diff --git a/kitty/simd-string-impl.h b/kitty/simd-string-impl.h
new file mode 100644
index 000000000..44e99588d
--- /dev/null
+++ b/kitty/simd-string-impl.h
@@ -0,0 +1,63 @@
+/*
+ * Copyright (C) 2023 Kovid Goyal <kovid at kovidgoyal.net>
+ *
+ * Distributed under terms of the GPL3 license.
+ */
+
+#ifndef BITS
+#define BITS 128
+#endif
+
+#ifdef __clang__
+_Pragma("clang diagnostic push") _Pragma("clang diagnostic ignored \"-Wbitwise-instead-of-logical\"")
+#endif
+#include <simde/x86/avx2.h>
+#ifdef __clang__
+_Pragma("clang diagnostic pop")
+#endif
+
+
+#if BITS == 128
+#define FUNC(name) name##_##128
+#define integer_t __m128i
+#define set1_epi8 simde_mm_set1_epi8
+#define load_unaligned simde_mm_loadu_si128
+#define cmpeq_epi8 simde_mm_cmpeq_epi8
+#define or_si simde_mm_or_si128
+#define movemask_epi8 simde_mm_movemask_epi8
+#else
+#define FUNC(name) name##_##256
+#define integer_t __m256i
+#define set1_epi8 simde_mm256_set1_epi8
+#define load_unaligned simde_mm256_loadu_si256
+#define cmpeq_epi8 simde_mm256_cmpeq_epi8
+#define or_si simde_mm256_or_si256
+#define movemask_epi8 simde_mm256_movemask_epi8
+#endif
+
+static inline const uint8_t*
+FUNC(find_either_of_two_bytes)(const uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b) {
+    integer_t a_vec = set1_epi8(a), b_vec = set1_epi8(b);
+    for (const uint8_t* limit = haystack + sz; haystack < limit; haystack += sizeof(integer_t)) {
+        const integer_t chunk = load_unaligned((integer_t*)haystack);
+        const integer_t a_cmp = cmpeq_epi8(chunk, a_vec);
+        const integer_t b_cmp = cmpeq_epi8(chunk, b_vec);
+        const integer_t matches = or_si(a_cmp, b_cmp);
+        const int mask = movemask_epi8(matches);
+        if (mask != 0) {
+            size_t pos = __builtin_ctz(mask);
+            const uint8_t *ans = haystack + pos;
+            if (ans < limit) return ans;
+        }
+    }
+    return NULL;
+}
+
+
+#undef FUNC
+#undef integer_t
+#undef set1_epi8
+#undef load_unaligned
+#undef cmpeq_epi8
+#undef or_si
+#undef movemask_epi8
diff --git a/kitty/simd-string.c b/kitty/simd-string.c
index aeea71f4a..b7997ba11 100644
--- a/kitty/simd-string.c
+++ b/kitty/simd-string.c
@@ -9,14 +9,10 @@
 #include "data-types.h"
 #include "charsets.h"
 #include "simd-string.h"
-#ifdef __clang__
-_Pragma("clang diagnostic push") _Pragma("clang diagnostic ignored \"-Wbitwise-instead-of-logical\"")
-#endif
-#include <simde/x86/avx2.h>
-#ifdef __clang__
-_Pragma("clang diagnostic pop")
-#endif
-
+#include "simd-string-impl.h"
+#undef BITS
+#define BITS 256
+#include "simd-string-impl.h"
 static bool has_sse4_2 = false, has_avx2 = false;
 
 // find_either_of_two_bytes {{{
@@ -27,49 +23,6 @@ find_either_of_two_bytes_scalar(const uint8_t *haystack, const size_t sz, const
     }
     return NULL;
 }
-#undef SHIFT_OP
-
-#define _mm128_set1_epi8 _mm_set1_epi8
-#define _mm128_load_si128 _mm_load_si128
-#define _mm128_loadu_si128 _mm_loadu_si128
-#define _mm128_cmpeq_epi8 _mm_cmpeq_epi8
-#define _mm128_or_si128 _mm_or_si128
-#define _mm128_movemask_epi8 _mm_movemask_epi8
-#define _mm128_cmpgt_epi8 _mm_cmpgt_epi8
-#define _mm128_and_si128 _mm_and_si128
-
-#define start_simd2(bits) \
-    __m##bits##i a_vec = _mm##bits##_set1_epi8(a); \
-    __m##bits##i b_vec = _mm##bits##_set1_epi8(b); \
-    for (const uint8_t* limit = haystack + sz; haystack < limit; haystack += sizeof(__m##bits##i))
-
-#define end_simd2 \
-    if (mask != 0) { \
-        size_t pos = __builtin_ctz(mask); \
-        if (haystack + pos < limit) return haystack + pos; \
-    }
-
-#define either_of_two(bits) \
-    start_simd2(bits) { \
-        __m##bits##i chunk = _mm##bits##_loadu_si##bits((__m##bits##i*)(haystack)); \
-        __m##bits##i a_cmp = _mm##bits##_cmpeq_epi8(chunk, a_vec); \
-        __m##bits##i b_cmp = _mm##bits##_cmpeq_epi8(chunk, b_vec); \
-        __m##bits##i matches = _mm##bits##_or_si##bits(a_cmp, b_cmp); \
-        const int mask = _mm##bits##_movemask_epi8(matches); \
-        end_simd2; \
-    } return NULL;
-
-static const uint8_t*
-find_either_of_two_bytes_sse4_2(const uint8_t *haystack, size_t sz, const uint8_t a, const uint8_t b) {
-    either_of_two(128);
-}
-
-
-static const uint8_t*
-find_either_of_two_bytes_avx2(const uint8_t *haystack, size_t sz, const uint8_t a, const uint8_t b) {
-    either_of_two(256);
-}
-
 
 static const uint8_t* (*find_either_of_two_bytes_impl)(const uint8_t*, const size_t, const uint8_t, const uint8_t) = find_either_of_two_bytes_scalar;
 
