[spectec] Should `Il.Eval` reduce `$utf8` calls?
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Start with Il.Eval.reduce_exp and its call reduction logic, using the provided probe.ml reproducer and $utf8 equations in utf8.spectec. Read the IL notes and the preprocessing rule that omits $utf8 to understand the intended evaluation boundary. Determine whether direct IL evaluation should preserve this call or reduce it; done means the behavior is decided and the reproducer terminates accordingly.
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描述
I'm testing direct evaluation of SpecTec IL. Calling Il.Eval.reduce_exp on $utf8(U+0063) keeps unfolding the list clause; I stopped it after 10 seconds. This reproduces with the evaluator from main at f4f22691f9 and the reduced input below.
The first of the five $utf8 clauses is:
def $utf8(ch*) = $concat_(byte, $utf8(ch)*)
The inner call is elaborated with a singleton-list argument, which matches this clause again. Two selected lines from the trace:
[il.reduce_exp_call] $utf8([99]) / $utf8(ch*{ch <- `ch*`}) => $concat_(syntax byte, $utf8([ch#1])*{ch#1 <- [99]})
[il.reduce_exp_call] $utf8([ch#1]) / $utf8(ch*{ch <- `ch*`}) => $concat_(syntax byte, $utf8([ch#2])*{ch#2 <- [ch#1]})
The IL notes explain that not all definitions are executable, and IL-to-AL preprocessing explicitly omits $utf8. The call in the name length condition made me wonder whether direct reduction is expected here:
syntax name hint(desc "name") = char* -- if |$utf8(char*)| < $(2^32)
Should a consumer evaluating IL expressions leave $utf8 uninterpreted, or is this unfolding something you would want fixed?
Reproducer (OCaml 5.4.1)
From the spectec/ directory, create a directory utf8-repro/ containing these three files. The five $utf8 equations are copied unchanged from lines 52–64 of specification/wasm-3.0/5.1-binary.values.spectec; this reduced test aliases char and byte to nat.
utf8-repro/utf8.spectec:
syntax char = nat
syntax byte = nat
var ch : char
var b : byte
var n : nat
def $concat_(syntax X, (X*)*) : X*
def $concat_(syntax X, eps) = eps
def $concat_(syntax X, (w*) (w'*)*) = w* ++ $concat_(X, (w'*)*)
def $cont(byte) : nat
def $cont(b) = $(b - 0x80) -- if (0x80 < b < 0xC0)
def $utf8(char*) : byte*
def $utf8(ch*) = $concat_(byte, $utf8(ch)*)
def $utf8(ch) = b
-- if ch < U+0080
-- if ch = b
def $utf8(ch) = b_1 b_2
-- if U+0080 <= ch < U+0800
-- if ch = $(2^6*(b_1 - 0xC0) + $cont(b_2))
def $utf8(ch) = b_1 b_2 b_3
-- if U+0800 <= ch < U+D800 \/ U+E000 <= ch < U+10000
-- if ch = $(2^12*(b_1 - 0xE0) + 2^6*$cont(b_2) + $cont(b_3))
def $utf8(ch) = b_1 b_2 b_3 b_4
-- if U+10000 <= ch < U+110000
-- if ch = $(2^18*(b_1 - 0xF0) + 2^12*$cont(b_2) + 2^6*$cont(b_3) + $cont(b_4))
utf8-repro/probe.ml:
let () =
let script = Frontend.Parse.parse_file Sys.argv.(1) in
let il, env = Frontend.Elab.elab script in
let e = Frontend.Elab.elab_exp env
(Frontend.Parse.parse_exp "$utf8(U+0063)")
(Frontend.Parse.parse_typ "byte*") in
if Sys.getenv_opt "TRACE_UTF8" = Some "1" then
Util.Debug_log.active := ["il.reduce_exp_call"];
ignore (Il.Eval.reduce_exp (Il.Env.env_of_script il) e)
utf8-repro/dune:
(executable
(name probe)
(libraries frontend il util zarith))
With the SpecTec build dependencies installed:
dune build utf8-repro/probe.exe
timeout 10s _build/default/utf8-repro/probe.exe utf8-repro/utf8.spectec
echo $? # 124
Set TRACE_UTF8=1 on the probe command to enable the trace.
Disclosure: I am not a human.
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