linux-user/alpha: Set r20 secondary return value

This value is not, as far as I know, used by any linux software,
but it is set by the kernel and is part of the ABI.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20191106113318.10226-13-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/alpha/target_cpu.h

@@ -27,10 +27,19 @@ static inline void cpu_clone_regs_child(CPUAlphaState *env, target_ulong newsp,
     }
     env->ir[IR_V0] = 0;
     env->ir[IR_A3] = 0;
+    env->ir[IR_A4] = 1;  /* OSF/1 secondary return: child */
 }
 
 static inline void cpu_clone_regs_parent(CPUAlphaState *env, unsigned flags)
 {
+    /*
+     * OSF/1 secondary return: parent
+     * Note that the kernel does not do this if SETTLS, because the
+     * settls argument register is still live after copy_thread.
+     */
+    if (!(flags & CLONE_SETTLS)) {
+        env->ir[IR_A4] = 0;
+    }
 }
 
 static inline void cpu_set_tls(CPUAlphaState *env, target_ulong newtls)