diff --git a/include/simdjson/arm64.h b/include/simdjson/arm64.h
index 2f49c963..81ad9e85 100644
--- a/include/simdjson/arm64.h
+++ b/include/simdjson/arm64.h
@@ -27,10 +27,6 @@ namespace arm64 {
 #include "simdjson/generic/atomparsing.h"
 #include "simdjson/arm64/stringparsing.h"
 #include "simdjson/arm64/numberparsing.h"
-#include "simdjson/generic/implementation_simdjson_result_base.h"
-
-// Inline definitions
-#include "simdjson/generic/implementation_simdjson_result_base-inl.h"
 #include "simdjson/arm64/end.h"
 
 #endif // SIMDJSON_IMPLEMENTATION_ARM64
diff --git a/include/simdjson/builtin.h b/include/simdjson/builtin.h
index 3879585b..2e0e1456 100644
--- a/include/simdjson/builtin.h
+++ b/include/simdjson/builtin.h
@@ -3,6 +3,37 @@
 
 #include "simdjson/implementations.h"
 
+// Determine the best builtin implementation
+#ifndef SIMDJSON_BUILTIN_IMPLEMENTATION
+#if SIMDJSON_CAN_ALWAYS_RUN_HASWELL
+#define SIMDJSON_BUILTIN_IMPLEMENTATION haswell
+#elif SIMDJSON_CAN_ALWAYS_RUN_WESTMERE
+#define SIMDJSON_BUILTIN_IMPLEMENTATION westmere
+#elif SIMDJSON_CAN_ALWAYS_RUN_ARM64
+#define SIMDJSON_BUILTIN_IMPLEMENTATION arm64
+#elif SIMDJSON_CAN_ALWAYS_RUN_PPC64
+#define SIMDJSON_BUILTIN_IMPLEMENTATION ppc64
+#elif SIMDJSON_CAN_ALWAYS_RUN_FALLBACK
+#define SIMDJSON_BUILTIN_IMPLEMENTATION fallback
+#else
+#error "All possible implementations (including fallback) have been disabled! simdjson will not run."
+#endif
+#endif // SIMDJSON_BUILTIN_IMPLEMENTATION
+
+#define SIMDJSON_IMPLEMENTATION SIMDJSON_BUILTIN_IMPLEMENTATION
+
+// ondemand is only compiled as part of the builtin implementation at present
+
+// Interface declarations
+#include "simdjson/generic/implementation_simdjson_result_base.h"
+#include "simdjson/generic/ondemand.h"
+
+// Inline definitions
+#include "simdjson/generic/implementation_simdjson_result_base-inl.h"
+#include "simdjson/generic/ondemand-inl.h"
+
+#undef SIMDJSON_IMPLEMENTATION
+
 namespace simdjson {
   /**
    * Represents the best statically linked simdjson implementation that can be used by the compiling
diff --git a/include/simdjson/fallback.h b/include/simdjson/fallback.h
index 182fa945..26a01c0d 100644
--- a/include/simdjson/fallback.h
+++ b/include/simdjson/fallback.h
@@ -24,13 +24,6 @@ namespace fallback {
 #include "simdjson/generic/atomparsing.h"
 #include "simdjson/fallback/stringparsing.h"
 #include "simdjson/fallback/numberparsing.h"
-#include "simdjson/generic/implementation_simdjson_result_base.h"
-#include "simdjson/generic/ondemand.h"
-
-// Inline definitions
-#include "simdjson/generic/implementation_simdjson_result_base-inl.h"
-#include "simdjson/generic/ondemand-inl.h"
-
 #include "simdjson/fallback/end.h"
 
 #endif // SIMDJSON_IMPLEMENTATION_FALLBACK
diff --git a/include/simdjson/haswell.h b/include/simdjson/haswell.h
index fc1541a2..48db97a0 100644
--- a/include/simdjson/haswell.h
+++ b/include/simdjson/haswell.h
@@ -5,12 +5,12 @@
 
