diff --git a/include/simdjson.h b/include/simdjson.h
index 2aa311ed..63e90c9a 100644
--- a/include/simdjson.h
+++ b/include/simdjson.h
@@ -34,30 +34,7 @@
     }
  */
 
-#include "simdjson/compiler_check.h"
-#include "simdjson/common_defs.h"
-
-SIMDJSON_PUSH_DISABLE_WARNINGS
-SIMDJSON_DISABLE_UNDESIRED_WARNINGS
-
-// Public API
-#include "simdjson/simdjson_version.h"
-#include "simdjson/error.h"
-#include "simdjson/minify.h"
-#include "simdjson/padded_string.h"
-#include "simdjson/padded_string_view.h"
-#include "simdjson/implementation.h"
-
-// Inline functions
-#include "simdjson/error-inl.h"
-#include "simdjson/padded_string-inl.h"
-#include "simdjson/padded_string_view-inl.h"
-
-// DOM
 #include "simdjson/dom.h"
-// Implementations
-#include "simdjson/implementations.h"
-
-SIMDJSON_POP_DISABLE_WARNINGS
+#include "simdjson/builtin.h"
 
 #endif // SIMDJSON_H
diff --git a/include/simdjson/arm64.h b/include/simdjson/arm64.h
index 907dd972..81ad9e85 100644
--- a/include/simdjson/arm64.h
+++ b/include/simdjson/arm64.h
@@ -1,22 +1,7 @@
 #ifndef SIMDJSON_ARM64_H
 #define SIMDJSON_ARM64_H
 
-#ifdef SIMDJSON_FALLBACK_H
-#error "arm64.h must be included before fallback.h"
-#endif
-
-#include "simdjson/portability.h"
-
-#ifndef SIMDJSON_IMPLEMENTATION_ARM64
-#define SIMDJSON_IMPLEMENTATION_ARM64 (SIMDJSON_IS_ARM64)
-#endif
-#define SIMDJSON_CAN_ALWAYS_RUN_ARM64 SIMDJSON_IMPLEMENTATION_ARM64 && SIMDJSON_IS_ARM64
-
-
-#include "simdjson/internal/isadetection.h"
-#include "simdjson/internal/jsoncharutils_tables.h"
-#include "simdjson/internal/numberparsing_tables.h"
-#include "simdjson/internal/simdprune_tables.h"
+#include "simdjson/implementation-base.h"
 
 #if SIMDJSON_IMPLEMENTATION_ARM64
 
@@ -42,12 +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"
-#include "simdjson/generic/ondemand.h"
-
-// Inline definitions
-#include "simdjson/generic/implementation_simdjson_result_base-inl.h"
-#include "simdjson/generic/ondemand-inl.h"
 #include "simdjson/arm64/end.h"
 
 #endif // SIMDJSON_IMPLEMENTATION_ARM64
diff --git a/include/simdjson/arm64/implementation.h b/include/simdjson/arm64/implementation.h
index 2e498ff4..7214011a 100644
--- a/include/simdjson/arm64/implementation.h
+++ b/include/simdjson/arm64/implementation.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_ARM64_IMPLEMENTATION_H
 #define SIMDJSON_ARM64_IMPLEMENTATION_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include "simdjson/internal/isadetection.h"
 
 namespace simdjson {
diff --git a/include/simdjson/arm64/intrinsics.h b/include/simdjson/arm64/intrinsics.h
index ceebaf24..6f6ac4e2 100644
--- a/include/simdjson/arm64/intrinsics.h
+++ b/include/simdjson/arm64/intrinsics.h
@@ -1,8 +1,6 @@
 #ifndef SIMDJSON_ARM64_INTRINSICS_H
 #define SIMDJSON_ARM64_INTRINSICS_H
 
