[libc][setjmp][x86] implement setjmp in terms of out of line asm

This fixes libc_setjmp_unittests for me.

This would consistently fail for me locally, to the point where I could not run
ninja libc-unit-tests without ninja libc_setjmp_unittests failing.

Turns out that since I enabled -ftrivial-auto-var-init=pattern in
commit 1d5c16d ("[libc] default enable -ftrivial-auto-var-init=pattern (llvm#78776)")
this has been a problem. Our x86_64 setjmp definition disabled -Wuninitialized,
so we wound up clobbering these registers and instead backing up
0xAAAAAAAAAAAAAAAA rather than the actual register value.

Adding out of line asm to llvm-libc is opening pandora's box. This should not
be merged without discussion and buy in from maintainers. We should have a
policy in place for _when_ it's acceptable to use out of line asm or not.

Link: llvm#87837
Link: llvm#88054
Link: https://discourse.llvm.org/t/hand-written-in-assembly-in-libc-setjmp-longjmp/73249

Author: nickdesaulniers

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(

libc/src/setjmp/x86_64/setjmp.S

@@ -0,0 +1,50 @@
+//===-- 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
+//
+//===----------------------------------------------------------------------===//
+
+#define paste(ns) _ZN22 ## ns ## 6setjmpEP9__jmp_buf
+#define expand(x) paste(x)
+#define LIBC_NAMESPACE_setjump expand(LIBC_NAMESPACE)
+// aka _ZN22__llvm_libc_19_0_0_git6setjmpEP9__jmp_buf
+// aka __llvm_libc_19_0_0_git::setjmp(__jmp_buf*)
+
+// Brittle! Changing the layout of __jmp_buf will break this!
+#define RBX_OFFSET 0
+#define RBP_OFFSET 8
+#define R12_OFFSET 16
+#define R13_OFFSET 24
+#define R14_OFFSET 32
+#define R15_OFFSET 40
+#define RSP_OFFSET 48
+#define RIP_OFFSET 56
+
+.global setjump
+.global LIBC_NAMESPACE_setjump
+
+.type setjump, @function
+.type LIBC_NAMESPACE_setjump, @function
+
+setjump:
+LIBC_NAMESPACE_setjump:
+
+  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
+
+.size setjump, . - setjump
+.size LIBC_NAMESPACE_setjump, . - LIBC_NAMESPACE_setjump
+
+.section .note.GNU-stack, "", @progbits

libc/src/setjmp/x86_64/setjmp.cpp

@@ -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
+  )

libc/test/include/CMakeLists.txt

@@ -0,0 +1,29 @@
+//===-- 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 "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), 0UL);
+  ASSERT_EQ(offsetof(__jmp_buf, rbp), 8UL);
+  ASSERT_EQ(offsetof(__jmp_buf, r12), 16UL);
+  ASSERT_EQ(offsetof(__jmp_buf, r13), 24UL);
+  ASSERT_EQ(offsetof(__jmp_buf, r14), 32UL);
+  ASSERT_EQ(offsetof(__jmp_buf, r15), 40UL);
+  ASSERT_EQ(offsetof(__jmp_buf, rsp), 48UL);
+  ASSERT_EQ(offsetof(__jmp_buf, rip), 56UL);
+  ASSERT_EQ(sizeof(__jmp_buf), 64UL);
+  ASSERT_EQ(alignof(__jmp_buf), 8UL);
+#endif // __x86_64__
+}