src/HOL/IMP/Compiler0.thy

tuned proofs;

(*  Title:      HOL/IMP/Compiler.thy
    ID:         $Id$
    Author:     Tobias Nipkow, TUM
    Copyright   1996 TUM

This is an early version of the compiler, where the abstract machine
has an explicit pc. This turned out to be awkward, and a second
development was started. See Machines.thy and Compiler.thy.
*)

header "A Simple Compiler"

theory Compiler0 imports Natural begin

subsection "An abstract, simplistic machine"

text {* There are only three instructions: *}
datatype instr = ASIN loc aexp | JMPF bexp nat | JMPB nat

text {* We describe execution of programs in the machine by
  an operational (small step) semantics:
*}

inductive_set
  stepa1 :: "instr list \<Rightarrow> ((state\<times>nat) \<times> (state\<times>nat))set"
  and stepa1' :: "[instr list,state,nat,state,nat] \<Rightarrow> bool"
    ("_ \<turnstile> (3\<langle>_,_\<rangle>/ -1\<rightarrow> \<langle>_,_\<rangle>)" [50,0,0,0,0] 50)
  for P :: "instr list"
where
  "P \<turnstile> \<langle>s,m\<rangle> -1\<rightarrow> \<langle>t,n\<rangle> == ((s,m),t,n) : stepa1 P"
| ASIN[simp]:
  "\<lbrakk> n<size P; P!n = ASIN x a \<rbrakk> \<Longrightarrow> P \<turnstile> \<langle>s,n\<rangle> -1\<rightarrow> \<langle>s[x\<mapsto> a s],Suc n\<rangle>"
| JMPFT[simp,intro]:
  "\<lbrakk> n<size P; P!n = JMPF b i;  b s \<rbrakk> \<Longrightarrow> P \<turnstile> \<langle>s,n\<rangle> -1\<rightarrow> \<langle>s,Suc n\<rangle>"
| JMPFF[simp,intro]:
  "\<lbrakk> n<size P; P!n = JMPF b i; ~b s; m=n+i \<rbrakk> \<Longrightarrow> P \<turnstile> \<langle>s,n\<rangle> -1\<rightarrow> \<langle>s,m\<rangle>"
| JMPB[simp]:
  "\<lbrakk> n<size P; P!n = JMPB i; i <= n; j = n-i \<rbrakk> \<Longrightarrow> P \<turnstile> \<langle>s,n\<rangle> -1\<rightarrow> \<langle>s,j\<rangle>"

abbreviation
  stepa :: "[instr list,state,nat,state,nat] \<Rightarrow> bool"
    ("_ \<turnstile>/ (3\<langle>_,_\<rangle>/ -*\<rightarrow> \<langle>_,_\<rangle>)" [50,0,0,0,0] 50)  where
  "P \<turnstile> \<langle>s,m\<rangle> -*\<rightarrow> \<langle>t,n\<rangle> == ((s,m),t,n) : ((stepa1 P)^*)"

abbreviation
  stepan :: "[instr list,state,nat,nat,state,nat] \<Rightarrow> bool"
    ("_ \<turnstile>/ (3\<langle>_,_\<rangle>/ -(_)\<rightarrow> \<langle>_,_\<rangle>)" [50,0,0,0,0,0] 50)  where
  "P \<turnstile> \<langle>s,m\<rangle> -(i)\<rightarrow> \<langle>t,n\<rangle> == ((s,m),t,n) : ((stepa1 P)^i)"

subsection "The compiler"

consts compile :: "com \<Rightarrow> instr list"
primrec
"compile \<SKIP> = []"
"compile (x:==a) = [ASIN x a]"
"compile (c1;c2) = compile c1 @ compile c2"
"compile (\<IF> b \<THEN> c1 \<ELSE> c2) =
 [JMPF b (length(compile c1) + 2)] @ compile c1 @
 [JMPF (%x. False) (length(compile c2)+1)] @ compile c2"
"compile (\<WHILE> b \<DO> c) = [JMPF b (length(compile c) + 2)] @ compile c @
 [JMPB (length(compile c)+1)]"

