[Patch 5/17] Add -fpermitted-flt-eval-methods=[c11|ts-18661-3]

[James Greenhalgh <james dot greenhalgh at arm dot com>]

---
This patch was approved here:
  https://gcc.gnu.org/ml/gcc-patches/2016-09/msg02405.html

Joseph had a comment in
https://gcc.gnu.org/ml/gcc-patches/2016-10/msg00335.html that the tests
should check FLT_EVAL_METHOD from <float.h> rather than
__FLT_EVAL_METHOD__. Rather than implement that suggestion, I added tests
to patch 6 which tested the <float.h> macro, and left the tests in this
patch testing the internal macro.
---

Hi,

This option is added to control which values of FLT_EVAL_METHOD the
compiler is allowed to set.

ISO/IEC TS 18661-3 defines new permissible values for
FLT_EVAL_METHOD that indicate that operations and constants with
a semantic type that is an interchange or extended format should be
evaluated to the precision and range of that type.  These new values are
a superset of those permitted under C99/C11, which does not specify the
meaning of other positive values of FLT_EVAL_METHOD.  As such, code
conforming to C11 may not have been written expecting the possibility of
the new values.

-fpermitted-flt-eval-methods specifies whether the compiler
should allow only the values of FLT_EVAL_METHOD specified in C99/C11,
or the extended set of values specified in ISO/IEC TS 18661-3.

The two possible values this option can take are "c11" or "ts-18661-3".

The default when in a standards compliant mode (-std=c11 or similar)
is -fpermitted-flt-eval-methods=c11.  The default when in a GNU
dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3.

I've added two testcases which test that when this option, or a C standards
dialect, would restrict the range of values to {-1, 0, 1, 2}, those are
the only values we see. At this stage in the patch series this trivially
holds for all targets.

Bootstrapped on x86_64 with no issues.

Thanks,
James

---
gcc/c-family/

2016-11-09  James Greenhalgh  <james.greenhalgh@arm.com>

	* c-opts.c (c_common_post_options): Add logic to handle the default
	case for -fpermitted-flt-eval-methods.

gcc/

2016-11-09  James Greenhalgh  <james.greenhalgh@arm.com>

	* common.opt (fpermitted-flt-eval-methods): New.
	* doc/invoke.texi (-fpermitted-flt-eval-methods): Document it.
	* flag_types.h (permitted_flt_eval_methods): New.

gcc/testsuite/

2016-11-09  James Greenhalgh  <james.greenhalgh@arm.com>

	* gcc.dg/fpermitted-flt-eval-methods_1.c: New.
	* gcc.dg/fpermitted-flt-eval-methods_2.c: New.

diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index de260e7..57717ff 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -788,6 +788,18 @@ c_common_post_options (const char **pfilename)
       && flag_unsafe_math_optimizations == 0)
     flag_fp_contract_mode = FP_CONTRACT_OFF;
 
+  /* If we are compiling C, and we are outside of a standards mode,
+     we can permit the new values from ISO/IEC TS 18661-3 for
+     FLT_EVAL_METHOD.  Otherwise, we must restrict the possible values to
+     the set specified in ISO C99/C11.  */
+  if (!flag_iso
+      && !c_dialect_cxx ()
+      && (global_options_set.x_flag_permitted_flt_eval_methods
+	  == PERMITTED_FLT_EVAL_METHODS_DEFAULT))
+    flag_permitted_flt_eval_methods = PERMITTED_FLT_EVAL_METHODS_TS_18661;
+  else
+    flag_permitted_flt_eval_methods = PERMITTED_FLT_EVAL_METHODS_C11;
+
   /* By default we use C99 inline semantics in GNU99 or C99 mode.  C99
      inline semantics are not supported in GNU89 or C89 mode.  */
   if (flag_gnu89_inline == -1)
diff --git a/gcc/common.opt b/gcc/common.opt
index 314145a..915c406 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1326,6 +1326,21 @@ Enum(excess_precision) String(fast) Value(EXCESS_PRECISION_FAST)
 EnumValue
 Enum(excess_precision) String(standard) Value(EXCESS_PRECISION_STANDARD)
 
+; Whether we permit the extended set of values for FLT_EVAL_METHOD
+; introduced in ISO/IEC TS 18661-3, or limit ourselves to those in C99/C11.
+fpermitted-flt-eval-methods=
+Common Joined RejectNegative Enum(permitted_flt_eval_methods) Var(flag_permitted_flt_eval_methods) Init(PERMITTED_FLT_EVAL_METHODS_DEFAULT)
+-fpermitted-flt-eval-methods=[c11|ts-18661]	Specify which values of FLT_EVAL_METHOD are permitted.
+
+Enum
+Name(permitted_flt_eval_methods) Type(enum permitted_flt_eval_methods) UnknownError(unknown specification for the set of FLT_EVAL_METHOD values to permit %qs)
+
+EnumValue
+Enum(permitted_flt_eval_methods) String(c11) Value(PERMITTED_FLT_EVAL_METHODS_C11)
+
+EnumValue
+Enum(permitted_flt_eval_methods) String(ts-18661-3) Value(PERMITTED_FLT_EVAL_METHODS_TS_18661)
+
 ffast-math
 Common Optimization
 
