diff --git a/kitty/simd-string.c b/kitty/simd-string.c
index b5dea4917..43b5123b6 100644
--- a/kitty/simd-string.c
+++ b/kitty/simd-string.c
@@ -66,8 +66,8 @@ byte_loader_skip(ByteLoader *self) {
 #define prepare_for_hasvalue(n) (~0ULL/255 * (n))
 #define hasvalue(x,n) (haszero((x) ^ (n)))
 
-static uint8_t*
-find_either_of_two_bytes_simple(uint8_t *haystack, const size_t sz, const uint8_t x, const uint8_t y) {
+static const uint8_t*
+find_either_of_two_bytes_simple(const uint8_t *haystack, const size_t sz, const uint8_t x, const uint8_t y) {
     ByteLoader it; byte_loader_init(&it, (uint8_t*)haystack, sz);
 
     // first align by testing the first few bytes one at a time
@@ -79,7 +79,7 @@ find_either_of_two_bytes_simple(uint8_t *haystack, const size_t sz, const uint8_
     const BYTE_LOADER_T a = prepare_for_hasvalue(x), b = prepare_for_hasvalue(y);
     while (it.num_left) {
         if (hasvalue(it.m, a) || hasvalue(it.m, b)) {
-            uint8_t *ans = haystack + sz - it.num_left, q = hasvalue(it.m, a) ? x : y;
+            const uint8_t *ans = haystack + sz - it.num_left, q = hasvalue(it.m, a) ? x : y;
             while (it.num_left) {
                 if (byte_loader_next(&it) == q) return ans;
                 ans++;
@@ -92,43 +92,19 @@ find_either_of_two_bytes_simple(uint8_t *haystack, const size_t sz, const uint8_
 }
 #undef SHIFT_OP
 
-static uint8_t*
-find_either_of_two_bytes_sse4_2_impl(uint8_t *haystack, const uint8_t* needle_, size_t sz) {
-    const size_t extra = (uintptr_t)haystack % sizeof(__m128i);
-    if (extra) { // need aligned loads for performance so search first few bytes by hand
-        const size_t es = MIN(sz, sizeof(__m128i) - extra);
-        uint8_t *ans = find_either_of_two_bytes_simple(haystack, es, needle_[0], needle_[1]);
-        if (ans) return ans;
-        sz -= es;
-        haystack += es;
-        if (!sz) return NULL;
-    }
-    const __m128i needle = _mm_load_si128((const __m128i *)needle_);
-    for (const uint8_t* limit = haystack + sz; haystack < limit; haystack += 16) {
-        const __m128i h = _mm_load_si128((const __m128i *)haystack);
-        int c = _mm_cmpistri(needle, h, _SIDD_CMP_EQUAL_ANY);
-        if (c != 16 && haystack + c < limit) {
-            return haystack + c;
-        }
-    }
-    return NULL;
-}
-
-static uint8_t*
-find_either_of_two_bytes_sse4_2(uint8_t *haystack, const size_t sz, const uint8_t x, const uint8_t y) {
-    uint8_t before = haystack[sz];
-    haystack[sz] = 0;
-    uint8_t needle[16] = {x, y, 0,0,0,0,0,0,0,0,0,0,0,0,0,0};
-    uint8_t *ans = find_either_of_two_bytes_sse4_2_impl(haystack, needle, sz);
-    haystack[sz] = before;
-    return ans;
-}
+#define _mm128_set1_epi8 _mm_set1_epi8
+#define _mm128_load_si128 _mm_load_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, aligner) \
     const size_t extra = (uintptr_t)haystack % sizeof(__m##bits##i); \
     if (extra) { /* do aligned loading */ \
         size_t es = MIN(sz, sizeof(__m##bits##i) - extra); \
-        uint8_t *ans = aligner; \
+        const uint8_t *ans = aligner; \
         if (ans) return ans; \
         sz -= es; \
         haystack += es; \
@@ -154,23 +130,29 @@ find_either_of_two_bytes_sse4_2(uint8_t *haystack, const size_t sz, const uint8_
         end_simd2; \
     } return NULL;
 
-static uint8_t*
-find_either_of_two_bytes_avx2(uint8_t *haystack, size_t sz, const uint8_t a, const uint8_t b) {
+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, find_either_of_two_bytes_simple(haystack, es, a, b));
+}
+
+
+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, (has_sse4_2 && es > 15) ? find_either_of_two_bytes_sse4_2(haystack, es, a, b) : find_either_of_two_bytes_simple(haystack, es, a, b));
 }
 