declare nth_append[simp]

subsection "Context lifting lemmas"

text {*
  Some lemmas for lifting an execution into a prefix and suffix
  of instructions; only needed for the first proof.
*}
lemma app_right_1:
  assumes "is1 \<turnstile> \<langle>s1,i1\<rangle> -1\<rightarrow> \<langle>s2,i2\<rangle>"
  shows "is1 @ is2 \<turnstile> \<langle>s1,i1\<rangle> -1\<rightarrow> \<langle>s2,i2\<rangle>"
  using assms
  by induct auto

lemma app_left_1:
  assumes "is2 \<turnstile> \<langle>s1,i1\<rangle> -1\<rightarrow> \<langle>s2,i2\<rangle>"
  shows "is1 @ is2 \<turnstile> \<langle>s1,size is1+i1\<rangle> -1\<rightarrow> \<langle>s2,size is1+i2\<rangle>"
  using assms
  by induct auto

declare rtrancl_induct2 [induct set: rtrancl]

lemma app_right:
  assumes "is1 \<turnstile> \<langle>s1,i1\<rangle> -*\<rightarrow> \<langle>s2,i2\<rangle>"
  shows "is1 @ is2 \<turnstile> \<langle>s1,i1\<rangle> -*\<rightarrow> \<langle>s2,i2\<rangle>"
  using assms
proof induct
  show "is1 @ is2 \<turnstile> \<langle>s1,i1\<rangle> -*\<rightarrow> \<langle>s1,i1\<rangle>" by simp
next
  fix s1' i1' s2 i2
  assume "is1 @ is2 \<turnstile> \<langle>s1,i1\<rangle> -*\<rightarrow> \<langle>s1',i1'\<rangle>"
    and "is1 \<turnstile> \<langle>s1',i1'\<rangle> -1\<rightarrow> \<langle>s2,i2\<rangle>"
  thus "is1 @ is2 \<turnstile> \<langle>s1,i1\<rangle> -*\<rightarrow> \<langle>s2,i2\<rangle>"
    by (blast intro: app_right_1 rtrancl_trans)
qed

lemma app_left:
  assumes "is2 \<turnstile> \<langle>s1,i1\<rangle> -*\<rightarrow> \<langle>s2,i2\<rangle>"
  shows "is1 @ is2 \<turnstile> \<langle>s1,size is1+i1\<rangle> -*\<rightarrow> \<langle>s2,size is1+i2\<rangle>"
using assms
proof induct
  show "is1 @ is2 \<turnstile> \<langle>s1,length is1 + i1\<rangle> -*\<rightarrow> \<langle>s1,length is1 + i1\<rangle>" by simp
next
  fix s1' i1' s2 i2
  assume "is1 @ is2 \<turnstile> \<langle>s1,length is1 + i1\<rangle> -*\<rightarrow> \<langle>s1',length is1 + i1'\<rangle>"
    and "is2 \<turnstile> \<langle>s1',i1'\<rangle> -1\<rightarrow> \<langle>s2,i2\<rangle>"
  thus "is1 @ is2 \<turnstile> \<langle>s1,length is1 + i1\<rangle> -*\<rightarrow> \<langle>s2,length is1 + i2\<rangle>"
    by (blast intro: app_left_1 rtrancl_trans)
qed

lemma app_left2:
  "\<lbrakk> is2 \<turnstile> \<langle>s1,i1\<rangle> -*\<rightarrow> \<langle>s2,i2\<rangle>; j1 = size is1+i1; j2 = size is1+i2 \<rbrakk> \<Longrightarrow>
    is1 @ is2 \<turnstile> \<langle>s1,j1\<rangle> -*\<rightarrow> \<langle>s2,j2\<rangle>"
  by (simp add: app_left)