 #if SIMDJSON_IMPLEMENTATION_HASWELL
 
-#ifndef SIMDJSON_CAN_ALWAYS_RUN_HASWELL
-#define SIMDJSON_TARGET_HASWELL SIMDJSON_TARGET_REGION("avx2,bmi,pclmul,lzcnt")
-#define SIMDJSON_UNTARGET_HASWELL SIMDJSON_UNTARGET_REGION
-#else
+#if SIMDJSON_CAN_ALWAYS_RUN_HASWELL
 #define SIMDJSON_TARGET_HASWELL
 #define SIMDJSON_UNTARGET_HASWELL
+#else
+#define SIMDJSON_TARGET_HASWELL SIMDJSON_TARGET_REGION("avx2,bmi,pclmul,lzcnt")
+#define SIMDJSON_UNTARGET_HASWELL SIMDJSON_UNTARGET_REGION
 #endif
 
 namespace simdjson {
@@ -41,13 +41,6 @@ namespace haswell {
 #include "simdjson/generic/atomparsing.h"
 #include "simdjson/haswell/stringparsing.h"
 #include "simdjson/haswell/numberparsing.h"
-#include "simdjson/generic/implementation_simdjson_result_base.h"
-#include "simdjson/generic/ondemand.h"
-
-// Inline definitions
-#include "simdjson/generic/implementation_simdjson_result_base-inl.h"
-#include "simdjson/generic/ondemand-inl.h"
-
 #include "simdjson/haswell/end.h"
 
 #endif // SIMDJSON_IMPLEMENTATION_HASWELL
diff --git a/include/simdjson/implementation-base.h b/include/simdjson/implementation-base.h
index 42710223..bcdf3ee3 100644
--- a/include/simdjson/implementation-base.h
+++ b/include/simdjson/implementation-base.h
@@ -51,21 +51,4 @@
 #endif
 #define SIMDJSON_CAN_ALWAYS_RUN_FALLBACK SIMDJSON_IMPLEMENTATION_FALLBACK
 
-// Determine the best builtin implementation
-#ifndef SIMDJSON_BUILTIN_IMPLEMENTATION
-#if SIMDJSON_CAN_ALWAYS_RUN_HASWELL
-#define SIMDJSON_BUILTIN_IMPLEMENTATION haswell
-#elif SIMDJSON_CAN_ALWAYS_RUN_WESTMERE
-#define SIMDJSON_BUILTIN_IMPLEMENTATION westmere
-#elif SIMDJSON_CAN_ALWAYS_RUN_ARM64
-#define SIMDJSON_BUILTIN_IMPLEMENTATION arm64
-#elif SIMDJSON_CAN_ALWAYS_RUN_PPC64
-#define SIMDJSON_BUILTIN_IMPLEMENTATION ppc64
-#elif SIMDJSON_CAN_ALWAYS_RUN_FALLBACK
-#define SIMDJSON_BUILTIN_IMPLEMENTATION fallback
-#else
-#error "All possible implementations (including fallback) have been disabled! simdjson will not run."
-#endif
-#endif // SIMDJSON_BUILTIN_IMPLEMENTATION
-
 #endif // SIMDJSON_IMPLEMENTATION_BASE_H
\ No newline at end of file
diff --git a/include/simdjson/implementations.h b/include/simdjson/implementations.h
index 2815d1ea..660021b4 100644
--- a/include/simdjson/implementations.h
+++ b/include/simdjson/implementations.h
@@ -1,6 +1,8 @@
 #ifndef SIMDJSON_IMPLEMENTATIONS_H
 #define SIMDJSON_IMPLEMENTATIONS_H
 
+#include "simdjson/implementation-base.h"
+
 SIMDJSON_PUSH_DISABLE_WARNINGS
 SIMDJSON_DISABLE_UNDESIRED_WARNINGS
 
@@ -10,6 +12,7 @@ SIMDJSON_DISABLE_UNDESIRED_WARNINGS
 #include "simdjson/westmere.h"
 #include "simdjson/ppc64.h"
 #include "simdjson/fallback.h"
+#include "simdjson/builtin.h"
 
 SIMDJSON_POP_DISABLE_WARNINGS
 
diff --git a/include/simdjson/ppc64.h b/include/simdjson/ppc64.h
index c8d76ca3..363c2af9 100644
--- a/include/simdjson/ppc64.h
+++ b/include/simdjson/ppc64.h
@@ -27,12 +27,6 @@ namespace ppc64 {
 #include "simdjson/generic/atomparsing.h"
 #include "simdjson/ppc64/stringparsing.h"
 #include "simdjson/ppc64/numberparsing.h"
-#include "simdjson/generic/implementation_simdjson_result_base.h"
-#include "simdjson/generic/ondemand.h"
-
-// Inline definitions
-#include "simdjson/generic/implementation_simdjson_result_base-inl.h"
-#include "simdjson/generic/ondemand-inl.h"
 #include "simdjson/ppc64/end.h"
 
