Implement C11 excess precision semantics for conversions (PR c/82071).

C11 semantics for excess precision (from N1531) are that an implicit
conversion (from the usual arithmetic conversions, not by assignment)
from integer to floating point has a result in the corresponding
evaluation format of that floating-point type, so possibly with excess
precision (whereas a cast or conversion by assignment from integer to
floating point must produce a value without excess range or precision,
as always).  This patch makes GCC support those semantics if
flag_isoc11 (which in turn means that conditional expressions need to
support generating a result with excess precision even if neither
operand had excess precision).

C99 is less than entirely clear in this regard, but my reading as
outlined at <https://gcc.gnu.org/ml/gcc-patches/2008-11/msg00105.html>
is that the results of conversions from integer to floating-point
types are always expected to be representable in the target type
without excess precision, and this patch conservatively keeps these
semantics for pre-C11 (i.e. if an older standard is explicitly
selected).

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

	PR c/82071

gcc/c:
	* c-typeck.c (ep_convert_and_check): Just call convert_and_check
	for C11.
	(build_conditional_expr): For C11, generate result with excess
	precision when one argument is an integer and the other is of a
	type using excess precision.

gcc/testsuite:
	* gcc.target/i386/excess-precision-8.c: New test.

From-SVN: r252847

1 files changed, 28 insertions, 2 deletions

diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 1956d45..73e7460 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -4866,7 +4866,9 @@ ep_convert_and_check (location_t loc, tree type, tree expr,
   if (TREE_TYPE (expr) == type)
     return expr;
 
-  if (!semantic_type)
+  /* For C11, integer conversions may have results with excess
+     precision.  */
+  if (flag_isoc11 || !semantic_type)
     return convert_and_check (loc, type, expr);
 
   if (TREE_CODE (TREE_TYPE (expr)) == INTEGER_TYPE
@@ -4994,7 +4996,31 @@ build_conditional_expr (location_t colon_loc, tree ifexp, bool ifexp_bcp,
 	   && (code2 == INTEGER_TYPE || code2 == REAL_TYPE
 	       || code2 == COMPLEX_TYPE))
     {
-      result_type = c_common_type (type1, type2);
+      /* In C11, a conditional expression between a floating-point
+	 type and an integer type should convert the integer type to
+	 the evaluation format of the floating-point type, with
+	 possible excess precision.  */
+      tree eptype1 = type1;
+      tree eptype2 = type2;
+      if (flag_isoc11)
+	{
+	  tree eptype;
+	  if (ANY_INTEGRAL_TYPE_P (type1)
+	      && (eptype = excess_precision_type (type2)) != NULL_TREE)
+	    {
+	      eptype2 = eptype;
+	      if (!semantic_result_type)
+		semantic_result_type = c_common_type (type1, type2);
+	    }
+	  else if (ANY_INTEGRAL_TYPE_P (type2)
+		   && (eptype = excess_precision_type (type1)) != NULL_TREE)
+	    {
+	      eptype1 = eptype;
+	      if (!semantic_result_type)
+		semantic_result_type = c_common_type (type1, type2);
+	    }
+	}
+      result_type = c_common_type (eptype1, eptype2);
       if (result_type == error_mark_node)
 	return error_mark_node;
       do_warn_double_promotion (result_type, type1, type2,