build2 (COMPOUND_EXPR, type,
-						    result_quo, result_rem));
+		  result = fold_build2 (COMPOUND_EXPR, type,
+					result_quo, result_rem);
+		  suppress_warning (result, OPT_Wunused_value);
+		  result = non_lvalue (result);
 		}
 	    }
 	}
diff --git a/gcc/testsuite/gcc.dg/pr106264.c b/gcc/testsuite/gcc.dg/pr106264.c
new file mode 100644
index 0000000..6b4af49
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr106264.c
@@ -0,0 +1,27 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -Wall" } */
+double frexp (double, int*);
+double modf (double, double*);
+double remquo (double, double, int*);
+
+int f (void)
+{
+  int y;
+  frexp (1.0, &y);
+  return y;
+}
+
+double g (void)
+{
+  double y;
+  modf (1.0, &y);
+  return y;
+}
+
+int h (void)
+{
+  int y;
+  remquo (1.0, 1.0, &y);
+  return y;
+}
+