-#include "simdjson.h"
-
 // This should be the correct header whether
 // you use visual studio or other compilers.
 #include <arm_neon.h>
diff --git a/include/simdjson/arm64/simd.h b/include/simdjson/arm64/simd.h
index 64a3a7e1..81764cf9 100644
--- a/include/simdjson/arm64/simd.h
+++ b/include/simdjson/arm64/simd.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_ARM64_SIMD_H
 #define SIMDJSON_ARM64_SIMD_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include "simdjson/internal/simdprune_tables.h"
 #include "simdjson/arm64/bitmanipulation.h"
 #include <type_traits>
diff --git a/include/simdjson/arm64/stringparsing.h b/include/simdjson/arm64/stringparsing.h
index e1c3b4d9..08c65487 100644
--- a/include/simdjson/arm64/stringparsing.h
+++ b/include/simdjson/arm64/stringparsing.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_ARM64_STRINGPARSING_H
 #define SIMDJSON_ARM64_STRINGPARSING_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include "simdjson/arm64/simd.h"
 #include "simdjson/arm64/bitmanipulation.h"
 
diff --git a/include/simdjson/base.h b/include/simdjson/base.h
new file mode 100644
index 00000000..c142f4a3
--- /dev/null
+++ b/include/simdjson/base.h
@@ -0,0 +1,26 @@
+#ifndef SIMDJSON_BASE_H
+#define SIMDJSON_BASE_H
+
+#include "simdjson/compiler_check.h"
+#include "simdjson/common_defs.h"
+#include "simdjson/portability.h"
+
+SIMDJSON_PUSH_DISABLE_WARNINGS
+SIMDJSON_DISABLE_UNDESIRED_WARNINGS
+
+// Public API
+#include "simdjson/simdjson_version.h"
+#include "simdjson/error.h"
+#include "simdjson/minify.h"
+#include "simdjson/padded_string.h"
+#include "simdjson/padded_string_view.h"
+#include "simdjson/implementation.h"
+
+// Inline functions
+#include "simdjson/error-inl.h"
+#include "simdjson/padded_string-inl.h"
+#include "simdjson/padded_string_view-inl.h"
+
+SIMDJSON_POP_DISABLE_WARNINGS
+
+#endif // SIMDJSON_BASE_H
\ No newline at end of file
diff --git a/include/simdjson/builtin.h b/include/simdjson/builtin.h
index 9353d90c..2e0e1456 100644
--- a/include/simdjson/builtin.h
+++ b/include/simdjson/builtin.h
@@ -1,8 +1,9 @@
 #ifndef SIMDJSON_BUILTIN_H
 #define SIMDJSON_BUILTIN_H
 
-#include "simdjson/portability.h"
+#include "simdjson/implementations.h"
 
+// Determine the best builtin implementation
 #ifndef SIMDJSON_BUILTIN_IMPLEMENTATION
 #if SIMDJSON_CAN_ALWAYS_RUN_HASWELL
 #define SIMDJSON_BUILTIN_IMPLEMENTATION haswell
@@ -19,6 +20,20 @@
 #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
@@ -32,7 +47,7 @@ namespace simdjson {
    */
   namespace builtin = SIMDJSON_BUILTIN_IMPLEMENTATION;
   /**
-   * @overload simdjson::SIMDJSON_BUILTIN_IMPLEMENTATION::ondemand
+   * @copydoc simdjson::SIMDJSON_BUILTIN_IMPLEMENTATION::ondemand
    */
   namespace ondemand = SIMDJSON_BUILTIN_IMPLEMENTATION::ondemand;
   /**
diff --git a/include/simdjson/dom.h b/include/simdjson/dom.h
index eba36013..b23745c5 100644
--- a/include/simdjson/dom.h
+++ b/include/simdjson/dom.h
@@ -1,6 +1,11 @@
 #ifndef SIMDJSON_DOM_H
 #define SIMDJSON_DOM_H
 
+#include "simdjson/base.h"
+
+SIMDJSON_PUSH_DISABLE_WARNINGS
+SIMDJSON_DISABLE_UNDESIRED_WARNINGS
+
 #include "simdjson/dom/array.h"
 #include "simdjson/dom/document_stream.h"
 #include "simdjson/dom/document.h"
@@ -25,4 +30,6 @@
 #include "simdjson/internal/tape_ref-inl.h"
 #include "simdjson/dom/serialization-inl.h"
 
+SIMDJSON_POP_DISABLE_WARNINGS
+
 #endif // SIMDJSON_DOM_H
diff --git a/include/simdjson/fallback.h b/include/simdjson/fallback.h
index bea92154..26a01c0d 100644
--- a/include/simdjson/fallback.h
+++ b/include/simdjson/fallback.h
@@ -1,13 +1,7 @@
 #ifndef SIMDJSON_FALLBACK_H
 #define SIMDJSON_FALLBACK_H
 