@@ -113,15 +66,73 @@ utf8_decode_to_sentinel_scalar(UTF8Decoder *d, const uint8_t *src, const size_t
     return num_consumed;
 }
 
+static unsigned
+utf8_decode_to_sentinel_sse4_2(UTF8Decoder *d, const uint8_t *src, const size_t src_sz, const uint8_t sentinel) {
+    (void)d; (void)src; (void)src_sz; (void)sentinel;
+    return 0;
+}
+
+static unsigned (*utf8_decode_to_sentinel_impl)(UTF8Decoder *d, const uint8_t *src, const size_t src_sz, const uint8_t sentinel) = utf8_decode_to_sentinel_scalar;
+
 unsigned
 utf8_decode_to_sentinel(UTF8Decoder *d, const uint8_t *src, const size_t src_sz, const uint8_t sentinel) {
-    return utf8_decode_to_sentinel_scalar(d, src, src_sz, sentinel);
+    return utf8_decode_to_sentinel_impl(d, src, src_sz, sentinel);
 }
 
 // }}}
+
+// Boilerplate {{{
+static void
+test_control_byte_callback(void *l, uint8_t ch) {
+    if (!PyErr_Occurred()) {
+        RAII_PyObject(c, PyLong_FromUnsignedLong((unsigned long)ch));
+        if (c) PyList_Append((PyObject*)l, c);
+    }
+}
+
+static void
+test_output_chars_callback(void *l, const uint32_t *chars, unsigned sz) {
+    if (!PyErr_Occurred()) {
+        RAII_PyObject(c, PyUnicode_FromKindAndData(PyUnicode_4BYTE_KIND, chars, (Py_ssize_t)sz));
+        if (c) PyList_Append((PyObject*)l, c);
+    }
+}
+
+static PyObject*
+test_utf8_decode_to_sentinel(PyObject *self UNUSED, PyObject *args) {
+    const uint8_t *src; Py_ssize_t src_sz;
+    int which_function = 0;
+    static UTF8Decoder d = {0};
+    unsigned char sentinel = 0x1b;
+    if (!PyArg_ParseTuple(args, "s#|iB", &src, &src_sz, &which_function, &sentinel)) return NULL;
+    RAII_PyObject(ans, PyList_New(0));
+    d.callback_data = ans;
+    d.control_byte_callback = test_control_byte_callback;
+    d.output_chars_callback = test_output_chars_callback;
+    unsigned long consumed;
+    switch(which_function) {
+        case -1:
+            zero_at_ptr(&d); Py_RETURN_NONE;
+        case 1:
+            consumed = utf8_decode_to_sentinel_scalar(&d, src, src_sz, sentinel); break;
+        case 2:
+            consumed = utf8_decode_to_sentinel_sse4_2(&d, src, src_sz, sentinel); break;
+        default:
+            consumed = utf8_decode_to_sentinel(&d, src, src_sz, sentinel); break;
+    }
+    return Py_BuildValue("kO", consumed, ans);
+}
+// }}}
+
+static PyMethodDef module_methods[] = {
+    METHODB(test_utf8_decode_to_sentinel, METH_VARARGS),
+    {NULL, NULL, 0, NULL}        /* Sentinel */
+};
+
 bool
 init_simd(void *x) {
     PyObject *module = (PyObject*)x;
+    if (PyModule_AddFunctions(module, module_methods) != 0) return false;
 #define A(x, val) { Py_INCREF(Py_##val); if (0 != PyModule_AddObject(module, #x, Py_##val)) return false; }
 #ifdef __APPLE__
 #ifdef __arm64__
@@ -141,13 +152,14 @@ init_simd(void *x) {
 #endif
     if (has_avx2) {
         A(has_avx2, True);
-        find_either_of_two_bytes_impl = find_either_of_two_bytes_avx2;
+        find_either_of_two_bytes_impl = find_either_of_two_bytes_256;
     } else {
         A(has_avx2, False);
     }
     if (has_sse4_2) {
         A(has_sse4_2, True);
-        if (find_either_of_two_bytes_impl == find_either_of_two_bytes_scalar) find_either_of_two_bytes_impl = find_either_of_two_bytes_sse4_2;
+        if (find_either_of_two_bytes_impl == find_either_of_two_bytes_scalar) find_either_of_two_bytes_impl = find_either_of_two_bytes_128;
+        /* if (utf8_decode_to_sentinel_impl == utf8_decode_to_sentinel_scalar) utf8_decode_to_sentinel_impl = utf8_decode_to_sentinel_sse4_2; */
     } else {
         A(has_sse4_2, False);
     }