 
-static uint8_t* (*find_either_of_two_bytes_impl)(uint8_t*, const size_t, const uint8_t, const uint8_t) = find_either_of_two_bytes_simple;
+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_simple;
 
-uint8_t*
-find_either_of_two_bytes(uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b) {
+const uint8_t*
+find_either_of_two_bytes(const uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b) {
     return (uint8_t*)find_either_of_two_bytes_impl(haystack, sz, a, b);
 }
 // }}}
 
 // find_byte_not_in_range {{{
-static uint8_t*
-find_byte_not_in_range_simple(uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b) {
+static const uint8_t*
+find_byte_not_in_range_simple(const uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b) {
     ByteLoader it; byte_loader_init(&it, haystack, sz);
     while (it.num_left) {
         const uint8_t ch = byte_loader_next(&it);
@@ -179,59 +161,31 @@ find_byte_not_in_range_simple(uint8_t *haystack, const size_t sz, const uint8_t
     return NULL;
 }
 
-static uint8_t*
-find_byte_not_in_range_sse4_2_impl(uint8_t *haystack, const uint8_t* needle_, size_t sz) {
-    const size_t extra = (uintptr_t)haystack % sizeof(__m128i);
-    if (extra) { // need aligned loads for performance so search first few bytes by hand
-        size_t es = MIN(sz, sizeof(__m128i) - extra);
-        uint8_t *ans = find_byte_not_in_range_simple(haystack, es, needle_[0], needle_[1]);
-        if (ans) return ans;
-        sz -= es;
-        haystack += es;
-        if (!sz) return NULL;
-    }
-    const __m128i needle = _mm_load_si128((const __m128i *)needle_);
-    for (const uint8_t* limit = haystack + sz; haystack < limit; haystack += sizeof(__m128i)) {
-        const __m128i h = _mm_load_si128((const __m128i *)haystack);
-        int c = _mm_cmpistri(needle, h, _SIDD_CMP_RANGES | _SIDD_NEGATIVE_POLARITY);
-        if (c != 16 && haystack + c < limit) {
-            return haystack + c;
-        }
-    }
-    return NULL;
-}
-
-
-static uint8_t*
-find_byte_not_in_range_sse4_2(uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b) {
-    uint8_t before = haystack[sz];
-    haystack[sz] = 0;
-    uint8_t needle[16] = {a, b, 0, 0, 0,0,0,0,0,0,0,0,0,0,0,0};
-    uint8_t *ans = (uint8_t*)find_byte_not_in_range_sse4_2_impl((uint8_t*)haystack, needle, sz);
-    haystack[sz] = before;
-    return ans;
-
-}
-
 #define not_in_range(bits, aligner) \
     start_simd2(bits, aligner) { \
-        __m256i chunk = _mm256_load_si256((__m256i*)(haystack)); \
-        __m256i above_lower = _mm256_cmpgt_epi8(chunk, a_vec); \
-        __m256i below_upper = _mm256_cmpgt_epi8(b_vec, chunk); \
-        __m256i in_range = _mm256_and_si256(above_lower, below_upper); \
-        const int mask = ~_mm256_movemask_epi8(in_range); /* ~ as we want not in range */ \
+        __m##bits##i chunk = _mm##bits##_load_si##bits((__m##bits##i*)(haystack)); \
+        __m##bits##i above_lower = _mm##bits##_cmpgt_epi8(chunk, a_vec); \
+        __m##bits##i below_upper = _mm##bits##_cmpgt_epi8(b_vec, chunk); \
+        __m##bits##i in_range = _mm##bits##_and_si##bits(above_lower, below_upper); \
+        const int mask = ~_mm##bits##_movemask_epi8(in_range); /* ~ as we want not in range */ \
         end_simd2; \
     } return NULL;
 
-static uint8_t*
-find_byte_not_in_range_avx2(uint8_t *haystack, size_t sz, const uint8_t a, const uint8_t b) {
+static const uint8_t*
+find_byte_not_in_range_sse4_2(const uint8_t *haystack, size_t sz, const uint8_t a, const uint8_t b) {
+    not_in_range(128, find_byte_not_in_range_simple(haystack, es, a, b));
+}
+
+
+static const uint8_t*
+find_byte_not_in_range_avx2(const uint8_t *haystack, size_t sz, const uint8_t a, const uint8_t b) {
     not_in_range(256, (has_sse4_2 && extra > 15) ? find_byte_not_in_range_sse4_2(haystack, es, a, b) : find_byte_not_in_range_simple(haystack, es, a, b));
 }
 
