diff --git a/hints/linux.sh b/hints/linux.sh
index 5202b28b099b..b3cd322dc805 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 "$optimize" || optimize='-O1'
     cccdlflags='-KPIC'
     lddlflags='-G -Bdynamic'
     # Sun C doesn't support gcc attributes, but, in many cases, doesn't
diff --git a/hints/solaris_2.sh b/hints/solaris_2.sh
index 15928c7d6d9d..e20d8d27a064 100644
--- a/hints/solaris_2.sh
+++ b/hints/solaris_2.sh
@@ -365,11 +365,7 @@ EOM
 			d_attribute_unused='undef'
 			d_attribute_warn_unused_result='undef'
 			case "$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