-#include "simdjson/portability.h"
-
-// Default Fallback to on unless a builtin implementation has already been selected.
-#ifndef SIMDJSON_IMPLEMENTATION_FALLBACK
-#define SIMDJSON_IMPLEMENTATION_FALLBACK 1 // (!SIMDJSON_CAN_ALWAYS_RUN_ARM64 && !SIMDJSON_CAN_ALWAYS_RUN_HASWELL && !SIMDJSON_CAN_ALWAYS_RUN_WESTMERE && !SIMDJSON_CAN_ALWAYS_RUN_PPC64)
-#endif
-#define SIMDJSON_CAN_ALWAYS_RUN_FALLBACK SIMDJSON_IMPLEMENTATION_FALLBACK
+#include "simdjson/implementation-base.h"
 
 #if SIMDJSON_IMPLEMENTATION_FALLBACK
 
@@ -30,13 +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/fallback/bitmanipulation.h b/include/simdjson/fallback/bitmanipulation.h
index 59117ef8..c4dae452 100644
--- a/include/simdjson/fallback/bitmanipulation.h
+++ b/include/simdjson/fallback/bitmanipulation.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_FALLBACK_BITMANIPULATION_H
 #define SIMDJSON_FALLBACK_BITMANIPULATION_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include <limits>
 
 namespace simdjson {
diff --git a/include/simdjson/fallback/stringparsing.h b/include/simdjson/fallback/stringparsing.h
index 97f04fd3..8a0951ef 100644
--- a/include/simdjson/fallback/stringparsing.h
+++ b/include/simdjson/fallback/stringparsing.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_FALLBACK_STRINGPARSING_H
 #define SIMDJSON_FALLBACK_STRINGPARSING_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 
 namespace simdjson {
 namespace SIMDJSON_IMPLEMENTATION {
diff --git a/include/simdjson/generic/dom_parser_implementation.h b/include/simdjson/generic/dom_parser_implementation.h
index ab0672c6..6b8eea32 100644
--- a/include/simdjson/generic/dom_parser_implementation.h
+++ b/include/simdjson/generic/dom_parser_implementation.h
@@ -1,4 +1,4 @@
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include "simdjson/internal/isadetection.h"
 
 namespace simdjson {
diff --git a/include/simdjson/haswell.h b/include/simdjson/haswell.h
index cc93482d..48db97a0 100644
--- a/include/simdjson/haswell.h
+++ b/include/simdjson/haswell.h
@@ -1,27 +1,17 @@
 #ifndef SIMDJSON_HASWELL_H
 #define SIMDJSON_HASWELL_H
 
-#ifdef SIMDJSON_WESTMERE_H
-#error "haswell.h must be included before westmere.h"
-#endif
-#ifdef SIMDJSON_FALLBACK_H
-#error "haswell.h must be included before fallback.h"
-#endif
-
-#include "simdjson/portability.h"
-
-// Default Haswell to on if this is x86-64. Even if we're not compiled for it, it could be selected
-// at runtime.
-#ifndef SIMDJSON_IMPLEMENTATION_HASWELL
-#define SIMDJSON_IMPLEMENTATION_HASWELL (SIMDJSON_IS_X86_64)
-#endif
-// To see why  (__BMI__) && (__PCLMUL__) && (__LZCNT__) are not part of this next line, see
-// https://github.com/simdjson/simdjson/issues/1247
-#define SIMDJSON_CAN_ALWAYS_RUN_HASWELL ((SIMDJSON_IMPLEMENTATION_HASWELL) && (SIMDJSON_IS_X86_64) && (__AVX2__))
+#include "simdjson/implementation-base.h"
 
 #if SIMDJSON_IMPLEMENTATION_HASWELL
 
+#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 {
 /**
@@ -32,7 +22,7 @@ namespace haswell {
 } // namespace simdjson
 