lemma app1_left:
  assumes "is \<turnstile> \<langle>s1,i1\<rangle> -*\<rightarrow> \<langle>s2,i2\<rangle>"
  shows "instr # is \<turnstile> \<langle>s1,Suc i1\<rangle> -*\<rightarrow> \<langle>s2,Suc i2\<rangle>"
proof -
  from app_left [OF assms, of "[instr]"]
  show ?thesis by simp
qed

subsection "Compiler correctness"

declare rtrancl_into_rtrancl[trans]
        converse_rtrancl_into_rtrancl[trans]
        rtrancl_trans[trans]

text {*
  The first proof; The statement is very intuitive,
  but application of induction hypothesis requires the above lifting lemmas
*}
theorem
  assumes "\<langle>c,s\<rangle> \<longrightarrow>\<^sub>c t"
  shows "compile c \<turnstile> \<langle>s,0\<rangle> -*\<rightarrow> \<langle>t,length(compile c)\<rangle>" (is "?P c s t")
  using assms
proof induct
  show "\<And>s. ?P \<SKIP> s s" by simp
next
  show "\<And>a s x. ?P (x :== a) s (s[x\<mapsto> a s])" by force
next
  fix c0 c1 s0 s1 s2
  assume "?P c0 s0 s1"
  hence "compile c0 @ compile c1 \<turnstile> \<langle>s0,0\<rangle> -*\<rightarrow> \<langle>s1,length(compile c0)\<rangle>"
    by (rule app_right)
  moreover assume "?P c1 s1 s2"
  hence "compile c0 @ compile c1 \<turnstile> \<langle>s1,length(compile c0)\<rangle> -*\<rightarrow>
    \<langle>s2,length(compile c0)+length(compile c1)\<rangle>"
  proof -
    show "\<And>is1 is2 s1 s2 i2.
      is2 \<turnstile> \<langle>s1,0\<rangle> -*\<rightarrow> \<langle>s2,i2\<rangle> \<Longrightarrow>
      is1 @ is2 \<turnstile> \<langle>s1,size is1\<rangle> -*\<rightarrow> \<langle>s2,size is1+i2\<rangle>"
      using app_left[of _ 0] by simp
  qed
  ultimately have "compile c0 @ compile c1 \<turnstile> \<langle>s0,0\<rangle> -*\<rightarrow>
      \<langle>s2,length(compile c0)+length(compile c1)\<rangle>"
    by (rule rtrancl_trans)
  thus "?P (c0; c1) s0 s2" by simp
next
  fix b c0 c1 s0 s1
  let ?comp = "compile(\<IF> b \<THEN> c0 \<ELSE> c1)"
  assume "b s0" and IH: "?P c0 s0 s1"
  hence "?comp \<turnstile> \<langle>s0,0\<rangle> -1\<rightarrow> \<langle>s0,1\<rangle>" by auto
  also from IH
  have "?comp \<turnstile> \<langle>s0,1\<rangle> -*\<rightarrow> \<langle>s1,length(compile c0)+1\<rangle>"
    by(auto intro:app1_left app_right)
  also have "?comp \<turnstile> \<langle>s1,length(compile c0)+1\<rangle> -1\<rightarrow> \<langle>s1,length ?comp\<rangle>"
    by(auto)
  finally show "?P (\<IF> b \<THEN> c0 \<ELSE> c1) s0 s1" .
next
  fix b c0 c1 s0 s1
  let ?comp = "compile(\<IF> b \<THEN> c0 \<ELSE> c1)"
  assume "\<not>b s0" and IH: "?P c1 s0 s1"
  hence "?comp \<turnstile> \<langle>s0,0\<rangle> -1\<rightarrow> \<langle>s0,length(compile c0) + 2\<rangle>" by auto
  also from IH
  have "?comp \<turnstile> \<langle>s0,length(compile c0)+2\<rangle> -*\<rightarrow> \<langle>s1,length ?comp\<rangle>"
    by (force intro!: app_left2 app1_left)
  finally show "?P (\<IF> b \<THEN> c0 \<ELSE> c1) s0 s1" .
next
  fix b c and s::state
  assume "\<not>b s"
  thus "?P (\<WHILE> b \<DO> c) s s" by force
next
  fix b c and s0::state and s1 s2
  let ?comp = "compile(\<WHILE> b \<DO> c)"
  assume "b s0" and
    IHc: "?P c s0 s1" and IHw: "?P (\<WHILE> b \<DO> c) s1 s2"
  hence "?comp \<turnstile> \<langle>s0,0\<rangle> -1\<rightarrow> \<langle>s0,1\<rangle>" by auto
  also from IHc
  have "?comp \<turnstile> \<langle>s0,1\<rangle> -*\<rightarrow> \<langle>s1,length(compile c)+1\<rangle>"
    by (auto intro: app1_left app_right)
  also have "?comp \<turnstile> \<langle>s1,length(compile c)+1\<rangle> -1\<rightarrow> \<langle>s1,0\<rangle>" by simp
  also note IHw
  finally show "?P (\<WHILE> b \<DO> c) s0 s2".
qed

