kleing@52396
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(* Author: Tobias Nipkow and Gerwin Klein *)
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header "Compiler for IMP"
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theory Compiler imports Big_Step Star
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begin
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subsection "List setup"
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text {*
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In the following, we use the length of lists as integers
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instead of natural numbers. Instead of converting @{typ nat}
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to @{typ int} explicitly, we tell Isabelle to coerce @{typ nat}
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automatically when necessary.
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*}
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declare [[coercion_enabled]]
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declare [[coercion "int :: nat \<Rightarrow> int"]]
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text {*
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Similarly, we will want to access the ith element of a list,
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where @{term i} is an @{typ int}.
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*}
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fun inth :: "'a list \<Rightarrow> int \<Rightarrow> 'a" (infixl "!!" 100) where
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"(x # xs) !! n = (if n = 0 then x else xs !! (n - 1))"
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text {*
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The only additional lemma we need about this function
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is indexing over append:
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*}
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lemma inth_append [simp]:
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"0 \<le> n \<Longrightarrow>
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(xs @ ys) !! n = (if n < size xs then xs !! n else ys !! (n - size xs))"
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by (induction xs arbitrary: n) (auto simp: algebra_simps)
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subsection "Instructions and Stack Machine"
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text_raw{*\snip{instrdef}{0}{1}{% *}
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datatype instr =
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LOADI int | LOAD vname | ADD | STORE vname |
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JMP int | JMPLESS int | JMPGE int
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text_raw{*}%endsnip*}
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type_synonym stack = "val list"
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type_synonym config = "int \<times> state \<times> stack"
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abbreviation "hd2 xs == hd(tl xs)"
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abbreviation "tl2 xs == tl(tl xs)"
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fun iexec :: "instr \<Rightarrow> config \<Rightarrow> config" where
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"iexec instr (i,s,stk) = (case instr of
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LOADI n \<Rightarrow> (i+1,s, n#stk) |
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LOAD x \<Rightarrow> (i+1,s, s x # stk) |
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ADD \<Rightarrow> (i+1,s, (hd2 stk + hd stk) # tl2 stk) |
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STORE x \<Rightarrow> (i+1,s(x := hd stk),tl stk) |
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JMP n \<Rightarrow> (i+1+n,s,stk) |
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JMPLESS n \<Rightarrow> (if hd2 stk < hd stk then i+1+n else i+1,s,tl2 stk) |
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JMPGE n \<Rightarrow> (if hd2 stk >= hd stk then i+1+n else i+1,s,tl2 stk))"
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definition
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exec1 :: "instr list \<Rightarrow> config \<Rightarrow> config \<Rightarrow> bool"
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("(_/ \<turnstile> (_ \<rightarrow>/ _))" [59,0,59] 60)
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where
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"P \<turnstile> c \<rightarrow> c' =
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(\<exists>i s stk. c = (i,s,stk) \<and> c' = iexec(P!!i) (i,s,stk) \<and> 0 \<le> i \<and> i < size P)"
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lemma exec1I [intro, code_pred_intro]:
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"c' = iexec (P!!i) (i,s,stk) \<Longrightarrow> 0 \<le> i \<Longrightarrow> i < size P
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\<Longrightarrow> P \<turnstile> (i,s,stk) \<rightarrow> c'"
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by (simp add: exec1_def)
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abbreviation
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exec :: "instr list \<Rightarrow> config \<Rightarrow> config \<Rightarrow> bool" ("(_/ \<turnstile> (_ \<rightarrow>*/ _))" 50)
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where
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"exec P \<equiv> star (exec1 P)"
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declare star.step[intro]
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lemmas exec_induct = star.induct [of "exec1 P", split_format(complete)]
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code_pred exec1 by (metis exec1_def)
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values
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"{(i,map t [''x'',''y''],stk) | i t stk.