 //
-// These two need to be included outside SIMDJSON_TARGET_REGION
+// These two need to be included outside SIMDJSON_TARGET_HASWELL
 //
 #include "simdjson/haswell/implementation.h"
 #include "simdjson/haswell/intrinsics.h"
@@ -51,13 +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/haswell/end.h b/include/simdjson/haswell/end.h
index e1e5634d..d90f3fdb 100644
--- a/include/simdjson/haswell/end.h
+++ b/include/simdjson/haswell/end.h
@@ -1,2 +1,2 @@
-SIMDJSON_UNTARGET_REGION
+SIMDJSON_UNTARGET_HASWELL
 #undef SIMDJSON_IMPLEMENTATION
diff --git a/include/simdjson/haswell/implementation.h b/include/simdjson/haswell/implementation.h
index 90c28830..99b7c97e 100644
--- a/include/simdjson/haswell/implementation.h
+++ b/include/simdjson/haswell/implementation.h
@@ -3,7 +3,7 @@
 
 #include "simdjson/implementation.h"
 
-// The constructor may be executed on any host, so we take care not to use SIMDJSON_TARGET_REGION
+// The constructor may be executed on any host, so we take care not to use SIMDJSON_TARGET_HASWELL
 namespace simdjson {
 namespace haswell {
 
diff --git a/include/simdjson/haswell/intrinsics.h b/include/simdjson/haswell/intrinsics.h
index e118c2c7..80146123 100644
--- a/include/simdjson/haswell/intrinsics.h
+++ b/include/simdjson/haswell/intrinsics.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_HASWELL_INTRINSICS_H
 #define SIMDJSON_HASWELL_INTRINSICS_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 
 #ifdef SIMDJSON_VISUAL_STUDIO
 // under clang within visual studio, this will include <x86intrin.h>
@@ -45,7 +45,7 @@ SIMDJSON_TARGET_HASWELL
 static simdjson_really_inline uint64_t _blsr_u64(uint64_t n) {
   return (n - 1) & n;
 }
-SIMDJSON_UNTARGET_REGION
+SIMDJSON_UNTARGET_HASWELL
 #endif //  _blsr_u64
 #endif // SIMDJSON_CLANG_VISUAL_STUDIO
 
diff --git a/include/simdjson/haswell/stringparsing.h b/include/simdjson/haswell/stringparsing.h
index 0141c780..ec56bc59 100644
--- a/include/simdjson/haswell/stringparsing.h
+++ b/include/simdjson/haswell/stringparsing.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_HASWELL_STRINGPARSING_H
 #define SIMDJSON_HASWELL_STRINGPARSING_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include "simdjson/haswell/simd.h"
 #include "simdjson/haswell/bitmanipulation.h"
 
diff --git a/include/simdjson/implementation-base.h b/include/simdjson/implementation-base.h
new file mode 100644
index 00000000..3d560750
--- /dev/null
+++ b/include/simdjson/implementation-base.h
@@ -0,0 +1,22 @@
+#ifndef SIMDJSON_IMPLEMENTATION_BASE_H
+#define SIMDJSON_IMPLEMENTATION_BASE_H
+
+/**
+ * @file
+ *
+ * Includes common stuff needed for implementations.
+ */
+
+#include "simdjson/base.h"
+#include "simdjson/implementation.h"
+
+// Implementation-internal files (must be included before the implementations themselves, to keep
+// amalgamation working--otherwise, the first time a file is included, it might be put inside the
+// #ifdef SIMDJSON_IMPLEMENTATION_ARM64/FALLBACK/etc., which means the other implementations can't
+// compile unless that implementation is turned on).
+#include "simdjson/internal/isadetection.h"
+#include "simdjson/internal/jsoncharutils_tables.h"
+#include "simdjson/internal/numberparsing_tables.h"
+#include "simdjson/internal/simdprune_tables.h"
+
+#endif // SIMDJSON_IMPLEMENTATION_BASE_H
\ No newline at end of file
diff --git a/include/simdjson/implementations.h b/include/simdjson/implementations.h
index 666262e4..5a1d8230 100644
--- a/include/simdjson/implementations.h
+++ b/include/simdjson/implementations.h
@@ -1,21 +1,57 @@
 #ifndef SIMDJSON_IMPLEMENTATIONS_H
 #define SIMDJSON_IMPLEMENTATIONS_H
 
