[PATCH v2][PR libgfortran/78314] Fix ieee_support_halting

[Szabolcs Nagy]

ieee_support_halting only checked the availability of status
flags, not trapping support.  On some targets the later can
only be checked at runtime: feenableexcept reports if
enabling traps failed.

So check trapping support by enabling/disabling it.

Updated the test that enabled trapping to check if it is
supported.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

gcc/testsuite/
2016-11-16  Szabolcs Nagy  <szabolcs.n...@arm.com>

        PR libgfortran/78314
        * gfortran.dg/ieee/ieee_6.f90: Use ieee_support_halting.

libgfortran/
2016-11-16  Szabolcs Nagy  <szabolcs.n...@arm.com>

        PR libgfortran/78314
        * config/fpu-glibc.h (support_fpu_trap): Use feenableexcept.


diff --git a/gcc/testsuite/gfortran.dg/ieee/ieee_6.f90 b/gcc/testsuite/gfortran.dg/ieee/ieee_6.f90
index 8fb4f6f..43aa3bf 100644
--- a/gcc/testsuite/gfortran.dg/ieee/ieee_6.f90
+++ b/gcc/testsuite/gfortran.dg/ieee/ieee_6.f90
@@ -9,7 +9,7 @@
   implicit none
 
   type(ieee_status_type) :: s1, s2
-  logical :: flags(5), halt(5)
+  logical :: flags(5), halt(5), haltworks
   type(ieee_round_type) :: mode
   real :: x
 
@@ -18,6 +18,7 @@
   call ieee_set_flag(ieee_all, .false.)
   call ieee_set_rounding_mode(ieee_down)
   call ieee_set_halting_mode(ieee_all, .false.)
+  haltworks = ieee_support_halting(ieee_overflow)
 
   call ieee_get_status(s1)
   call ieee_set_status(s1)
@@ -46,7 +47,7 @@
   call ieee_get_rounding_mode(mode)
   if (mode /= ieee_to_zero) call abort
   call ieee_get_halting_mode(ieee_all, halt)
-  if ((.not. halt(1)) .or. any(halt(2:))) call abort
+  if ((haltworks .and. .not. halt(1)) .or. any(halt(2:))) call abort
 
   call ieee_set_status(s2)
 
@@ -58,7 +59,7 @@
   call ieee_get_rounding_mode(mode)
   if (mode /= ieee_to_zero) call abort
   call ieee_get_halting_mode(ieee_all, halt)
-  if ((.not. halt(1)) .or. any(halt(2:))) call abort
+  if ((haltworks .and. .not. halt(1)) .or. any(halt(2:))) call abort
 
   call ieee_set_status(s1)
 
@@ -79,6 +80,6 @@
   call ieee_get_rounding_mode(mode)
   if (mode /= ieee_to_zero) call abort
   call ieee_get_halting_mode(ieee_all, halt)
-  if ((.not. halt(1)) .or. any(halt(2:))) call abort
+  if ((haltworks .and. .not. halt(1)) .or. any(halt(2:))) call abort
 
 end
diff --git a/libgfortran/config/fpu-glibc.h b/libgfortran/config/fpu-glibc.h
index 6e505da..8b29a76 100644
--- a/libgfortran/config/fpu-glibc.h
+++ b/libgfortran/config/fpu-glibc.h
@@ -121,7 +121,41 @@ get_fpu_trap_exceptions (void)
 int
 support_fpu_trap (int flag)
 {
-  return support_fpu_flag (flag);
+  int exceptions = 0;
+  int old;
+
+  if (!support_fpu_flag (flag))
+    return 0;
+
+#ifdef FE_INVALID
+  if (flag & GFC_FPE_INVALID) exceptions |= FE_INVALID;
+#endif
+
+#ifdef FE_DIVBYZERO
+  if (flag & GFC_FPE_ZERO) exceptions |= FE_DIVBYZERO;
+#endif
+
+#ifdef FE_OVERFLOW
+  if (flag & GFC_FPE_OVERFLOW) exceptions |= FE_OVERFLOW;
+#endif
+
+#ifdef FE_UNDERFLOW
+  if (flag & GFC_FPE_UNDERFLOW) exceptions |= FE_UNDERFLOW;
+#endif
+
+#ifdef FE_DENORMAL
+  if (flag & GFC_FPE_DENORMAL) exceptions |= FE_DENORMAL;
+#endif
+
+#ifdef FE_INEXACT
+  if (flag & GFC_FPE_INEXACT) exceptions |= FE_INEXACT;
+#endif
+
+  old = feenableexcept (exceptions);
+  if (old == -1)
+    return 0;
+  fedisableexcept (exceptions & ~old);
+  return 1;
 }