 #endif // SIMDJSON_IMPLEMENTATION_PPC64
diff --git a/include/simdjson/westmere.h b/include/simdjson/westmere.h
index 3bec30f5..4419b012 100644
--- a/include/simdjson/westmere.h
+++ b/include/simdjson/westmere.h
@@ -5,12 +5,12 @@
 
 #if SIMDJSON_IMPLEMENTATION_WESTMERE
 
-#ifndef SIMDJSON_CAN_ALWAYS_RUN_WESTMERE
-#define SIMDJSON_TARGET_WESTMERE SIMDJSON_TARGET_REGION("sse4.2,pclmul")
-#define SIMDJSON_UNTARGET_WESTMERE SIMDJSON_UNTARGET_REGION
-#else
+#if SIMDJSON_CAN_ALWAYS_RUN_WESTMERE
 #define SIMDJSON_TARGET_WESTMERE
 #define SIMDJSON_UNTARGET_WESTMERE
+#else
+#define SIMDJSON_TARGET_WESTMERE SIMDJSON_TARGET_REGION("sse4.2,pclmul")
+#define SIMDJSON_UNTARGET_WESTMERE SIMDJSON_UNTARGET_REGION
 #endif
 
 namespace simdjson {
@@ -41,13 +41,6 @@ namespace westmere {
 #include "simdjson/generic/atomparsing.h"
 #include "simdjson/westmere/stringparsing.h"
 #include "simdjson/westmere/numberparsing.h"
-#include "simdjson/generic/implementation_simdjson_result_base.h"
-#include "simdjson/generic/ondemand.h"
-
-// Inline definitions
-#include "simdjson/generic/implementation_simdjson_result_base-inl.h"
-#include "simdjson/generic/ondemand-inl.h"
-
 #include "simdjson/westmere/end.h"
 
 #endif // SIMDJSON_IMPLEMENTATION_WESTMERE
diff --git a/singleheader/amalgamate.py b/singleheader/amalgamate.py
index d1c746fc..6691efec 100755
--- a/singleheader/amalgamate.py
+++ b/singleheader/amalgamate.py
@@ -83,7 +83,8 @@ def dofile(fid, prepath, filename):
     # Last lines are always ignored. Files should end by an empty lines.
     print(f"/* begin file {RELFILE} */", file=fid)
     includepattern = re.compile('^#include "(.*)"')
-    redefines_simdjson_implementation = re.compile('^#define SIMDJSON_IMPLEMENTATION (.*)')
+    redefines_simdjson_implementation = re.compile('^#define\s+SIMDJSON_IMPLEMENTATION\s+(.*)')
+    undefines_simdjson_implementation = re.compile('^#undef\s+SIMDJSON_IMPLEMENTATION\s*$')
     uses_simdjson_implementation = re.compile('SIMDJSON_IMPLEMENTATION([^_a-zA-Z0-9]|$)')
     with open(file, 'r') as fid2:
         for line in fid2:
@@ -106,6 +107,9 @@ def dofile(fid, prepath, filename):
                 if s:
                     current_implementation=s.group(1)
                     print(f"// redefining SIMDJSON_IMPLEMENTATION to \"{current_implementation}\"\n// {line}", file=fid)
+                elif undefines_simdjson_implementation.search(line):
+                    # Don't include #undef SIMDJSON_IMPLEMENTATION since we're handling it ourselves
+                    print(f"// {line}")
                 else:
                     # copy the line, with SIMDJSON_IMPLEMENTATION replace to what it is currently defined to
                     print(uses_simdjson_implementation.sub(current_implementation+"\\1",line), file=fid)
diff --git a/src/implementation.cpp b/src/implementation.cpp
index 7bd68b78..4c0017a4 100644
--- a/src/implementation.cpp
+++ b/src/implementation.cpp
@@ -155,6 +155,7 @@ simdjson_warn_unused bool validate_utf8(const char *buf, size_t len) noexcept {
 
 const implementation * builtin_implementation() {
   static const implementation * builtin_impl = available_implementations[STRINGIFY(SIMDJSON_BUILTIN_IMPLEMENTATION)];
+  assert(builtin_impl);
   return builtin_impl;
 }