-// Implementation-internal files (must be included before the implementations themselves, to keep
-// amalgamation working--otherwise, the first time a file is included, it might be put inside the
-// #ifdef SIMDJSON_IMPLEMENTATION_ARM64/FALLBACK/etc., which means the other implementations can't
-// compile unless that implementation is turned on).
-#include "simdjson/internal/isadetection.h"
-#include "simdjson/internal/jsoncharutils_tables.h"
-#include "simdjson/internal/numberparsing_tables.h"
-#include "simdjson/internal/simdprune_tables.h"
+#include "simdjson/implementation-base.h"
+
+//
+// First, figure out which implementations can be run. Doing it here makes it so we don't have to worry about the order
+// in which we include them.
+//
+
+#ifndef SIMDJSON_IMPLEMENTATION_ARM64
+#define SIMDJSON_IMPLEMENTATION_ARM64 (SIMDJSON_IS_ARM64)
+#endif
+#define SIMDJSON_CAN_ALWAYS_RUN_ARM64 SIMDJSON_IMPLEMENTATION_ARM64 && SIMDJSON_IS_ARM64
+
+// Default Haswell to on if this is x86-64. Even if we're not compiled for it, it could be selected
+// at runtime.
+#ifndef SIMDJSON_IMPLEMENTATION_HASWELL
+#define SIMDJSON_IMPLEMENTATION_HASWELL (SIMDJSON_IS_X86_64)
+#endif
+// To see why  (__BMI__) && (__PCLMUL__) && (__LZCNT__) are not part of this next line, see
+// https://github.com/simdjson/simdjson/issues/1247
+#define SIMDJSON_CAN_ALWAYS_RUN_HASWELL ((SIMDJSON_IMPLEMENTATION_HASWELL) && (SIMDJSON_IS_X86_64) && (__AVX2__))
+
+// Default Westmere to on if this is x86-64, unless we'll always select Haswell.
+#ifndef SIMDJSON_IMPLEMENTATION_WESTMERE
+#define SIMDJSON_IMPLEMENTATION_WESTMERE (SIMDJSON_IS_X86_64 && !SIMDJSON_REQUIRES_HASWELL)
+#endif
+#define SIMDJSON_CAN_ALWAYS_RUN_WESTMERE (SIMDJSON_IMPLEMENTATION_WESTMERE && SIMDJSON_IS_X86_64 && __SSE4_2__ && __PCLMUL__)
+
+#ifndef SIMDJSON_IMPLEMENTATION_PPC64
+#define SIMDJSON_IMPLEMENTATION_PPC64 (SIMDJSON_IS_PPC64)
+#endif
+#define SIMDJSON_CAN_ALWAYS_RUN_PPC64 SIMDJSON_IMPLEMENTATION_PPC64 && SIMDJSON_IS_PPC64
+
+// Default Fallback to on unless a builtin implementation has already been selected.
+#ifndef SIMDJSON_IMPLEMENTATION_FALLBACK
+#define SIMDJSON_IMPLEMENTATION_FALLBACK 1 // (!SIMDJSON_CAN_ALWAYS_RUN_ARM64 && !SIMDJSON_CAN_ALWAYS_RUN_HASWELL && !SIMDJSON_CAN_ALWAYS_RUN_WESTMERE && !SIMDJSON_CAN_ALWAYS_RUN_PPC64)
+#endif
+#define SIMDJSON_CAN_ALWAYS_RUN_FALLBACK SIMDJSON_IMPLEMENTATION_FALLBACK
+
+SIMDJSON_PUSH_DISABLE_WARNINGS
+SIMDJSON_DISABLE_UNDESIRED_WARNINGS
 
 // Implementations
 #include "simdjson/arm64.h"
-#include "simdjson/haswell.h"
-#include "simdjson/westmere.h"
-#include "simdjson/ppc64.h"
 #include "simdjson/fallback.h"
+#include "simdjson/haswell.h"
+#include "simdjson/ppc64.h"
+#include "simdjson/westmere.h"
+
+// Builtin implementation
 #include "simdjson/builtin.h"
 
+SIMDJSON_POP_DISABLE_WARNINGS
+
 #endif // SIMDJSON_IMPLEMENTATIONS_H
\ No newline at end of file
diff --git a/include/simdjson/internal/jsoncharutils_tables.h b/include/simdjson/internal/jsoncharutils_tables.h
index a53551cc..bc412b86 100644
--- a/include/simdjson/internal/jsoncharutils_tables.h
+++ b/include/simdjson/internal/jsoncharutils_tables.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_INTERNAL_JSONCHARUTILS_TABLES_H
 #define SIMDJSON_INTERNAL_JSONCHARUTILS_TABLES_H
 
