powerpc: Do not raise exception traps for fesetexcept/fesetexceptflag (BZ 30988)

According to ISO C23 (7.6.4.4), fesetexcept is supposed to set
floating-point exception flags without raising a trap (unlike
feraiseexcept, which is supposed to raise a trap if feenableexcept was
called with the appropriate argument).

This is a side-effect of how we implement the GNU extension
feenableexcept, where feenableexcept/fesetenv/fesetmode/feupdateenv
might issue prctl (PR_SET_FPEXC, PR_FP_EXC_PRECISE) depending of the
argument.  And on PR_FP_EXC_PRECISE, setting a floating-point exception
flag triggers a trap.

To make the both functions follow the C23, fesetexcept and
fesetexceptflag now fail if the argument may trigger a trap.

The math tests now check for an value different than 0, instead
of bail out as unsupported for EXCEPTION_SET_FORCES_TRAP.

Checked on powerpc64le-linux-gnu.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

4 files changed, 38 insertions, 16 deletions

diff --git a/math/test-fesetexcept-traps.c b/math/test-fesetexcept-traps.c
index 71b6e45b33..7efcd0343a 100644
--- a/math/test-fesetexcept-traps.c
+++ b/math/test-fesetexcept-traps.c
@@ -39,16 +39,24 @@ do_test (void)
       return result;
     }
 
-  if (EXCEPTION_SET_FORCES_TRAP)
-    {
-      puts ("setting exceptions traps, cannot test on this architecture");
-      return 77;
-    }
-  /* Verify fesetexcept does not cause exception traps.  */
+  /* Verify fesetexcept does not cause exception traps.  For architectures
+     where setting the exception might result in traps the function should
+     return a nonzero value.  */
   ret = fesetexcept (FE_ALL_EXCEPT);
+
+  _Static_assert (!(EXCEPTION_SET_FORCES_TRAP && !EXCEPTION_TESTS(float)),
+		  "EXCEPTION_SET_FORCES_TRAP only makes sense if the "
+		  "architecture suports exceptions");
+
   if (ret == 0)
-    puts ("fesetexcept (FE_ALL_EXCEPT) succeeded");
-  else
+    {
+      if (EXCEPTION_SET_FORCES_TRAP)
+	{
+	  puts ("unexpected fesetexcept success");
+	  result = 1;
+	}
+    }
+  else if (!EXCEPTION_SET_FORCES_TRAP)
     {
       puts ("fesetexcept (FE_ALL_EXCEPT) failed");
       if (EXCEPTION_TESTS (float))
diff --git a/math/test-fexcept-traps.c b/math/test-fexcept-traps.c
index 9701c3c320..998c241058 100644
--- a/math/test-fexcept-traps.c
+++ b/math/test-fexcept-traps.c
@@ -63,14 +63,16 @@ do_test (void)
       result = 1;
     }
 
-  if (EXCEPTION_SET_FORCES_TRAP)
-    {
-      puts ("setting exceptions traps, cannot test on this architecture");
-      return 77;
-    }
-  /* The test is that this does not cause exception traps.  */
+  /* The test is that this does not cause exception traps.  For architectures
+     where setting the exception might result in traps the function should
+     return a nonzero value.  */
   ret = fesetexceptflag (&saved, FE_ALL_EXCEPT);
-  if (ret != 0)
+
+  _Static_assert (!(EXCEPTION_SET_FORCES_TRAP && !EXCEPTION_TESTS(float)),
+		  "EXCEPTION_SET_FORCES_TRAP only makes sense if the "
+		  "architecture suports exceptions");
+
+  if (ret != 0 && !EXCEPTION_SET_FORCES_TRAP)
     {
       puts ("fesetexceptflag failed");
       result = 1;
diff --git a/sysdeps/powerpc/fpu/fesetexcept.c b/sysdeps/powerpc/fpu/fesetexcept.c
index 609a148a95..2850156d3a 100644
--- a/sysdeps/powerpc/fpu/fesetexcept.c
+++ b/sysdeps/powerpc/fpu/fesetexcept.c
@@ -31,6 +31,11 @@ fesetexcept (int excepts)
 	    & FE_INVALID_SOFTWARE));
   if (n.l != u.l)
     {
+      if (n.l & fenv_exceptions_to_reg (excepts))
+	/* Setting the exception flags may trigger a trap.  ISO C 23 § 7.6.4.4
+	    does not allow it.   */
+	return -1;
+
       fesetenv_register (n.fenv);
 
       /* Deal with FE_INVALID_SOFTWARE not being implemented on some chips.  */
diff --git a/sysdeps/powerpc/fpu/fsetexcptflg.c b/sysdeps/powerpc/fpu/fsetexcptflg.c
index 2b22f913c0..6517e8ea03 100644
--- a/sysdeps/powerpc/fpu/fsetexcptflg.c
+++ b/sysdeps/powerpc/fpu/fsetexcptflg.c
@@ -44,7 +44,14 @@ __fesetexceptflag (const fexcept_t *flagp, int excepts)
      This may cause floating-point exceptions if the restored state
      requests it.  */
   if (n.l != u.l)
-    fesetenv_register (n.fenv);
+    {
+      if (n.l & fenv_exceptions_to_reg (excepts))
+	/* Setting the exception flags may trigger a trap.  ISO C 23 § 7.6.4.4
+	    does not allow it.   */
+	return -1;
+
+      fesetenv_register (n.fenv);
+    }
 
   /* Deal with FE_INVALID_SOFTWARE not being implemented on some chips.  */
   if (flag & FE_INVALID)