diff --git a/acinclude.m4 b/acinclude.m4
index 1b2007c721..c6a669be7f 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -462,6 +462,8 @@ case $system in
 		MKAFS_OSTYPE=SOLARIS
                 AC_MSG_RESULT(sun4)
 		SOLARIS_PATH_CC
+		SOLARIS_CC_TAKES_XVECTOR_NONE
+		AC_SUBST(SOLARIS_CC_KOPTS)
 		SOLARIS_UFSVFS_HAS_DQRWLOCK
 		SOLARIS_FS_HAS_FS_ROLLED
 		SOLARIS_SOLOOKUP_TAKES_SOCKPARAMS
diff --git a/src/cf/osconf.m4 b/src/cf/osconf.m4
index 7e30a99ff7..39ca73e980 100644
--- a/src/cf/osconf.m4
+++ b/src/cf/osconf.m4
@@ -540,6 +540,7 @@ case $AFS_SYSNAME in
 		CFLAGS="$CFLAGS ${XARCHFLAGS}"
 		LD="/usr/ccs/bin/ld"
 		MT_CFLAGS='-mt'
+		KERN_OPTMZ="-xO3"
 		PAM_CFLAGS="-KPIC"
 		PAM_LIBS="-lc -lpam -lsocket -lnsl -lm"
 		SHLIB_CFLAGS="-KPIC"
@@ -567,10 +568,12 @@ if test "x$enable_optimize_kernel" = "x" ; then
   AS_CASE([$AFS_SYSNAME],
     [sunx86_510|sunx86_511],
       dnl Somewhere around Solaris Studio 12.*, the compiler started adding SSE
-      dnl instructions to optimized code, without any ability to turn it off.
-      dnl So just default to not optimizing kernel code for the relevant
-      dnl platforms, until we get a better autoconf test for this.
-      [enable_optimize_kernel=no],
+      dnl instructions to optimized code, without any known way to turn it off.
+      dnl To cope, this condition was added to change the default to
+      dnl 'no'.
+      dnl Now that we have an autoconf test to allow disabling the SSE
+      dnl optimizations, it's safe to once more default to 'yes' here.
+      [enable_optimize_kernel=yes],
     [enable_optimize_kernel=yes])
 fi
 
diff --git a/src/cf/solaris-test1.m4 b/src/cf/solaris-test1.m4
new file mode 100644
index 0000000000..a0b572ee26
--- /dev/null
+++ b/src/cf/solaris-test1.m4
@@ -0,0 +1,38 @@
+dnl This test is for the Solaris x86 kernel module
+dnl build.  They prevent newer Solaris compilers (12.3
+dnl and up) from using XMM registers (SIMD instructions)
+dnl and floating point registers, which are invalid in
+dnl Solaris kernel code.
+dnl Without this, Solaris may panic in libafs with:
+dnl   BAD TRAP: type=7 (#nm Device not available)
+dnl
+dnl
+AC_DEFUN([SOLARIS_CC_TAKES_XVECTOR_NONE], [
+  AC_CACHE_CHECK([if $CC accepts -xvector=%none],
+    [ac_cv_solaris_cc_takes_xvector_none],
+    [save_CFLAGS="$CFLAGS"
+     CFLAGS="$CFLAGS -xvector=%none"
+     AC_TRY_COMPILE([],
+		    [int x;],
+		    [ac_cv_solaris_cc_takes_xvector_none=yes],
+		    [ac_cv_solaris_cc_takes_xvector_none=no])
+     CFLAGS="$save_CFLAGS"
+    ])
+
+dnl -xvector=%none first appeared in Studio 11, but has only been
+dnl documented as required for Solaris x86 kernel code since Studio
+dnl 12.3.  Studio 12.3 is when the compiler started making more
+dnl aggressive optimizations by using SSE/SIMD instructions with XMM
+dnl (floating point/ SIMD) registers.  Although -xvector=%none is
+dnl required to prevent these optimizations, it is not sufficient.
+dnl Experiments have shown that -xregs=no%float is also needed to
+dnl 1) eliminate a few optimizations not squelched by -xvector=%none,
+dnl and 2) prevent actual use of floating point types in the kernel
+dnl module.  -xregs=no%float has been present since before Studio 8, so
+dnl it is safe to assume its presence when -xvector=%none is present.
+dnl
+
+  AS_IF([test "$ac_cv_solaris_cc_takes_xvector_none" = "yes"],
+        [SOLARIS_CC_KOPTS="-xvector=%none -xregs=no%float "])
+])
+
diff --git a/src/crypto/hcrypto/kernel/config.h b/src/crypto/hcrypto/kernel/config.h
index c7b22d9368..81c2944d50 100644
--- a/src/crypto/hcrypto/kernel/config.h
+++ b/src/crypto/hcrypto/kernel/config.h
@@ -97,3 +97,8 @@ static_inline int close(int d) {return -1;}
 #endif
 static_inline int gettimeofday(struct timeval *tp, void *tzp)
     {if (tp == NULL) return -1; tp->tv_sec = osi_Time(); tp->tv_usec = 0; return 0;}
+
+#ifdef AFS_SUN5_ENV
+/* workaround to allow --disable-optimize-kernel on Solaris */
+#define double int
+#endif
diff --git a/src/libafs/MakefileProto.SOLARIS.in b/src/libafs/MakefileProto.SOLARIS.in
index c9f936ff37..01f86a5fa7 100644
--- a/src/libafs/MakefileProto.SOLARIS.in
+++ b/src/libafs/MakefileProto.SOLARIS.in
@@ -51,7 +51,7 @@ KDEFS_32 =
 KDEFS_64 = -xarch=amd64 -xmodel=kernel
 
 <all>
-CFLAGS=-I. -I.. -I${TOP_OBJDIR}/src/config ${FSINCLUDES} $(DEFINES) $(KDEFS) $(KOPTS) ${DBUG}
+CFLAGS=-I. -I.. -I${TOP_OBJDIR}/src/config ${FSINCLUDES} $(DEFINES) $(KDEFS) @SOLARIS_CC_KOPTS@ ${DBUG}
 
 <sun4x_58 sunx86_58 sun4x_59 sunx86_59>
 LDFLAGS=-r -dy -N drv/ip -N drv/udp -N strmod/rpcmod