diff --git a/src/doomtype.h b/src/doomtype.h
index 198162852..fc55c9999 100644
--- a/src/doomtype.h
+++ b/src/doomtype.h
@@ -199,6 +199,7 @@ typedef int32_t boolean;
 	#if (__GNUC__ > 3) || (__GNUC__ == 3 && __GNUC_MINOR__ >= 1) // >= GCC 3.1
 		#define FUNCDEAD __attribute__ ((deprecated))
 		#define FUNCINLINE __attribute__((always_inline))
+		#define PUREFUNC __attribute__((pure))
 		#define FUNCNONNULL __attribute__((nonnull))
 	#endif
 
@@ -264,6 +265,9 @@ typedef int32_t boolean;
 #ifndef ATTRNOINLINE
 #define ATTRNOINLINE
 #endif
+#ifndef PUREFUNC
+#define PUREFUNC
+#endif
 
 /* Miscellaneous types that don't fit anywhere else (Can this be changed?) */
 
diff --git a/src/fastcmp.h b/src/fastcmp.h
index 833b75330..91ac07211 100644
--- a/src/fastcmp.h
+++ b/src/fastcmp.h
@@ -7,14 +7,14 @@ extern "C" {
 
 // returns false if s != c
 // returns true if s == c
-FUNCINLINE static ATTRINLINE boolean fasticmp(const char *s, const char *c)
+FUNCINLINE static ATTRINLINE PUREFUNC boolean fasticmp(const char *s, const char *c)
 {
 	for (; *s && toupper(*s) == toupper(*c); s++, c++) ;
 	return (*s == *c); // make sure both strings ended
 }
 
 // case-sensitive of the above
-FUNCINLINE static ATTRINLINE boolean fastcmp(const char *s, const char *c)
+FUNCINLINE static ATTRINLINE PUREFUNC boolean fastcmp(const char *s, const char *c)
 {
 	for (; *s && *s == *c; s++, c++) ;
 	return (*s == *c); // make sure both strings ended
@@ -22,7 +22,7 @@ FUNCINLINE static ATTRINLINE boolean fastcmp(const char *s, const char *c)
 
 // length-limited of the above
 // only true if both strings are at least l characters long AND match, case-sensitively!
-FUNCINLINE static ATTRINLINE boolean fastncmp(const char *s, const char *c, UINT16 l)
+FUNCINLINE static ATTRINLINE PUREFUNC boolean fastncmp(const char *s, const char *c, UINT16 l)
 {
 	for (; *s && *s == *c && --l; s++, c++) ;
 	return !l; // make sure you reached the end