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index f133b3a..75ff8ec 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -378,7 +378,8 @@ Objective-C and Objective-C++ Dialects}.
 -flto-partition=@var{alg} -fmerge-all-constants @gol
 -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves @gol
 -fmove-loop-invariants -fno-branch-count-reg @gol
--fno-defer-pop -fno-fp-int-builtin-inexact -fno-function-cse @gol
+-fno-defer-pop -fno-fp-int-builtin-inexact @gol
+-fpermitted-flt-eval-methods=@var{standard} -fno-function-cse @gol
 -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole @gol
 -fno-peephole2 -fno-sched-interblock -fno-sched-spec -fno-signed-zeros @gol
 -fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss @gol
@@ -9155,6 +9156,30 @@ Even if @option{-fno-fp-int-builtin-inexact} is used, if the functions
 generate a call to a library function then the ``inexact'' exception
 may be raised if the library implementation does not follow TS 18661.
 
+@item -fpermitted-flt-eval-methods=@var{style}
+@opindex fpermitted-flt-eval-methods
+@opindex fpermitted-flt-eval-methods=c11
+@opindex fpermitted-flt-eval-methods=ts-18661-3
+ISO/IEC TS 18661-3 defines new permissible values for
+@code{FLT_EVAL_METHOD} that indicate that operations and constants with
+a semantic type that is an interchange or extended format should be
+evaluated to the precision and range of that type.  These new values are
+a superset of those permitted under C99/C11, which does not specify the
+meaning of other positive values of @code{FLT_EVAL_METHOD}.  As such, code
+conforming to C11 may not have been written expecting the possibility of
+the new values.
+
+@option{-fpermitted-flt-eval-methods} specifies whether the compiler
+should allow only the values of @code{FLT_EVAL_METHOD} specified in C99/C11,
+or the extended set of values specified in ISO/IEC TS 18661-3.
+
+@var{style} is either @code{c11} or @code{ts-18661-3} as appropriate.
+
+The default when in a standards compliant mode (@option{-std=c11} or similar)
+is @option{-fpermitted-flt-eval-methods=c11}.  The default when in a GNU
+dialect (@option{-std=gnu11} or similar) is
+@option{-fpermitted-flt-eval-methods=ts-18661-3}.
+
 @item -fsingle-precision-constant
 @opindex fsingle-precision-constant
 Treat floating-point constants as single precision instead of
diff --git a/gcc/flag-types.h b/gcc/flag-types.h
index 6c5a4cc..d69f8f4 100644
--- a/gcc/flag-types.h
+++ b/gcc/flag-types.h
@@ -158,6 +158,14 @@ enum excess_precision
   EXCESS_PRECISION_STANDARD
 };
 
+/* The options for which values of FLT_EVAL_METHOD are permissible.  */
+enum permitted_flt_eval_methods
+{
+  PERMITTED_FLT_EVAL_METHODS_DEFAULT,
+  PERMITTED_FLT_EVAL_METHODS_TS_18661,
+  PERMITTED_FLT_EVAL_METHODS_C11
+};
+
 /* Type of stack check.  */
 enum stack_check_type
 {
diff --git a/gcc/testsuite/gcc.dg/fpermitted-flt-eval-methods_1.c b/gcc/testsuite/gcc.dg/fpermitted-flt-eval-methods_1.c
new file mode 100644
index 0000000..c022f86
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fpermitted-flt-eval-methods_1.c
@@ -0,0 +1,19 @@
+/* { dg-do run } */
+/* { dg-options "-std=c11" } */
+
+/* Test that we only see the C99/C11 values for __FLT_EVAL_METHOD__ if
+   we are compiling with -std=c11.  */
+
+int main (int argc, char** argv)
+{
+  switch (__FLT_EVAL_METHOD__)
+    {
+      case 0:
+      case 1:
+      case 2:
+      case -1:
+	return 0;
+      default:
+	return 1;
+    }
+}
diff --git a/gcc/testsuite/gcc.dg/fpermitted-flt-eval-methods_2.c b/gcc/testsuite/gcc.dg/fpermitted-flt-eval-methods_2.c
new file mode 100644
index 0000000..a76ea7e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fpermitted-flt-eval-methods_2.c
@@ -0,0 +1,19 @@
+/* { dg-do run } */
+/* { dg-options "-fpermitted-flt-eval-methods=c11" } */
+
+/* Test that we only see the C99/C11 values for __FLT_EVAL_METHOD__ if
+   we are compiling with -fpermitted-flt-eval-methods=c11.  */
+
+int main (int argc, char** argv)
+{
+  switch (__FLT_EVAL_METHOD__)
+    {
+      case 0:
+      case 1:
+      case 2:
+      case -1:
+	return 0;
+      default:
+	return 1;
+    }
+}