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[LOAD ''y'', STORE ''x''] \<turnstile>
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(0, <''x'' := 3, ''y'' := 4>, []) \<rightarrow>* (i,t,stk)}"
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subsection{* Verification infrastructure *}
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text{* Below we need to argue about the execution of code that is embedded in
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larger programs. For this purpose we show that execution is preserved by
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appending code to the left or right of a program. *}
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lemma iexec_shift [simp]:
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"((n+i',s',stk') = iexec x (n+i,s,stk)) = ((i',s',stk') = iexec x (i,s,stk))"
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by(auto split:instr.split)
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lemma exec1_appendR: "P \<turnstile> c \<rightarrow> c' \<Longrightarrow> P@P' \<turnstile> c \<rightarrow> c'"
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by (auto simp: exec1_def)
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lemma exec_appendR: "P \<turnstile> c \<rightarrow>* c' \<Longrightarrow> P@P' \<turnstile> c \<rightarrow>* c'"
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by (induction rule: star.induct) (fastforce intro: exec1_appendR)+
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lemma exec1_appendL:
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fixes i i' :: int
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shows
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"P \<turnstile> (i,s,stk) \<rightarrow> (i',s',stk') \<Longrightarrow>
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P' @ P \<turnstile> (size(P')+i,s,stk) \<rightarrow> (size(P')+i',s',stk')"
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unfolding exec1_def
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by (auto simp del: iexec.simps)
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lemma exec_appendL:
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fixes i i' :: int
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shows
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"P \<turnstile> (i,s,stk) \<rightarrow>* (i',s',stk') \<Longrightarrow>
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P' @ P \<turnstile> (size(P')+i,s,stk) \<rightarrow>* (size(P')+i',s',stk')"
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by (induction rule: exec_induct) (blast intro!: exec1_appendL)+
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text{* Now we specialise the above lemmas to enable automatic proofs of
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@{prop "P \<turnstile> c \<rightarrow>* c'"} where @{text P} is a mixture of concrete instructions and
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pieces of code that we already know how they execute (by induction), combined
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by @{text "@"} and @{text "#"}. Backward jumps are not supported.
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The details should be skipped on a first reading.
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If we have just executed the first instruction of the program, drop it: *}
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lemma exec_Cons_1 [intro]:
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"P \<turnstile> (0,s,stk) \<rightarrow>* (j,t,stk') \<Longrightarrow>
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instr#P \<turnstile> (1,s,stk) \<rightarrow>* (1+j,t,stk')"
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by (drule exec_appendL[where P'="[instr]"]) simp
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lemma exec_appendL_if[intro]:
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fixes i i' :: int
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shows
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"size P' <= i
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\<Longrightarrow> P \<turnstile> (i - size P',s,stk) \<rightarrow>* (i',s',stk')
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\<Longrightarrow> P' @ P \<turnstile> (i,s,stk) \<rightarrow>* (size P' + i',s',stk')"
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by (drule exec_appendL[where P'=P']) simp
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text{* Split the execution of a compound program up into the excution of its
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parts: *}
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lemma exec_append_trans[intro]:
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fixes i' i'' j'' :: int
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shows
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"P \<turnstile> (0,s,stk) \<rightarrow>* (i',s',stk') \<Longrightarrow>
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size P \<le> i' \<Longrightarrow>
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P' \<turnstile> (i' - size P,s',stk') \<rightarrow>* (i'',s'',stk'') \<Longrightarrow>
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j'' = size P + i''
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\<Longrightarrow>
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P @ P' \<turnstile> (0,s,stk) \<rightarrow>* (j'',s'',stk'')"
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by(metis star_trans[OF exec_appendR exec_appendL_if])
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declare Let_def[simp]
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subsection "Compilation"
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fun acomp :: "aexp \<Rightarrow> instr list" where
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"acomp (N n) = [LOADI n]" |
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"acomp (V x) = [LOAD x]" |
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"acomp (Plus a1 a2) = acomp a1 @ acomp a2 @ [ADD]"
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lemma acomp_correct[intro]:
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"acomp a \<turnstile> (0,s,stk) \<rightarrow>* (size(acomp a),s,aval a s#stk)"
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by (induction a arbitrary: stk) fastforce+
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fun bcomp :: "bexp \<Rightarrow> bool \<Rightarrow> int \<Rightarrow> instr list" where
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"bcomp (Bc v) c n = (if v=c then [JMP n] else [])" |
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"bcomp (Not b) c n = bcomp b (\<not>c) n" |
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"bcomp (And b1 b2) c n =
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(let cb2 = bcomp b2 c n;
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m = (if c then size cb2 else (size cb2::int)+n);
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cb1 = bcomp b1 False m
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in cb1 @ cb2)" |
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"bcomp (Less a1 a2) c n =
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acomp a1 @ acomp a2 @ (if c then [JMPLESS n] else [JMPGE n])"
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value
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"bcomp (And (Less (V ''x'') (V ''y'')) (Not(Less (V ''u'') (V ''v''))))