-#include "simdjson.h"
+#include "simdjson/common_defs.h"
 
 #ifdef JSON_TEST_STRINGS
 void found_string(const uint8_t *buf, const uint8_t *parsed_begin,
diff --git a/include/simdjson/internal/numberparsing_tables.h b/include/simdjson/internal/numberparsing_tables.h
index 9f66b685..1762056f 100644
--- a/include/simdjson/internal/numberparsing_tables.h
+++ b/include/simdjson/internal/numberparsing_tables.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_INTERNAL_NUMBERPARSING_TABLES_H
 #define SIMDJSON_INTERNAL_NUMBERPARSING_TABLES_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 
 namespace simdjson {
 namespace internal {
diff --git a/include/simdjson/ppc64.h b/include/simdjson/ppc64.h
index 181cad21..363c2af9 100644
--- a/include/simdjson/ppc64.h
+++ b/include/simdjson/ppc64.h
@@ -1,22 +1,7 @@
 #ifndef SIMDJSON_PPC64_H
 #define SIMDJSON_PPC64_H
 
-#ifdef SIMDJSON_FALLBACK_H
-#error "ppc64.h must be included before fallback.h"
-#endif
-
-#include "simdjson/portability.h"
-
-#ifndef SIMDJSON_IMPLEMENTATION_PPC64
-#define SIMDJSON_IMPLEMENTATION_PPC64 (SIMDJSON_IS_PPC64)
-#endif
-#define SIMDJSON_CAN_ALWAYS_RUN_PPC64 SIMDJSON_IMPLEMENTATION_PPC64 && SIMDJSON_IS_PPC64
-
-
-#include "simdjson/internal/isadetection.h"
-#include "simdjson/internal/jsoncharutils_tables.h"
-#include "simdjson/internal/numberparsing_tables.h"
-#include "simdjson/internal/simdprune_tables.h"
+#include "simdjson/implementation-base.h"
 
 #if SIMDJSON_IMPLEMENTATION_PPC64
 
@@ -42,12 +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/ppc64/implementation.h b/include/simdjson/ppc64/implementation.h
index 0620f3cc..77b0e830 100644
--- a/include/simdjson/ppc64/implementation.h
+++ b/include/simdjson/ppc64/implementation.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_PPC64_IMPLEMENTATION_H
 #define SIMDJSON_PPC64_IMPLEMENTATION_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include "simdjson/internal/isadetection.h"
 
 namespace simdjson {
diff --git a/include/simdjson/ppc64/intrinsics.h b/include/simdjson/ppc64/intrinsics.h
index ab86ed84..00dc79c1 100644
--- a/include/simdjson/ppc64/intrinsics.h
+++ b/include/simdjson/ppc64/intrinsics.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_PPC64_INTRINSICS_H
 #define SIMDJSON_PPC64_INTRINSICS_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 
 // This should be the correct header whether
 // you use visual studio or other compilers.
diff --git a/include/simdjson/ppc64/simd.h b/include/simdjson/ppc64/simd.h
index 6ebecf1e..47a7b9a3 100644
--- a/include/simdjson/ppc64/simd.h
+++ b/include/simdjson/ppc64/simd.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_PPC64_SIMD_H
 #define SIMDJSON_PPC64_SIMD_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include "simdjson/internal/simdprune_tables.h"
 #include "simdjson/ppc64/bitmanipulation.h"
 #include <type_traits>
diff --git a/include/simdjson/ppc64/stringparsing.h b/include/simdjson/ppc64/stringparsing.h
index ca78df50..6654d3ff 100644
--- a/include/simdjson/ppc64/stringparsing.h
+++ b/include/simdjson/ppc64/stringparsing.h
@@ -1,7 +1,7 @@
 #ifndef SIMDJSON_PPC64_STRINGPARSING_H
 #define SIMDJSON_PPC64_STRINGPARSING_H
 
