diff --git a/CMakeLists.txt b/CMakeLists.txt
index 54ae1b1a..500b6a89 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -35,6 +35,18 @@ set(LLHTTP_SOURCES
   ${CMAKE_CURRENT_SOURCE_DIR}/src/api.c
 )
 
+# On x86-64, compile a second instantiation of the parser with SSE4.2 enabled so
+# llhttp_execute() can select it at runtime (see src/api.c). The default build
+# passes no -march, so the SSE4.2 span scanners in the generated code are
+# otherwise never compiled in. Skipped on MSVC (no __builtin_cpu_supports).
+if(NOT MSVC AND CMAKE_SYSTEM_PROCESSOR MATCHES "^(x86_64|amd64|AMD64)$")
+  list(APPEND LLHTTP_SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/src/llhttp_sse42.c)
+  set_source_files_properties(
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/llhttp_sse42.c
+    PROPERTIES COMPILE_OPTIONS "-msse4.2"
+  )
+endif()
+
 set(LLHTTP_HEADERS
   ${CMAKE_CURRENT_SOURCE_DIR}/include/llhttp.h
 )
diff --git a/Makefile b/Makefile
index 16fe819f..f92fbe65 100644
--- a/Makefile
+++ b/Makefile
@@ -11,6 +11,15 @@ endif
 
 INCLUDES += -Ibuild/
 
+# On x86-64, also build a second instantiation of the generated parser with
+# -msse4.2 so llhttp_execute() can select the SSE4.2 span scanners at runtime
+# (see src/native/api.c). The default build passes no -march, so without this
+# the SSE4.2 code generated under `#ifdef __SSE4_2__` is never compiled in.
+ARCH ?= $(shell uname -m)
+ifneq (,$(filter $(ARCH),x86_64 amd64 AMD64))
+	SSE42_OBJ = build/c/llhttp_sse42.o
+endif
+
 INSTALL ?= install
 PREFIX ?= /usr/local
 LIBDIR = $(PREFIX)/lib
@@ -23,17 +32,20 @@ clean:
 	rm -rf build/
 	rm -rf test/tmp
 
-build/libllhttp.so: build/c/llhttp.o build/native/api.o \
+build/libllhttp.so: build/c/llhttp.o $(SSE42_OBJ) build/native/api.o \
 		build/native/http.o
 	$(CLANG) -shared $^ -o $@
 
-build/libllhttp.a: build/c/llhttp.o build/native/api.o \
+build/libllhttp.a: build/c/llhttp.o $(SSE42_OBJ) build/native/api.o \
 		build/native/http.o
-	$(AR) rcs $@ build/c/llhttp.o build/native/api.o build/native/http.o
+	$(AR) rcs $@ build/c/llhttp.o $(SSE42_OBJ) build/native/api.o build/native/http.o
 
 build/c/llhttp.o: build/c/llhttp.c
 	$(CLANG) $(CFLAGS) $(INCLUDES) -c $< -o $@
 
+build/c/llhttp_sse42.o: build/c/llhttp_sse42.c build/c/llhttp.c
+	$(CLANG) $(CFLAGS) -msse4.2 $(INCLUDES) -c $< -o $@
+
 build/native/%.o: src/native/%.c build/llhttp.h src/native/api.h \
 		build/native
 	$(CLANG) $(CFLAGS) $(INCLUDES) -c $< -o $@
@@ -52,6 +64,8 @@ release: clean all generate
 	mkdir -p release/include
 	cp -rf build/llhttp.h release/include/
 	cp -rf build/c/llhttp.c release/src/
+	cp -rf build/c/llhttp_sse42.c release/src/
+	cp -rf build/c/llhttp_sse42_rename.h release/src/
 	cp -rf src/native/*.c release/src/
 	cp -rf src/llhttp.gyp release/
 	cp -rf src/common.gypi release/
diff --git a/bin/generate.ts b/bin/generate.ts
old mode 100755
new mode 100644
index 727269dd..dbb4c993
--- a/bin/generate.ts
+++ b/bin/generate.ts
@@ -11,6 +11,8 @@ const semverRE = /^(?<major>0|[1-9]\d*)\.(?<minor>0|[1-9]\d*)\.(?<patch>0|[1-9]\
 
