diff --git a/libc/src/__support/assembly.h b/libc/src/__support/assembly.h
new file mode 100644
index 0000000000000..5b1d4cb230886
--- /dev/null
+++ b/libc/src/__support/assembly.h
@@ -0,0 +1,34 @@
+//===-- assembly.h - libc assembler support macros based on compiler-rt's -===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_LIBC_SRC___SUPPORT_ASSEMBLY_H
+#define LLVM_LIBC_SRC___SUPPORT_ASSEMBLY_H
+
+#ifndef __ASSEMBLER__
+#error "No not include assembly.h in non-asm sources"
+#endif
+
+#define GLUE2_(a, b) a##b
+#define GLUE(a, b) GLUE2_(a, b)
+#define SYMBOL_NAME(name) GLUE(__USER_LABEL_PREFIX__, name)
+
+#if defined(__ELF__) && (defined(__GNU__) || defined(__FreeBSD__) ||           \
+                         defined(__Fuchsia__) || defined(__linux__))
+
+// clang-format off
+#define NO_EXEC_STACK_DIRECTIVE .section .note.GNU-stack, "", @progbits
+#define SYMBOL_IS_FUNC(name) .type SYMBOL_NAME(name), %function
+#define END_FUNC(name) .size SYMBOL_NAME(name), . - SYMBOL_NAME(name)
+// clang-format on
+
+#else // !ELF
+#define NO_EXEC_STACK_DIRECTIVE
+#define SYMBOL_IS_FUNC(name)
+#define END_FUNC(name)
+#endif // ELF
+
+#endif // LLVM_LIBC_SRC___SUPPORT_ASSEMBLY_H
diff --git a/libc/src/setjmp/x86_64/CMakeLists.txt b/libc/src/setjmp/x86_64/CMakeLists.txt
index 9899c00e7c4a6..38f2b872e673c 100644
--- a/libc/src/setjmp/x86_64/CMakeLists.txt
+++ b/libc/src/setjmp/x86_64/CMakeLists.txt
@@ -1,14 +1,9 @@
 add_entrypoint_object(
   setjmp
   SRCS
-    setjmp.cpp
+    setjmp.S
   HDRS
     ../setjmp_impl.h
-  DEPENDS
-    libc.include.setjmp
-  COMPILE_OPTIONS
-    -O3
-    -fno-omit-frame-pointer
 )
 
 add_entrypoint_object(
diff --git a/libc/src/setjmp/x86_64/setjmp.S b/libc/src/setjmp/x86_64/setjmp.S
new file mode 100644
index 0000000000000..10a68f0f700a6
--- /dev/null
+++ b/libc/src/setjmp/x86_64/setjmp.S
@@ -0,0 +1,42 @@
+//===-- Implementation of setjmp --------------------------------*- ASM -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/assembly.h"
+#include "src/setjmp/x86_64/setjmp.h"
+
+#define paste(ns) _ZN22 ## ns ## 6setjmpEP9__jmp_buf
+#define expand(x) paste(x)
+#define LIBC_NAMESPACE_setjmp expand(LIBC_NAMESPACE)
+// aka _ZN22__llvm_libc_19_0_0_git6setjmpEP9__jmp_buf
+// aka __llvm_libc_19_0_0_git::setjmp(__jmp_buf*)
+
+.global SYMBOL_NAME(setjmp)
+.global SYMBOL_NAME(LIBC_NAMESPACE_setjmp)
+
+SYMBOL_IS_FUNC(setjmp)
+SYMBOL_IS_FUNC(LIBC_NAMESPACE_setjmp)
+
+SYMBOL_NAME(setjmp):
+SYMBOL_NAME(LIBC_NAMESPACE_setjmp):
+
+  mov %rbx, RBX_OFFSET(%rdi)
+  mov %rbp, RBP_OFFSET(%rdi)
+  mov %r12, R12_OFFSET(%rdi)
+  mov %r13, R13_OFFSET(%rdi)
+  mov %r14, R14_OFFSET(%rdi)
+  mov %r15, R15_OFFSET(%rdi)
+  lea 8(%rsp), %rax
+  mov %rax, RSP_OFFSET(%rdi)
+  mov (%rsp), %rax
+  mov %rax, RIP_OFFSET(%rdi)
+  xor %eax, %eax
+  ret
+END_FUNC(setjmp)
+END_FUNC(LIBC_NAMESPACE_setjmp)
+
+NO_EXEC_STACK_DIRECTIVE
diff --git a/libc/src/setjmp/x86_64/setjmp.cpp b/libc/src/setjmp/x86_64/setjmp.cpp
deleted file mode 100644
index 8b6981d4cc34a..0000000000000
--- a/libc/src/setjmp/x86_64/setjmp.cpp
+++ /dev/null
@@ -1,56 +0,0 @@
-//===-- Implementation of setjmp ------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-#include "src/__support/common.h"
-#include "src/setjmp/setjmp_impl.h"
-
-#if !defined(LIBC_TARGET_ARCH_IS_X86_64)
-#error "Invalid file include"
-#endif
-
-namespace LIBC_NAMESPACE {
-
-LLVM_LIBC_FUNCTION(int, setjmp, (__jmp_buf * buf)) {
-  register __UINT64_TYPE__ rbx __asm__("rbx");
-  register __UINT64_TYPE__ r12 __asm__("r12");
-  register __UINT64_TYPE__ r13 __asm__("r13");
-  register __UINT64_TYPE__ r14 __asm__("r14");
-  register __UINT64_TYPE__ r15 __asm__("r15");
-
-  // We want to store the register values as is. So, we will suppress the
-  // compiler warnings about the uninitialized variables declared above.
