Model external calls as destroying all caller-save registers

The semantics of external function calls in LTL, Linear, Mach and Asm
now consider that all caller-save registers are set to Vundef by the call.
This models that fact that the external function can modify those registers
arbitrarily.

Update the proofs of the Allocation, Tunneling, Stacking and Asmgen passes
accordingly.

2 files changed, 12 insertions, 3 deletions

diff --git a/x86/Asm.v b/x86/Asm.v
index 8b873e48..7b4a29f4 100644
--- a/x86/Asm.v
+++ b/x86/Asm.v
@@ -1047,6 +1047,15 @@ Definition preg_of (r: mreg) : preg :=
   | FP0 => ST0
   end.
 
+(** Undefine all registers except SP and callee-save registers *)
+
+Definition undef_caller_save_regs (rs: regset) : regset :=
+  fun r =>
+    if preg_eq r SP
+    || In_dec preg_eq r (List.map preg_of (List.filter is_callee_save all_mregs))
+    then rs r
+    else Vundef.
+
 (** Extract the values of the arguments of an external call.
     We exploit the calling conventions from module [Conventions], except that
     we use machine registers instead of locations. *)
@@ -1106,7 +1115,7 @@ Inductive step: state -> trace -> state -> Prop :=
       Genv.find_funct_ptr ge b = Some (External ef) ->
       extcall_arguments rs m (ef_sig ef) args ->
       external_call ef ge args m t res m' ->
-      rs' = (set_pair (loc_external_result (ef_sig ef)) res rs) #PC <- (rs RA) ->
+      rs' = (set_pair (loc_external_result (ef_sig ef)) res (undef_caller_save_regs rs)) #PC <- (rs RA) ->
       step (State rs m) t (State rs' m').
 
 End RELSEM.
diff --git a/x86/Asmgenproof.v b/x86/Asmgenproof.v
index 38816fd2..3aa87a4c 100644
--- a/x86/Asmgenproof.v
+++ b/x86/Asmgenproof.v
@@ -861,8 +861,8 @@ Transparent destroyed_at_function_entry.
   apply plus_one. eapply exec_step_external; eauto.
   eapply external_call_symbols_preserved; eauto. apply senv_preserved.
   econstructor; eauto.
-  unfold loc_external_result.
-  apply agree_set_other; auto. apply agree_set_pair; auto.
+  unfold loc_external_result. apply agree_set_other; auto. apply agree_set_pair; auto.
+  apply agree_undef_caller_save_regs; auto. 
 
 - (* return *)
   inv STACKS. simpl in *.