diff --git a/qpms/indexing.h b/qpms/indexing.h
index f32679e..59e90eb 100644
--- a/qpms/indexing.h
+++ b/qpms/indexing.h
@@ -6,6 +6,7 @@
 
 #include "qpms_types.h"
 #include <math.h>
+#include "optim.h"
 
 
 
@@ -72,8 +73,8 @@ static inline qpms_errno_t qpms_uvswfi2tmn(qpms_uvswfi_t u,
 	qpms_y_sc_t y_sc = u / 4;
 	qpms_y2mn_sc_p(y_sc, m, n);
 	// Test validity
-	if (*t == 3) return QPMS_ERROR; // VSWF type code invalid, TODO WARN
-	if (*t && !y_sc) return QPMS_ERROR; // l == 0 for transversal wave, TODO WARN
+	if (QPMS_UNLIKELY(*t == 3)) return QPMS_ERROR; // VSWF type code invalid, TODO WARN
+	if (QPMS_UNLIKELY(*t && !y_sc)) return QPMS_ERROR; // l == 0 for transversal wave, TODO WARN
 	return QPMS_SUCCESS;
 }
 
@@ -83,8 +84,8 @@ static inline qpms_errno_t qpms_uvswfi2ty(qpms_uvswfi_t u,
 		qpms_vswf_type_t *t, qpms_y_t *y) {
 	*t = u & 3;
 	*y = u / 4 - 1;
-	if (*t == 0 || *t == 3) return QPMS_ERROR;
-	if (*y < 0) return QPMS_ERROR;
+	if (QPMS_UNLIKELY(*t == 0 || *t == 3)) return QPMS_ERROR;
+	if (QPMS_UNLIKELY(*y < 0)) return QPMS_ERROR;
 	return QPMS_SUCCESS;
 }
 
@@ -96,8 +97,8 @@ static inline qpms_errno_t qpms_uvswfi2ty_l(qpms_uvswfi_t u,
 		qpms_vswf_type_t *t, qpms_y_t *y) {
 	*t = u & 3;
 	*y = u / 4 - 1;
-	if (*t == 3) return QPMS_ERROR;
-	if (*y < 0) return QPMS_ERROR;
+	if (QPMS_UNLIKELY(*t == 3)) return QPMS_ERROR;
+	if (QPMS_UNLIKELY(*y < 0)) return QPMS_ERROR;
 	return QPMS_SUCCESS;
 }
 
diff --git a/qpms/normalisation.h b/qpms/normalisation.h
index 5ab7099..f84f546 100644
--- a/qpms/normalisation.h
+++ b/qpms/normalisation.h
@@ -11,11 +11,12 @@
 #include <math.h>
 #include <complex.h>
 #include "indexing.h"
+#include "optim.h"
 
 /// Returns the (real positive) common norm factor of a given normalisation compared to the reference convention.
 /** Does NOT perform the inversion if QPMS_NORMALISATION_INVERSE is set. */
 static inline double qpms_normalisation_normfactor(qpms_normalisation_t norm, qpms_l_t l, qpms_m_t m) {
-	switch (norm & QPMS_NORMALISATION_NORM_BITS) {
+	switch (QPMS_EXPECT(norm & QPMS_NORMALISATION_NORM_BITS, norm & QPMS_NORMALISATION_DEFAULT)) {
 		case QPMS_NORMALISATION_NORM_POWER:
 			return 1;
 		case QPMS_NORMALISATION_NORM_SPHARM:
@@ -37,9 +38,9 @@ static inline double qpms_normalisation_normfactor(qpms_normalisation_t norm, qp
  */
 static inline complex double qpms_normalisation_factor_M_noCS(qpms_normalisation_t norm, qpms_l_t l, qpms_m_t m) {
 	complex double fac = qpms_normalisation_normfactor(norm, l, m);
-	if (norm & QPMS_NORMALISATION_M_MINUS) fac *= -1;
-	if (norm & QPMS_NORMALISATION_M_I) fac *= I;
-	if (norm & QPMS_NORMALISATION_INVERSE) fac = 1/fac;
+	if (QPMS_UNLIKELY(norm & QPMS_NORMALISATION_M_MINUS)) fac *= -1;
+	if (QPMS_UNLIKELY(norm & QPMS_NORMALISATION_M_I)) fac *= I;
+	if (QPMS_UNLIKELY(norm & QPMS_NORMALISATION_INVERSE)) fac = 1/fac;
 	return fac;
 }
 
