diff mbox

Disable floating-point contraction in ISO C conformance modes

Message ID Pine.LNX.4.64.1311061652450.13465@digraph.polyomino.org.uk
State New
Headers show

Commit Message

Joseph Myers Nov. 6, 2013, 4:53 p.m. UTC 
In <http://gcc.gnu.org/ml/gcc-patches/2013-10/msg01131.html>, I noted
that, as I'd previously noted in
<http://gcc.gnu.org/ml/gcc-patches/2010-10/msg01951.html>, ISO C
conformance modes should set -ffp-contract=on (= -ffp-contract=off)
rather than the default -ffp-contract=fast, given the ISO C
restriction to contraction taking place within source language
expressions.  No-one indicated any reason not to do this, so I've now
applied this patch to do so (and to make an explicit
-ffp-contract=fast in ISO C mode imply __GCC_IEC_559=0).  Bootstrapped
with no regressions on x86_64-unknown-linux-gnu.

c-family:
2013-11-06  Joseph Myers  <joseph@codesourcery.com>

	* c-opts.c (c_common_post_options): Set -ffp-contract=off in C
	standards modes.
	* c-cppbuiltin.c (cpp_iec_559_value): Consider -ffp-contract=fast
	to mean lack of IEEE 754 support.

testsuite:
2013-11-06  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/torture/c99-contract-1.c: New test.
diff mbox

Patch

Index: gcc/c-family/c-cppbuiltin.c
===================================================================
--- gcc/c-family/c-cppbuiltin.c	(revision 204453)
+++ gcc/c-family/c-cppbuiltin.c	(working copy)
@@ -726,16 +726,19 @@  cpp_iec_559_value (void)
     ret = 0;
 
   /* In strict C standards conformance mode, consider unpredictable
-     excess precision to mean lack of IEEE 754 support.  ??? The same
-     should apply to unpredictable contraction, but at present
-     standards conformance options do not enable conforming
-     contraction.  For C++, and outside strict conformance mode, do
-     not consider these options to mean lack of IEEE 754 support.  */
+     excess precision to mean lack of IEEE 754 support.  The same
+     applies to unpredictable contraction.  For C++, and outside
+     strict conformance mode, do not consider these options to mean
+     lack of IEEE 754 support.  */
   if (flag_iso
       && !c_dialect_cxx ()
       && TARGET_FLT_EVAL_METHOD != 0
       && flag_excess_precision_cmdline != EXCESS_PRECISION_STANDARD)
     ret = 0;
+  if (flag_iso
+      && !c_dialect_cxx ()
+      && flag_fp_contract_mode == FP_CONTRACT_FAST)
+    ret = 0;
 
   /* Various options are contrary to IEEE 754 semantics.  */
   if (flag_unsafe_math_optimizations
Index: gcc/c-family/c-opts.c
===================================================================
--- gcc/c-family/c-opts.c	(revision 204453)
+++ gcc/c-family/c-opts.c	(working copy)
@@ -827,6 +827,15 @@  c_common_post_options (const char **pfilename)
 				     ? EXCESS_PRECISION_STANDARD
 				     : EXCESS_PRECISION_FAST);
 
+  /* ISO C restricts floating-point expression contraction to within
+     source-language expressions (-ffp-contract=on, currently an alias
+     for -ffp-contract=off).  */
+  if (flag_iso
+      && !c_dialect_cxx ()
+      && (global_options_set.x_flag_fp_contract_mode
+	  == (enum fp_contract_mode) 0))
+    flag_fp_contract_mode = FP_CONTRACT_OFF;
+
   /* 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)
Index: gcc/testsuite/gcc.dg/torture/c99-contract-1.c
===================================================================
--- gcc/testsuite/gcc.dg/torture/c99-contract-1.c	(revision 0)
+++ gcc/testsuite/gcc.dg/torture/c99-contract-1.c	(revision 0)
@@ -0,0 +1,21 @@ 
+/* Test floating-point contraction occurs only within source language
+   expressions.  */
+/* { dg-do run } */
+/* { dg-options "-std=c99 -pedantic-errors" } */
+
+extern void abort (void);
+extern void exit (int);
+
+volatile float a = 1 + 0x1p-23f, b = 1 - 0x1p-23f, c = -1;
+
+int
+main (void)
+{
+  float av = a, bv = b, cv = c;
+  float p = av * bv;
+  float r = p + cv;
+  if (r == 0)
+    exit (0);
+  else
+    abort ();
+}