text {*
  Second proof; statement is generalized to cater for prefixes and suffixes;
  needs none of the lifting lemmas, but instantiations of pre/suffix.
  *}
(*
theorem assumes A: "\<langle>c,s\<rangle> \<longrightarrow>\<^sub>c t"
shows "\<And>a z. a@compile c@z \<turnstile> \<langle>s,size a\<rangle> -*\<rightarrow> \<langle>t,size a + size(compile c)\<rangle>"
      (is "\<And>a z. ?P c s t a z")
proof -
  from A show "\<And>a z. ?thesis a z"
  proof induct
    case Skip thus ?case by simp
  next
    case Assign thus ?case by (force intro!: ASIN)
  next
    fix c1 c2 s s' s'' a z
    assume IH1: "\<And>a z. ?P c1 s s' a z" and IH2: "\<And>a z. ?P c2 s' s'' a z"
    from IH1 IH2[of "a@compile c1"]
    show "?P (c1;c2) s s'' a z"
      by(simp add:add_assoc[THEN sym])(blast intro:rtrancl_trans)
  next
(* at this point I gave up converting to structured proofs *)
(* \<IF> b \<THEN> c0 \<ELSE> c1; case b is true *)
   apply(intro strip)
   (* instantiate assumption sufficiently for later: *)
   apply(erule_tac x = "a@[?I]" in allE)
   apply(simp)
   (* execute JMPF: *)
   apply(rule converse_rtrancl_into_rtrancl)
    apply(force intro!: JMPFT)
   (* execute compile c0: *)
   apply(rule rtrancl_trans)
    apply(erule allE)
    apply assumption
   (* execute JMPF: *)
   apply(rule r_into_rtrancl)
   apply(force intro!: JMPFF)
(* end of case b is true *)
  apply(intro strip)
  apply(erule_tac x = "a@[?I]@compile c0@[?J]" in allE)
  apply(simp add:add_assoc)
  apply(rule converse_rtrancl_into_rtrancl)
   apply(force intro!: JMPFF)
  apply(blast)
 apply(force intro: JMPFF)
apply(intro strip)
apply(erule_tac x = "a@[?I]" in allE)
apply(erule_tac x = a in allE)
apply(simp)
apply(rule converse_rtrancl_into_rtrancl)
 apply(force intro!: JMPFT)
apply(rule rtrancl_trans)
 apply(erule allE)
 apply assumption
apply(rule converse_rtrancl_into_rtrancl)
 apply(force intro!: JMPB)
apply(simp)
done
*)
text {* Missing: the other direction! I did much of it, and although
the main lemma is very similar to the one in the new development, the
lemmas surrounding it seemed much more complicated. In the end I gave
up. *}

end