Implement keepGoing in asm if musttail is not available

CMakeLists.txt

@@ -38,11 +38,6 @@ if(NOT APPLE)
   add_compile_options(-g -fno-omit-frame-pointer)
 endif()
 
-# work around lack of musttail for gcc
-if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_BUILD_TYPE STREQUAL "Debug")
-  add_compile_options(-O1 -foptimize-sibling-calls)
-endif()
-
 set(full_relro_flags "-pie;LINKER:-z,relro,-z,now,-z,noexecstack")
 cmake_push_check_state()
 list(APPEND CMAKE_REQUIRED_LINK_OPTIONS ${full_relro_flags})

ConflictSet.cpp

@@ -3262,10 +3262,29 @@ struct CheckContext {
   ReadContext *tls;
 };
 
+#if __has_attribute(musttail)
 FLATTEN PRESERVE_NONE void keepGoing(CheckJob *job, CheckContext *context) {
   job = job->next;
   MUSTTAIL return job->continuation(job, context);
 }
+#else
+static_assert(offsetof(CheckJob, next) == 0x38);
+static_assert(offsetof(CheckJob, continuation) == 0x28);
+extern "C" void keepGoing(CheckJob *, CheckContext *);
+
+#if defined(__x86_64__)
+asm("keepGoing:\n"
+    "   mov 0x38(%rdi),%rdi\n"
+    "   jmp *0x28(%rdi)\n");
+#elif defined(__aarch64__)
+asm("keepGoing:\n"
+    "   ldr x0, [x0, #0x38]\n"
+    "   ldr x16, [x0, #0x28]\n"
+    "   br  x16\n");
+#else
+#error "Only x64 and aarch64 are supported"
+#endif
+#endif
 
 FLATTEN PRESERVE_NONE void complete(CheckJob *job, CheckContext *context) {
   if (context->started == context->count) {