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

[Joseph Myers <joseph at codesourcery dot com>]

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.  Applied to 
mainline.

gcc/c:
2017-09-15  Joseph Myers  <joseph@codesourcery.com>

	PR c/82071
	* 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:
2017-09-15  Joseph Myers  <joseph@codesourcery.com>

	PR c/82071
	* gcc.target/i386/excess-precision-8.c: New test.

Index: gcc/c/c-typeck.c
===================================================================
--- gcc/c/c-typeck.c	(revision 252806)
+++ gcc/c/c-typeck.c	(working copy)
@@ -4866,7 +4866,9 @@ ep_convert_and_check (location_t loc, tree type, t
   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
 	   && (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,
Index: gcc/testsuite/gcc.target/i386/excess-precision-8.c
===================================================================
--- gcc/testsuite/gcc.target/i386/excess-precision-8.c	(nonexistent)
+++ gcc/testsuite/gcc.target/i386/excess-precision-8.c	(working copy)
@@ -0,0 +1,61 @@
+/* Excess precision tests.  Test C11 semantics for conversions from
+   integers to floating point: no excess precision for either explicit
+   conversions, but excess precision for implicit conversions.  */
+/* { dg-do run } */
+/* { dg-options "-std=c11 -mfpmath=387 -fexcess-precision=standard" } */
+
+extern void abort (void);
+extern void exit (int);
+
+int
+main (void)
+{
+  float f = 1.0f;
+  int i;
+
+  i = 0x10001234;
+  if ((float) i != 0x10001240)
+    abort ();
+
+  i = 0x10001234;
+  i += f;
+  if (i != 0x10001235)
+    abort ();
+
+  i = 0x10001234;
+  i += 1.0f;
+  if (i != 0x10001235)
+    abort ();
+
+  i = 0x10001234;
+  i = i + f;
+  if (i != 0x10001235)
+    abort ();
+
+  i = 0x10001234;
+  i = i + 1.0f;
+  if (i != 0x10001235)
+    abort ();
+
+  i = 0x10001235;
+  i = (1 ? i : 1.0f);
+  if (i != 0x10001235)
+    abort ();
+
+  i = 0x10001235;
+  i = (1 ? i : f);
+  if (i != 0x10001235)
+    abort ();
+
+  i = 0x10001235;
+  i = (0 ? 1.0f :i);
+  if (i != 0x10001235)
+    abort ();
+
+  i = 0x10001235;
+  i = (0 ? f : i);
+  if (i != 0x10001235)
+    abort ();
+
+  exit (0);
+}

-- 
Joseph S. Myers
joseph@codesourcery.com