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 <jos...@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 <jos...@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 jos...@codesourcery.com