 const C_FILE = resolve(__dirname, '../build/c/llhttp.c');
 const HEADER_FILE = resolve(__dirname, '../build/llhttp.h');
+const SSE42_RENAME_FILE = resolve(__dirname, '../build/c/llhttp_sse42_rename.h');
+const SSE42_C_FILE = resolve(__dirname, '../build/c/llhttp_sse42.c');
 
 const pkg = JSON.parse(
   readFileSync(resolve(__dirname, '..', 'package.json')).toString(),
@@ -49,3 +51,32 @@ ${nativeHeaders}
 mkdirSync(dirname(C_FILE), { recursive: true });
 writeFileSync(HEADER_FILE, headers);
 writeFileSync(C_FILE, generated.c);
+
+// A second instantiation of the generated parser, compiled with -msse4.2, lets
+// llhttp_execute() use the SSE4.2 span scanners at runtime on default x86-64
+// builds (which pass no -march, so the `#ifdef __SSE4_2__` code is otherwise
+// dead). The parser defines symbols prefixed `llhttp__internal`; the callbacks
+// it imports are prefixed `llhttp__on`/`llhttp__before`/`llhttp__after` and stay
+// shared. Rename only the defined `llhttp__internal*` symbols so the second copy
+// links beside the first. Derived from the generated source, so it stays correct
+// across grammar changes.
+const internalSymbols = Array.from(
+  new Set(generated.c.match(/\bllhttp__internal[0-9A-Za-z_]*/g) ?? []),
+).sort();
+const renameHeader = `/* Generated by bin/generate.ts. Renames the defined symbols of llhttp.c so a
+ * second, -msse4.2 instantiation (llhttp_sse42.c) can link beside the first. */
+#ifndef INCLUDE_LLHTTP_SSE42_RENAME_H_
+#define INCLUDE_LLHTTP_SSE42_RENAME_H_
+${internalSymbols.map((s) => `#define ${s} ${s}_sse42`).join('\n')}
+#endif  /* INCLUDE_LLHTTP_SSE42_RENAME_H_ */
+`;
+const sse42Wrapper = `/* Generated by bin/generate.ts. See src/native/api.c: a second instantiation of
+ * the generated parser with the SSE4.2 span scanners enabled, selected once at
+ * runtime by llhttp_execute(). Built with -msse4.2 by the build system. */
+#if defined(__x86_64__) || defined(_M_X64)
+#include "llhttp_sse42_rename.h"
+#include "llhttp.c"
+#endif
+`;
+writeFileSync(SSE42_RENAME_FILE, renameHeader);
+writeFileSync(SSE42_C_FILE, sse42Wrapper);
diff --git a/src/native/api.c b/src/native/api.c
index ae5e862d..3c483659 100644
--- a/src/native/api.c
+++ b/src/native/api.c
@@ -122,9 +122,33 @@ void llhttp_reset(llhttp_t* parser) {
 }
 
 
+#if (defined(__x86_64__) || defined(_M_X64)) && !defined(__SSE4_2__) && \
+    (defined(__GNUC__) || defined(__clang__))
+/* A second instantiation of the generated parser, compiled with -msse4.2 (see
+ * llhttp_sse42.c), which turns on the SSE4.2 span scanners that the generated
+ * code guards with `#ifdef __SSE4_2__`. The default x86-64 build (no -march)
+ * never contains them, so pick the SSE4.2 build once at runtime. Both
+ * instantiations come from the same generated source, so the parser state
+ * layout is identical and a parser may be driven by either. */
+llhttp_errno_t llhttp__internal_execute_sse42(llhttp_t* state, const char* p,
+                                              const char* endp);
+
+static int llhttp__sse42 = -1;
+
+llhttp_errno_t llhttp_execute(llhttp_t* parser, const char* data, size_t len) {
+  if (llhttp__sse42 < 0) {
+    llhttp__sse42 = __builtin_cpu_supports("sse4.2") ? 1 : 0;
+  }
+  if (llhttp__sse42) {
+    return llhttp__internal_execute_sse42(parser, data, data + len);
+  }
+  return llhttp__internal_execute(parser, data, data + len);
+}
+#else
 llhttp_errno_t llhttp_execute(llhttp_t* parser, const char* data, size_t len) {
   return llhttp__internal_execute(parser, data, data + len);
 }
+#endif
 
 
 void llhttp_settings_init(llhttp_settings_t* settings) {