@@ -63,9 +64,9 @@ static inline complex double qpms_normalisation_factor_M(qpms_normalisation_t no
  */
 static inline complex double qpms_normalisation_factor_N_noCS(qpms_normalisation_t norm, qpms_l_t l, qpms_m_t m) {
 	complex double fac = qpms_normalisation_normfactor(norm, l, m);
-	if (norm & QPMS_NORMALISATION_N_MINUS) fac *= -1;
-	if (norm & QPMS_NORMALISATION_N_I) fac *= I;
-	if (norm & QPMS_NORMALISATION_INVERSE) fac = 1/fac;
+	if (QPMS_UNLIKELY(norm & QPMS_NORMALISATION_N_MINUS)) fac *= -1;
+	if (QPMS_UNLIKELY(norm & QPMS_NORMALISATION_N_I)) fac *= I;
+	if (QPMS_UNLIKELY(norm & QPMS_NORMALISATION_INVERSE)) fac = 1/fac;
 	return fac;
 }
 
@@ -96,9 +97,9 @@ static inline complex double qpms_normalisation_factor_N_M(qpms_normalisation_t
  */
 static inline complex double qpms_normalisation_factor_L_noCS(qpms_normalisation_t norm, qpms_l_t l, qpms_m_t m) {
 	complex double fac = qpms_normalisation_normfactor(norm, l, m);
-	if (norm & QPMS_NORMALISATION_L_MINUS) fac *= -1;
-	if (norm & QPMS_NORMALISATION_L_I) fac *= I;
-	if (norm & QPMS_NORMALISATION_INVERSE) fac = 1/fac;
+	if (QPMS_UNLIKELY(norm & QPMS_NORMALISATION_L_MINUS)) fac *= -1;
+	if (QPMS_UNLIKELY(norm & QPMS_NORMALISATION_L_I)) fac *= I;
+	if (QPMS_UNLIKELY(norm & QPMS_NORMALISATION_INVERSE)) fac = 1/fac;
 	return fac;
 }
 
@@ -156,7 +157,8 @@ static inline qpms_normalisation_t qpms_normalisation_dual(qpms_normalisation_t
  * \f]
  */
 static inline complex double qpms_spharm_azimuthal_part(qpms_normalisation_t norm, qpms_m_t m, double phi) {
-	switch(norm & (QPMS_NORMALISATION_REVERSE_AZIMUTHAL_PHASE | QPMS_NORMALISATION_SPHARM_REAL)) {
+	switch(QPMS_EXPECT(norm, QPMS_NORMALISATION_DEFAULT) 
+			& (QPMS_NORMALISATION_REVERSE_AZIMUTHAL_PHASE | QPMS_NORMALISATION_SPHARM_REAL)) {
 		case 0:
 			return cexp(I*m*phi);
 		case QPMS_NORMALISATION_REVERSE_AZIMUTHAL_PHASE:
@@ -194,7 +196,8 @@ static inline complex double qpms_spharm_azimuthal_part(qpms_normalisation_t nor
  */
 static inline complex double qpms_spharm_azimuthal_part_derivative_div_m(qpms_normalisation_t norm, qpms_m_t m, double phi) {
 	if(m==0) return 0;
-	switch(norm & (QPMS_NORMALISATION_REVERSE_AZIMUTHAL_PHASE | QPMS_NORMALISATION_SPHARM_REAL)) {
+	switch(QPMS_EXPECT(norm, QPMS_NORMALISATION_DEFAULT) 
+			& (QPMS_NORMALISATION_REVERSE_AZIMUTHAL_PHASE | QPMS_NORMALISATION_SPHARM_REAL)) {
 		case 0:
 			return I*cexp(I*m*phi);
 		case QPMS_NORMALISATION_REVERSE_AZIMUTHAL_PHASE:
diff --git a/qpms/optim.h b/qpms/optim.h
new file mode 100644
index 0000000..44e14c8
--- /dev/null
+++ b/qpms/optim.h
@@ -0,0 +1,22 @@
+/** \file optim.h
+ * \brief Macros for compiler optimisation.
+ */
+#ifndef QPMS_OPTIM_H
+#define QPMS_OPTIM_H
+
+
+#if ((defined __GNUC__) || (defined __clang__)) && !(defined QPMS_NO_BUILTIN_EXPECT)
+/// Wrapper over gcc's and clang's __builtin_expect.
+/** If expands to __builtin_expect if gcc or clang are used,
+ * else expands only to the first argument.
+ */
+#define QPMS_EXPECT(exp, c) __builtin_expect(exp, c)
+#define QPMS_LIKELY(x) __builtin_expect(!!(x), 1)
+#define QPMS_UNLIKELY(x) __builtin_expect(!!(x), 0)
+#else
+#define QPMS_LIKELY(x) (x)
+#define QPMS_UNLIKELY(x)
+#define QPMS_EXPECT(exp,c) (exp)
+#endif
+
+#endif // OPTIM_H