[libc] Fix longjmp's fake return in x86_64

This implements the fake return after a successful longjmp(), as
described in the linux man page:

Following  a  successful  longjmp(), execution continues as if setjmp()
had returned for a second time.  This  "fake"  return  can  be  distin-
guished from a true setjmp() call because the "fake" return returns the
value provided in val.  If the programmer mistakenly passes the value 0
in val, the "fake" return will instead return 1.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D146981

libc/src/setjmp/x86_64/longjmp.cpp

@@ -29,6 +29,7 @@ LLVM_LIBC_FUNCTION(void, longjmp, (__jmp_buf * buf, int val)) {
   // |val| in rax. Note that this has to happen before we restore the registers
   // from values in |buf|. Otherwise, once rsp and rbp are updated, we cannot
   // read |val|.
+  val = val == 0 ? 1 : val;
   LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(rax) : "m"(val) :);
   LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(rbx) : "m"(buf->rbx) :);
   LIBC_INLINE_ASM("mov %1, %0\n\t" : "=r"(rbp) : "m"(buf->rbp) :);

libc/test/src/setjmp/setjmp_test.cpp

@@ -10,21 +10,38 @@
 #include "src/setjmp/setjmp_impl.h"
 #include "test/UnitTest/Test.h"
 
-jmp_buf buf;
 constexpr int MAX_LOOP = 123;
+int longjmp_called = 0;
 
-void jump_back(int n) {
+void jump_back(jmp_buf buf, int n) {
+  longjmp_called++;
   __llvm_libc::longjmp(buf, n); // Will return |n| out of setjmp
 }
 
 TEST(LlvmLibcSetJmpTest, SetAndJumpBack) {
+  jmp_buf buf;
+  longjmp_called = 0;
+
   // Local variables in setjmp scope should be declared volatile.
   volatile int n = 0;
   // The first time setjmp is called, it should return 0.
   // Subsequent calls will return the value passed to jump_back below.
   if (__llvm_libc::setjmp(buf) <= MAX_LOOP) {
     ++n;
-    jump_back(n);
+    jump_back(buf, n);
   }
+  ASSERT_EQ(longjmp_called, n);
   ASSERT_EQ(n, MAX_LOOP + 1);
 }
+
+TEST(LlvmLibcSetJmpTest, SetAndJumpBackValOne) {
+  jmp_buf buf;
+  longjmp_called = 0;
+
+  int val = __llvm_libc::setjmp(buf);
+  if (val == 0)
+    jump_back(buf, val);
+
+  ASSERT_EQ(longjmp_called, 1);
+  ASSERT_EQ(val, 1);
+}