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wuninitialized"
-  LIBC_INLINE_ASM("mov %1, %0\n\t" : "=m"(buf->rbx) : "r"(rbx) :);
-  LIBC_INLINE_ASM("mov %1, %0\n\t" : "=m"(buf->r12) : "r"(r12) :);
-  LIBC_INLINE_ASM("mov %1, %0\n\t" : "=m"(buf->r13) : "r"(r13) :);
-  LIBC_INLINE_ASM("mov %1, %0\n\t" : "=m"(buf->r14) : "r"(r14) :);
-  LIBC_INLINE_ASM("mov %1, %0\n\t" : "=m"(buf->r15) : "r"(r15) :);
-#pragma GCC diagnostic pop
-
-  // We want the rbp of the caller, which is what __builtin_frame_address(1)
-  // should return. But, compilers generate a warning that calling
-  // __builtin_frame_address with non-zero argument is unsafe. So, we use
-  // the knowledge of the x86_64 ABI to fetch the callers rbp. As per the ABI,
-  // the rbp of the caller is pushed on to the stack and then new top is saved
-  // in this function's rbp. So, we fetch it from location at which this
-  // functions's rbp is pointing.
-  buf->rbp = *reinterpret_cast<__UINTPTR_TYPE__ *>(__builtin_frame_address(0));
-
-  // The callers stack address is exactly 2 pointer widths ahead of the current
-  // frame pointer - between the current frame pointer and the rsp of the caller
-  // are the return address (pushed by the x86_64 call instruction) and the
-  // previous stack pointer as required by the x86_64 ABI.
-  // The stack pointer is ahead because the stack grows down on x86_64.
-  buf->rsp = reinterpret_cast<__UINTPTR_TYPE__>(__builtin_frame_address(0)) +
-             sizeof(__UINTPTR_TYPE__) * 2;
-  buf->rip = reinterpret_cast<__UINTPTR_TYPE__>(__builtin_return_address(0));
-  return 0;
-}
-
-} // namespace LIBC_NAMESPACE
diff --git a/libc/src/setjmp/x86_64/setjmp.h b/libc/src/setjmp/x86_64/setjmp.h
new file mode 100644
index 0000000000000..71aae43fd09cf
--- /dev/null
+++ b/libc/src/setjmp/x86_64/setjmp.h
@@ -0,0 +1,28 @@
+//===-- setjmp.h - shared constants between C++ and asm sources (x86_64) --===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SETJMP_X86_64_SETJMP_H
+#define LLVM_LIBC_SRC_SETJMP_X86_64_SETJMP_H
+
+#ifdef __ASSEMBLER__
+#define UL(x) x
+#else
+#define UL(x) x##UL
+#endif
+
+// Brittle! Changing the layout of __jmp_buf will break this!
+#define RBX_OFFSET UL(0)
+#define RBP_OFFSET UL(8)
+#define R12_OFFSET UL(16)
+#define R13_OFFSET UL(24)
+#define R14_OFFSET UL(32)
+#define R15_OFFSET UL(40)
+#define RSP_OFFSET UL(48)
+#define RIP_OFFSET UL(56)
+
+#endif // LLVM_LIBC_SRC_SETJMP_X86_64_SETJMP_H
diff --git a/libc/test/include/CMakeLists.txt b/libc/test/include/CMakeLists.txt
index 8d8dff53169f6..4d42f348d5e00 100644
--- a/libc/test/include/CMakeLists.txt
+++ b/libc/test/include/CMakeLists.txt
@@ -69,3 +69,14 @@ add_libc_test(
   DEPENDS
     libc.include.llvm-libc-macros.stdckdint_macros
 )
+
+add_libc_test(
+  setjmp_test
+  SUITE
+    libc_include_tests
+  SRCS
+    setjmp_test.cpp
+  DEPENDS
+    libc.include.llvm-libc-macros.offsetof_macro
+    libc.include.llvm-libc-types.jmp_buf
+  )
diff --git a/libc/test/include/setjmp_test.cpp b/libc/test/include/setjmp_test.cpp
new file mode 100644
index 0000000000000..14d1e89b317d2
--- /dev/null
+++ b/libc/test/include/setjmp_test.cpp
@@ -0,0 +1,30 @@
+//===-- Unittests for setjmp ----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDSList-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "include/llvm-libc-macros/offsetof-macro.h"
+#include "include/llvm-libc-types/jmp_buf.h"
+#include "src/setjmp/x86_64/setjmp.h"
+#include "test/UnitTest/Test.h"
+
+// If this test fails, then *_OFFSET macro definitions in
+// libc/src/setjmp/x86_64/setjmp.S need to be fixed. This is a simple change
+// detector.
+TEST(LlvmLibcSetjmpTest, JmpBufLayout) {
+#ifdef __x86_64__
+  ASSERT_EQ(offsetof(__jmp_buf, rbx), RBX_OFFSET);
+  ASSERT_EQ(offsetof(__jmp_buf, rbp), RBP_OFFSET);
+  ASSERT_EQ(offsetof(__jmp_buf, r12), R12_OFFSET);
+  ASSERT_EQ(offsetof(__jmp_buf, r13), R13_OFFSET);
+  ASSERT_EQ(offsetof(__jmp_buf, r14), R14_OFFSET);
+  ASSERT_EQ(offsetof(__jmp_buf, r15), R15_OFFSET);
+  ASSERT_EQ(offsetof(__jmp_buf, rsp), RSP_OFFSET);
+  ASSERT_EQ(offsetof(__jmp_buf, rip), RIP_OFFSET);
+  ASSERT_EQ(sizeof(__jmp_buf), 64UL);
+  ASSERT_EQ(alignof(__jmp_buf), 8UL);
+#endif // __x86_64__
+}