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include "simdjson/ppc64/bitmanipulation.h"
 #include "simdjson/ppc64/simd.h"
 
diff --git a/include/simdjson/westmere.h b/include/simdjson/westmere.h
index 2f202c71..4419b012 100644
--- a/include/simdjson/westmere.h
+++ b/include/simdjson/westmere.h
@@ -1,21 +1,17 @@
 #ifndef SIMDJSON_WESTMERE_H
 #define SIMDJSON_WESTMERE_H
 
-#ifdef SIMDJSON_FALLBACK_H
-#error "westmere.h must be included before fallback.h"
-#endif
-
-#include "simdjson/portability.h"
-
-// Default Westmere to on if this is x86-64, unless we'll always select Haswell.
-#ifndef SIMDJSON_IMPLEMENTATION_WESTMERE
-#define SIMDJSON_IMPLEMENTATION_WESTMERE (SIMDJSON_IS_X86_64 && !SIMDJSON_REQUIRES_HASWELL)
-#endif
-#define SIMDJSON_CAN_ALWAYS_RUN_WESTMERE (SIMDJSON_IMPLEMENTATION_WESTMERE && SIMDJSON_IS_X86_64 && __SSE4_2__ && __PCLMUL__)
+#include "simdjson/implementation-base.h"
 
 #if SIMDJSON_IMPLEMENTATION_WESTMERE
 
+#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 {
 /**
@@ -26,7 +22,7 @@ namespace westmere {
 } // namespace simdjson
 
 //
-// These two need to be included outside SIMDJSON_TARGET_REGION
+// These two need to be included outside SIMDJSON_TARGET_WESTMERE
 //
 #include "simdjson/westmere/implementation.h"
 #include "simdjson/westmere/intrinsics.h"
@@ -45,13 +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/include/simdjson/westmere/end.h b/include/simdjson/westmere/end.h
index e1e5634d..4af0dc54 100644
--- a/include/simdjson/westmere/end.h
+++ b/include/simdjson/westmere/end.h
@@ -1,2 +1,2 @@
-SIMDJSON_UNTARGET_REGION
+SIMDJSON_UNTARGET_WESTMERE
 #undef SIMDJSON_IMPLEMENTATION
diff --git a/include/simdjson/westmere/implementation.h b/include/simdjson/westmere/implementation.h
index c4573adc..42bc7657 100644
--- a/include/simdjson/westmere/implementation.h
+++ b/include/simdjson/westmere/implementation.h
@@ -3,7 +3,7 @@
 
 #include "simdjson/implementation.h"
 
-// The constructor may be executed on any host, so we take care not to use SIMDJSON_TARGET_REGION
+// The constructor may be executed on any host, so we take care not to use SIMDJSON_TARGET_WESTMERE
 namespace simdjson {
 namespace 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 4301b324..4c0017a4 100644
--- a/src/implementation.cpp
+++ b/src/implementation.cpp
@@ -1,4 +1,4 @@
-#include "simdjson.h"
+#include "simdjson/base.h"
 #include <initializer_list>
 
 namespace simdjson {
@@ -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;
 }
 
diff --git a/src/internal/error_tables.cpp b/src/internal/error_tables.cpp
index ce7da7bc..e8792603 100644
--- a/src/internal/error_tables.cpp
+++ b/src/internal/error_tables.cpp
@@ -1,4 +1,4 @@
-#include "simdjson.h"
+#include "simdjson/base.h"
 
 namespace simdjson {
 namespace internal {
diff --git a/src/internal/jsoncharutils_tables.cpp b/src/internal/jsoncharutils_tables.cpp
index 2cbe1199..deae5489 100644
--- a/src/internal/jsoncharutils_tables.cpp
+++ b/src/internal/jsoncharutils_tables.cpp
@@ -1,4 +1,4 @@
-#include "simdjson.h"
+#include "simdjson/base.h"
 
 namespace simdjson {
 namespace internal {
diff --git a/src/internal/numberparsing_tables.cpp b/src/internal/numberparsing_tables.cpp
index 4c98607c..4db818b4 100644
--- a/src/internal/numberparsing_tables.cpp
+++ b/src/internal/numberparsing_tables.cpp
@@ -1,4 +1,4 @@
-#include "simdjson.h"
+#include "simdjson/base.h"
 
 namespace simdjson {
 namespace internal {