diff --git a/ext/POSIX/t/math.t b/ext/POSIX/t/math.t
index f3a162a99bf9..ce9090cb48e5 100644
--- a/ext/POSIX/t/math.t
+++ b/ext/POSIX/t/math.t
@@ -83,9 +83,6 @@ SKIP: {
     if ($^O =~ /VMS/) {
         skip "running in $^O, C99 math support uneven";
     }
-    if ($Config{cc} =~ /\b(?:cl|icl)/) {
-        skip "Microsoft compiler - C99 math support uneven";
-    }
 
     near(M_SQRT2, 1.4142135623731, "M_SQRT2", 1e-9);
     near(M_E, 2.71828182845905, "M_E", 1e-9);
@@ -293,7 +290,10 @@ SKIP: {
     like(NAN, qr/^NaN/, "NAN is Perl's NaN");
     cmp_ok(NAN, '!=', NAN, "NAN != NAN");
     ok(!(NAN == NAN), "NAN == NAN");
-    ok(!signbit(NAN), "signbit(NAN)");
+    # we have a fallback copysign(), but it doesn't work for NaN
+    skip('no copysign', 2) unless $Config{d_copysign};
+    ok(!signbit(copysign(NAN, 1.0)), "signbit(copysign(NAN, 1.0)))");
+    ok(signbit(copysign(NAN, -1.0)), "signbit(copysign(NAN, -1.0)))");
 }
 
 done_testing();
diff --git a/hints/linux.sh b/hints/linux.sh
index 5202b28b099b..a3e476e4f6a9 100644
--- a/hints/linux.sh
+++ b/hints/linux.sh
@@ -111,7 +111,9 @@ case "`${cc:-cc} -V 2>&1`" in
     # 'Sun' and the 'C':  Examples:
     # cc: Sun C 5.9 Linux_i386 Patch 124871-01 2007/07/31
     # cc: Sun Ceres C 5.10 Linux_i386 2008/07/10
-    test "$optimize" || optimize='-xO2'
+    # cc: Studio 12.6 Sun C 5.15 Linux_i386 2017/05/30
+    # GH #21535 - apparent optimization bug in workshop cc
+    test "$optimize" || optimize='-O1'
     cccdlflags='-KPIC'
     lddlflags='-G -Bdynamic'
     # Sun C doesn't support gcc attributes, but, in many cases, doesn't
@@ -123,6 +125,15 @@ case "`${cc:-cc} -V 2>&1`" in
     d_attribute_pure='undef'
     d_attribute_unused='undef'
     d_attribute_warn_unused_result='undef'
+    case "$cc" in
+    *c99)   # Without -Xa c99 errors on some Linux system headers
+            # in particular zero sized arrays at the end of structs
+	    case "$ccflags" in
+		*-Xa*)	;;
+		*) ccflags="$ccflags -Xa" ;;
+	    esac
+	    ;;
+    esac
     ;;
 esac
 
diff --git a/hints/solaris_2.sh b/hints/solaris_2.sh
index 15928c7d6d9d..e20d8d27a064 100644
--- a/hints/solaris_2.sh
+++ b/hints/solaris_2.sh
@@ -365,11 +365,7 @@ EOM
 			d_attribute_unused='undef'
 			d_attribute_warn_unused_result='undef'
 			case "$cc" in
-			*c99)	# c99 rejects bare '-O'.
-				case "$optimize" in
-				''|-O) optimize=-O3 ;;
-				esac
-				# Without -Xa c99 doesn't see
+			*c99)	# Without -Xa c99 doesn't see
 				# many OS interfaces.
 				case "$ccflags" in
 				*-Xa*)	;;
@@ -377,6 +373,10 @@ EOM
 				esac
 				;;
 			esac
+                        # GH #21535 - apparent optimization bug in workshop cc
+			case "$optimize" in
+			''|-O) optimize=-O1 ;;
+			esac
 			;;
 		esac
 	fi
diff --git a/win32/config.vc b/win32/config.vc
index 7ee5bd98e5d2..b22466a35c64 100644
--- a/win32/config.vc
+++ b/win32/config.vc
@@ -93,14 +93,14 @@ d__fwalk='undef'
 d_accept4='undef'
 d_access='define'
 d_accessx='undef'
-d_acosh='undef'
+d_acosh='define'
 d_aintl='undef'
 d_alarm='define'
 d_archlib='define'
 d_asctime64='undef'
 d_asctime_r='undef'
-d_asinh='undef'
-d_atanh='undef'
+d_asinh='define'
+d_atanh='define'
 d_atolf='undef'
 d_atoll='undef'
 d_attribute_always_inline='undef'
@@ -133,8 +133,8 @@ d_class='undef'
 d_clearenv='undef'
 d_closedir='define'
 d_cmsghdr_s='undef'
-d_copysign='undef'
-d_copysignl='undef'
+d_copysign='define'
+d_copysignl='define'
 d_cplusplus='undef'
 d_crypt='define'
 d_crypt_r='undef'
@@ -186,7 +186,7 @@ d_eofnblk='define'
 d_erf='undef'
 d_erfc='undef'
 d_eunice='undef'
-d_exp2='undef'
+d_exp2='define'
 d_expm1='undef'
 d_faststdio='define'
 d_fchdir='undef'
@@ -529,7 +529,7 @@ d_siginfo_si_pid='undef'
 d_siginfo_si_status='undef'
 d_siginfo_si_uid='undef'
 d_siginfo_si_value='undef'
-d_signbit='undef'
+d_signbit='define'
 d_sigprocmask='undef'
 d_sigsetjmp='undef'
 d_sin6_scope_id='define'
@@ -593,7 +593,7 @@ d_tcgetpgrp='undef'
 d_tcsetpgrp='undef'
 d_telldir='define'
 d_telldirproto='define'
-d_tgamma='undef'
+d_tgamma='define'
 d_thread_local='undef'
 d_thread_safe_nl_langinfo_l='undef'
 d_time='define'