-static uint8_t* (*find_byte_not_in_range_impl)(uint8_t *haystack, size_t sz, const uint8_t a, const uint8_t b) = find_byte_not_in_range_simple;
+static const uint8_t* (*find_byte_not_in_range_impl)(const uint8_t *haystack, size_t sz, const uint8_t a, const uint8_t b) = find_byte_not_in_range_simple;
 
-uint8_t*
-find_byte_not_in_range(uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b) {
+const uint8_t*
+find_byte_not_in_range(const uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b) {
     return (uint8_t*)find_byte_not_in_range_impl(haystack, sz, a, b);
 }
 // }}}
diff --git a/kitty/simd-string.h b/kitty/simd-string.h
index 7b7c1fb5f..a8450d8ba 100644
--- a/kitty/simd-string.h
+++ b/kitty/simd-string.h
@@ -8,6 +8,7 @@
 
 #include <stdint.h>
 #include <stddef.h>
+#include <stdbool.h>
 
 #define BYTE_LOADER_T unsigned long long
 typedef struct ByteLoader {
@@ -24,13 +25,12 @@ uint8_t byte_loader_next(ByteLoader *self);
 // Pass a PyModule PyObject* as the argument. Must be called once at application startup
 bool init_simd(void* module);
 
-// Requires haystack[sz] to be writable and 7 bytes to the left of haystack to
-// be readable. Returns pointer to first position in haystack that contains
-// either of the two chars or NULL if not found.
-uint8_t* find_either_of_two_bytes(uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b);
+// Requires 7 bytes to the left of haystack to be readable. Returns pointer to
+// first position in haystack that contains either of the two chars or NULL if
+// not found.
+const uint8_t* find_either_of_two_bytes(const uint8_t *haystack, const size_t sz, const uint8_t a, const uint8_t b);
 
-// Requires haystack[sz] to be writable and 7 bytes to the left of haystack to
-// be readable. Returns pointer to first position in haystack that contains
-// a char that is not in [a, b]. a must be <= b
-uint8_t*
-find_byte_not_in_range(uint8_t *haystack, const size_t sz, const uint8_t a1, const uint8_t b);
+// Requires 7 bytes to the left of haystack to be readable. Returns pointer to
+// first position in haystack that contains a char that is not in [a, b].
+// a must be <= b
+const uint8_t* find_byte_not_in_range(const uint8_t *haystack, const size_t sz, const uint8_t a1, const uint8_t b);
diff --git a/kitty/vt-parser.c b/kitty/vt-parser.c
index 766df6121..4047631c5 100644
--- a/kitty/vt-parser.c
+++ b/kitty/vt-parser.c
@@ -284,7 +284,7 @@ consume_normal(PS *self) {
     do {
         if (self->utf8.state == UTF8_ACCEPT) {
             size_t sz = self->read.sz - self->read.pos;
-            uint8_t *p = find_byte_not_in_range(self->buf + self->read.pos, sz, 32, 126);
+            const uint8_t *p = find_byte_not_in_range(self->buf + self->read.pos, sz, 32, 126);
             if (p != NULL) sz = p - (self->buf + self->read.pos);
             if (sz) dispatch_printable_ascii(self, sz);
             else dispatch_normal_mode_byte(self, self->buf[self->read.pos++]);
@@ -408,7 +408,7 @@ consume_esc(PS *self) {
 static bool
 find_st_terminator(PS *self, size_t *end_pos) {
     const size_t sz = self->read.sz - self->read.pos;
-    uint8_t *q = find_either_of_two_bytes(self->buf + self->read.pos, sz, BEL, ESC_ST);
+    const uint8_t *q = find_either_of_two_bytes(self->buf + self->read.pos, sz, BEL, ESC_ST);
     if (q == NULL) {
         self->read.pos += sz;
         return false;
@@ -1459,7 +1459,7 @@ consume_input(PS *self) {
 static bool
 find_pending_stop_csi(PS *self) {
     const size_t sz = self->read.sz - self->read.pos;
-    uint8_t *q = find_either_of_two_bytes(self->buf + self->read.pos, sz, ESC, 'l');
+    const uint8_t *q = find_either_of_two_bytes(self->buf + self->read.pos, sz, ESC, 'l');
     if (q == NULL) {
         self->read.pos += sz;
         return false;