RISC-V port and assorted changes

This commits adds code generation for the RISC-V architecture, both in 32- and 64-bit modes.

The generated code was lightly tested using the simulator and cross-binutils from https://riscv.org/software-tools/

This port required the following additional changes:

- Integers: More properties about shrx

- SelectOp: now provides smart constructors for mulhs and mulhu

- SelectDiv, 32-bit integer division and modulus: implement constant propagation, use the new smart constructors mulhs and mulhu.

- Runtime library: if no asm implementation is provided, run the reference C implementation through CompCert.  Since CompCert rejects the definitions of names of special functions such as __i64_shl, the reference implementation now uses "i64_" names, e.g. "i64_shl", and a renaming "i64_ -> __i64_" is performed over the generated assembly file, before assembling and building the runtime library.

- test/: add SIMU make variable to run tests through a simulator

- test/regression/alignas.c: make sure _Alignas and _Alignof are not #define'd by C headers

commit da14495c01cf4f66a928c2feff5c53f09bde837f
Author: Xavier Leroy <xavier.leroy@inria.fr>
Date:   Thu Apr 13 17:36:10 2017 +0200

    RISC-V port, continued

    Now working on Asmgen.

commit 36f36eb3a5abfbb8805960443d087b6a83e86005
Author: Xavier Leroy <xavier.leroy@inria.fr>
Date:   Wed Apr 12 17:26:39 2017 +0200

    RISC-V port, first steps

    This port is based on Prashanth Mundkur's experimental RV32 port and brings it up to date with CompCert, and adds 64-bit support (RV64).  Work in progress.

1 files changed, 40 insertions, 18 deletions

diff --git a/backend/SelectDiv.vp b/backend/SelectDiv.vp
index dc85fb25..96b07e28 100644
--- a/backend/SelectDiv.vp
+++ b/backend/SelectDiv.vp
@@ -19,6 +19,12 @@ Require Import Op CminorSel SelectOp SplitLong SelectLong.
 
 Local Open Scope cminorsel_scope.
 
+Definition is_intconst (e: expr) : option int :=
+  match e with
+  | Eop (Ointconst n) _ => Some n
+  | _ => None
+  end.
+
 (** We try to turn divisions by a constant into a multiplication by
   a pseudo-inverse of the divisor.  The approach is described in
 - Torbjörn Granlund, Peter L. Montgomery: "Division by Invariant
@@ -101,7 +107,7 @@ Definition divlu_mul_params (d: Z) : option (Z * Z) :=
   end.
 
 Definition divu_mul (p: Z) (m: Z) :=
-  shruimm (Eop Omulhu (Eletvar O ::: Eop (Ointconst (Int.repr m)) Enil ::: Enil))
+  shruimm (mulhu (Eletvar O) (Eop (Ointconst (Int.repr m)) Enil))
           (Int.repr p).
 
 Definition divuimm (e1: expr) (n2: int) :=
@@ -117,10 +123,14 @@ Definition divuimm (e1: expr) (n2: int) :=
         end
   end.
 
-Nondetfunction divu (e1: expr) (e2: expr) := 
-  match e2 with
-  | Eop (Ointconst n2) Enil => divuimm e1 n2
-  | _ => divu_base e1 e2
+Definition divu (e1: expr) (e2: expr) := 
+  match is_intconst e2, is_intconst e1 with
+  | Some n2, Some n1 =>
+      if Int.eq n2 Int.zero
+      then divu_base e1 e2
+      else Eop (Ointconst (Int.divu n1 n2)) Enil
+  | Some n2, _ => divuimm e1 n2
+  | _, _ => divu_base e1 e2
   end.
 
 Definition mod_from_div (equo: expr) (n: int) :=
@@ -139,15 +149,19 @@ Definition moduimm (e1: expr) (n2: int) :=
         end
   end.
 
-Nondetfunction modu (e1: expr) (e2: expr) := 
-  match e2 with
-  | Eop (Ointconst n2) Enil => moduimm e1 n2
-  | _ => modu_base e1 e2
+Definition modu (e1: expr) (e2: expr) := 
+  match is_intconst e2, is_intconst e1 with
+  | Some n2, Some n1 =>
+      if Int.eq n2 Int.zero
+      then modu_base e1 e2
+      else Eop (Ointconst (Int.modu n1 n2)) Enil
+  | Some n2, _ => moduimm e1 n2
+  | _, _ => modu_base e1 e2
   end.
 
 Definition divs_mul (p: Z) (m: Z) :=
   let e2 :=
-    Eop Omulhs (Eletvar O ::: Eop (Ointconst (Int.repr m)) Enil ::: Enil) in
+    mulhs (Eletvar O) (Eop (Ointconst (Int.repr m)) Enil) in
   let e3 :=
     if zlt m Int.half_modulus then e2 else add e2 (Eletvar O) in
   add (shrimm e3 (Int.repr p))
@@ -169,10 +183,14 @@ Definition divsimm (e1: expr) (n2: int) :=
         end
   end.
 
-Nondetfunction divs (e1: expr) (e2: expr) := 
-  match e2 with
-  | Eop (Ointconst n2) Enil => divsimm e1 n2
-  | _ => divs_base e1 e2
+Definition divs (e1: expr) (e2: expr) := 
+  match is_intconst e2, is_intconst e1 with
+  | Some n2, Some n1 =>
+      if Int.eq n2 Int.zero
+      then divs_base e1 e2
+      else Eop (Ointconst (Int.divs n1 n2)) Enil
+  | Some n2, _ => divsimm e1 n2
+  | _, _ => divs_base e1 e2
   end.
 
 Definition modsimm (e1: expr) (n2: int) :=
@@ -191,10 +209,14 @@ Definition modsimm (e1: expr) (n2: int) :=
         end
   end.
 
-Nondetfunction mods (e1: expr) (e2: expr) := 
-  match e2 with
-  | Eop (Ointconst n2) Enil => modsimm e1 n2
-  | _ => mods_base e1 e2
+Definition mods (e1: expr) (e2: expr) := 
+  match is_intconst e2, is_intconst e1 with
+  | Some n2, Some n1 =>
+      if Int.eq n2 Int.zero
+      then mods_base e1 e2
+      else Eop (Ointconst (Int.mods n1 n2)) Enil
+  | Some n2, _ => modsimm e1 n2
+  | _, _ => mods_base e1 e2
   end.
 
 (** 64-bit integer divisions *)