Give formal semantics to some built-in functions and run-time functions

This commit adds mechanisms to
- recognize certain built-in and run-time functions by name and signature;
- associate semantics to these functions, as a partial function from
  list of values to values;
- interpret external calls to these functions according to this semantics
  (pure function from values to values, memory unchanged, no observable
  events in the trace);
- external calls to unknown built-in and run-time functions remain
  interpreted as generating observable events and possibly changing
  memory, like before.

The description of the built-ins is split into a target-independent
part (in common/Builtins0.v) and a target-specific part (in
$ARCH/Builtins1.v).

Instruction selection uses the new mechanism in order to
- recognize some built-in functions and turn them into operations
  of the target processor.  Currently, this is done for
  __builtin_sel and __builtin_fabs; more to come.
- remove the axioms about int64 helper functions from the standard
  library.  More precisely, the behavior of these functions is
  still axiomatized, but now it is specified using the more general
  machinery introduced in this commit, rather than ad-hoc axioms
  in backend/SplitLongproof.

The only built-ins currently described are __builtin_fsqrt (for all platforms)
and __builtin_fmin / __builtin_fmax (for x86).  More built-ins will be
added later.

1 files changed, 58 insertions, 0 deletions

diff --git a/common/Builtins.v b/common/Builtins.v
new file mode 100644
index 00000000..c9097e86
--- /dev/null
+++ b/common/Builtins.v
@@ -0,0 +1,58 @@
+(* *********************************************************************)
+(*                                                                     *)
+(*              The Compcert verified compiler                         *)
+(*                                                                     *)
+(*          Xavier Leroy, Collège de France and Inria Paris            *)
+(*                                                                     *)
+(*  Copyright Institut National de Recherche en Informatique et en     *)
+(*  Automatique.  All rights reserved.  This file is distributed       *)
+(*  under the terms of the GNU General Public License as published by  *)
+(*  the Free Software Foundation, either version 2 of the License, or  *)
+(*  (at your option) any later version.  This file is also distributed *)
+(*  under the terms of the INRIA Non-Commercial License Agreement.     *)
+(*                                                                     *)
+(* *********************************************************************)
+
+(** Known built-in functions *)
+
+Require Import String Coqlib.
+Require Import AST Integers Floats Values.
+Require Export Builtins0 Builtins1.
+
+Inductive builtin_function : Type :=
+  | BI_standard (b: standard_builtin)
+  | BI_platform (b: platform_builtin).
+
+Definition builtin_function_sig (b: builtin_function) : signature :=
+  match b with
+  | BI_standard b => standard_builtin_sig b
+  | BI_platform b => platform_builtin_sig b
+  end.
+
+Definition builtin_function_sem (b: builtin_function) : builtin_sem (proj_sig_res (builtin_function_sig b)) :=
+  match b with
+  | BI_standard b => standard_builtin_sem b
+  | BI_platform b => platform_builtin_sem b
+  end.
+
+Definition lookup_builtin_function (name: string) (sg: signature) : option builtin_function :=
+  match lookup_builtin standard_builtin_sig name sg standard_builtin_table with
+  | Some b => Some (BI_standard b)
+  | None => 
+  match lookup_builtin platform_builtin_sig name sg platform_builtin_table with
+  | Some b => Some (BI_platform b)
+  | None => None
+  end end.
+
+Lemma lookup_builtin_function_sig:
+  forall name sg b, lookup_builtin_function name sg = Some b -> builtin_function_sig b = sg.
+Proof.
+  unfold lookup_builtin_function; intros.
+  destruct (lookup_builtin standard_builtin_sig name sg standard_builtin_table) as [bs|] eqn:E.
+  inv H. simpl. eapply lookup_builtin_sig; eauto.
+  destruct (lookup_builtin platform_builtin_sig name sg platform_builtin_table) as [bp|] eqn:E'.
+  inv H. simpl. eapply lookup_builtin_sig; eauto.
+  discriminate.
+Qed.
+
+