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False 3"
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lemma bcomp_correct[intro]:
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fixes n :: int
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shows
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"0 \<le> n \<Longrightarrow>
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bcomp b c n \<turnstile>
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(0,s,stk) \<rightarrow>* (size(bcomp b c n) + (if c = bval b s then n else 0),s,stk)"
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proof(induction b arbitrary: c n)
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case Not
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from Not(1)[where c="~c"] Not(2) show ?case by fastforce
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next
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case (And b1 b2)
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from And(1)[of "if c then size(bcomp b2 c n) else size(bcomp b2 c n) + n"
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"False"]
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And(2)[of n "c"] And(3)
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show ?case by fastforce
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qed fastforce+
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fun ccomp :: "com \<Rightarrow> instr list" where
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"ccomp SKIP = []" |
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"ccomp (x ::= a) = acomp a @ [STORE x]" |
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wenzelm@54152
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"ccomp (c\<^sub>1;;c\<^sub>2) = ccomp c\<^sub>1 @ ccomp c\<^sub>2" |
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wenzelm@54152
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"ccomp (IF b THEN c\<^sub>1 ELSE c\<^sub>2) =
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wenzelm@54152
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(let cc\<^sub>1 = ccomp c\<^sub>1; cc\<^sub>2 = ccomp c\<^sub>2; cb = bcomp b False (size cc\<^sub>1 + 1)
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wenzelm@54152
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in cb @ cc\<^sub>1 @ JMP (size cc\<^sub>2) # cc\<^sub>2)" |
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"ccomp (WHILE b DO c) =
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kleing@52396
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(let cc = ccomp c; cb = bcomp b False (size cc + 1)
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in cb @ cc @ [JMP (-(size cb + size cc + 1))])"
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value "ccomp
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(IF Less (V ''u'') (N 1) THEN ''u'' ::= Plus (V ''u'') (N 1)
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ELSE ''v'' ::= V ''u'')"
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value "ccomp (WHILE Less (V ''u'') (N 1) DO (''u'' ::= Plus (V ''u'') (N 1)))"
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Jean@45966
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subsection "Preservation of semantics"
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lemma ccomp_bigstep:
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"(c,s) \<Rightarrow> t \<Longrightarrow> ccomp c \<turnstile> (0,s,stk) \<rightarrow>* (size(ccomp c),t,stk)"
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nipkow@45886
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proof(induction arbitrary: stk rule: big_step_induct)
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case (Assign x a s)
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show ?case by (fastforce simp:fun_upd_def cong: if_cong)
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next
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nipkow@48689
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case (Seq c1 s1 s2 c2 s3)
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let ?cc1 = "ccomp c1" let ?cc2 = "ccomp c2"
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have "?cc1 @ ?cc2 \<turnstile> (0,s1,stk) \<rightarrow>* (size ?cc1,s2,stk)"
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nipkow@48689
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using Seq.IH(1) by fastforce
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moreover
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have "?cc1 @ ?cc2 \<turnstile> (size ?cc1,s2,stk) \<rightarrow>* (size(?cc1 @ ?cc2),s3,stk)"
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nipkow@48689
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using Seq.IH(2) by fastforce
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kleing@54052
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ultimately show ?case by simp (blast intro: star_trans)
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next
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case (WhileTrue b s1 c s2 s3)
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let ?cc = "ccomp c"
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let ?cb = "bcomp b False (size ?cc + 1)"
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let ?cw = "ccomp(WHILE b DO c)"
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kleing@52396
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have "?cw \<turnstile> (0,s1,stk) \<rightarrow>* (size ?cb,s1,stk)"
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nipkow@51148
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using `bval b s1` by fastforce
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nipkow@51148
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moreover
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kleing@52396
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have "?cw \<turnstile> (size ?cb,s1,stk) \<rightarrow>* (size ?cb + size ?cc,s2,stk)"
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nipkow@51148
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using WhileTrue.IH(1) by fastforce
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nipkow@43982
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moreover
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kleing@52396
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have "?cw \<turnstile> (size ?cb + size ?cc,s2,stk) \<rightarrow>* (0,s2,stk)"
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nipkow@45761
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by fastforce
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nipkow@43982
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moreover
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kleing@52396
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have "?cw \<turnstile> (0,s2,stk) \<rightarrow>* (size ?cw,s3,stk)" by(rule WhileTrue.IH(2))
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kleing@54052
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ultimately show ?case by(blast intro: star_trans)
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nipkow@45761
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qed fastforce+
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nipkow@13095
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webertj@20217
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end
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