accel/tcg: Split out cpu_exec_{setjmp,loop}

Recently the g_assert(cpu == current_cpu) test has been
intermittently failing with gcc.  Reorg the code around
the setjmp to minimize the lifetime of the cpu variable
affected by the setjmp.

This appears to fix the existing issue with clang as well.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1147
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/cpu-exec.c

@@ -909,64 +909,10 @@ static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
 
 /* main execution loop */
 
-int cpu_exec(CPUState *cpu)
+static int __attribute__((noinline))
+cpu_exec_loop(CPUState *cpu, SyncClocks *sc)
 {
     int ret;
-    SyncClocks sc = { 0 };
-
-    /* replay_interrupt may need current_cpu */
-    current_cpu = cpu;
-
-    if (cpu_handle_halt(cpu)) {
-        return EXCP_HALTED;
-    }
-
-    rcu_read_lock();
-
-    cpu_exec_enter(cpu);
-
-    /* Calculate difference between guest clock and host clock.
-     * This delay includes the delay of the last cycle, so
-     * what we have to do is sleep until it is 0. As for the
-     * advance/delay we gain here, we try to fix it next time.
-     */
-    init_delay_params(&sc, cpu);
-
-    /* prepare setjmp context for exception handling */
-    if (sigsetjmp(cpu->jmp_env, 0) != 0) {
-#if defined(__clang__)
-        /*
-         * Some compilers wrongly smash all local variables after
-         * siglongjmp (the spec requires that only non-volatile locals
-         * which are changed between the sigsetjmp and siglongjmp are
-         * permitted to be trashed). There were bug reports for gcc
-         * 4.5.0 and clang.  The bug is fixed in all versions of gcc
-         * that we support, but is still unfixed in clang:
-         *   https://bugs.llvm.org/show_bug.cgi?id=21183
-         *
-         * Reload an essential local variable here for those compilers.
-         * Newer versions of gcc would complain about this code (-Wclobbered),
-         * so we only perform the workaround for clang.
-         */
-        cpu = current_cpu;
-#else
-        /* Non-buggy compilers preserve this; assert the correct value. */
-        g_assert(cpu == current_cpu);
-#endif
-
-#ifndef CONFIG_SOFTMMU
-        clear_helper_retaddr();
-        if (have_mmap_lock()) {
-            mmap_unlock();
-        }
-#endif
-        if (qemu_mutex_iothread_locked()) {
-            qemu_mutex_unlock_iothread();
-        }
-        qemu_plugin_disable_mem_helpers(cpu);
-
-        assert_no_pages_locked();
-    }
 
     /* if an exception is pending, we execute it here */
     while (!cpu_handle_exception(cpu, &ret)) {
@@ -1033,9 +979,60 @@ int cpu_exec(CPUState *cpu)
 
             /* Try to align the host and virtual clocks
                if the guest is in advance */
-            align_clocks(&sc, cpu);
+            align_clocks(sc, cpu);
         }
     }
+    return ret;
+}
+
+static int cpu_exec_setjmp(CPUState *cpu, SyncClocks *sc)
+{
+    /* Prepare setjmp context for exception handling. */
+    if (unlikely(sigsetjmp(cpu->jmp_env, 0) != 0)) {
+        /* Non-buggy compilers preserve this; assert the correct value. */
+        g_assert(cpu == current_cpu);
+
+#ifndef CONFIG_SOFTMMU
+        clear_helper_retaddr();
+        if (have_mmap_lock()) {
+            mmap_unlock();
+        }
+#endif
+        if (qemu_mutex_iothread_locked()) {
+            qemu_mutex_unlock_iothread();
+        }
+        qemu_plugin_disable_mem_helpers(cpu);
+
+        assert_no_pages_locked();
+    }
+
+    return cpu_exec_loop(cpu, sc);
+}
+
+int cpu_exec(CPUState *cpu)
+{
+    int ret;
+    SyncClocks sc = { 0 };
+
+    /* replay_interrupt may need current_cpu */
+    current_cpu = cpu;
+
+    if (cpu_handle_halt(cpu)) {
+        return EXCP_HALTED;
+    }
+
+    rcu_read_lock();
+    cpu_exec_enter(cpu);
+
+    /*
+     * Calculate difference between guest clock and host clock.
+     * This delay includes the delay of the last cycle, so
+     * what we have to do is sleep until it is 0. As for the
+     * advance/delay we gain here, we try to fix it next time.
+     */
+    init_delay_params(&sc, cpu);
+
+    ret = cpu_exec_setjmp(cpu, &sc);
 
     cpu_exec_exit(cpu);
     rcu_read_unlock();