2023/04/13

:immutable slot option

Immutability is far more valued nowadays than when CLOS was designed. Back then, basically the system allows programmers to change almost everything, assuming he knows what he was doing.

However, specifying something immutable is not only to protect users from shooting their own foot; it communicates to the readers. Here the readers can be a programmer who reads and uses the code years later (who can be the same programmer who wrote it bot has forgotten the details), or a program-processing programs such as optimizer to take advantage of immutablity.

CLOS (and Gauche's object system) have enough flexibility to implement immutable slots, but it is somewhat awkward. It's not as simple as having a custom setter that throws an error; for, the slot setter is also called in the instance initializer which runs at the instance construction. You have to distinguish whether the slot setter is invoked during initialization or outside initialization, but such dynamic introspection would be costly.

We came up an alternative mechanism which is effectively realizes immutable slots in practical situations, but does not require to distinguish whether it's in initialization or not.

If a slot has a true value for :immutable slot option, the slot can only be initialized once--that is, the setter sets the value if the slot is previously unbound, but throws an error if not. If you give the slot an initial value, either with :init-keyword or :init-value etc., then that one chance to set the value is used within initializer. Uninitialized immutable slots don't make much sense, so we expect almost always immutable slots are initialized this way.

It is possible that the initializer leaves the slot unbound, and later the user call slot-set! to set it once. It can be viewed as delayed initialization.

(We first named this option init-once, for the slot can be set once, but changed our mind for it could be confusing.)

Tag: ObjectSystem

2022/10/05

Source info propagation with macro expansion

Gauche tracks source code location information and shows it in the stack trace. However, what if the source is generated by macros? In 0.9.12, the macro expander re-attached the original source info to the outermost form of the macro output. However, if a runtime error occurred in constructed code other than the outermost one, stack trace couldn't find the info and had to show "[unknown location]". It was annoying especially when the code was the result of nested macro expansions, that you didn't get a clue about where the error came from.

I was annoyed enough, so from 0.9.13, you can have better stack trace. (Well, if you're familiar with other Scheme that employs syntax-case macro expander, you're already familiar with such a feature. Yes, Gauche finally caught up.)

Let's show it with a somewhat contrived example. The following cxr macro expands to cxxx...xxr according to the given sequence of a or d.

;; (cxr a r obj) == (car obj)
;; (cxr a a r obj) == (caar obj)
;; (cxr a d a r obj) == (cadar obj)
;;etc.
(define-syntax cxr
  (syntax-rules (a d r)
    [(_ r obj) obj]
    [(_ a xs ...) (car (cxr xs ...))]
    [(_ d xs ...) (cdr (cxr xs ...))]
    [(_ . xs) (syntax-error "Malformed cxr:" (cxr . xs))]))

In 0.9.12, if you pass something that causes a runtime error, you get the annoying unknown location:

gosh$ (cxr a a a a r '(1 2 3 4))
*** ERROR: pair required, but got 1
Stack Trace:
_______________________________________
  0  (car (cxr a r '(1 2 3 4)))
        [unknown location]
  1  (eval expr env)
        at "/usr/share/gauche-0.98/0.9.12/lib/gauche/interactive.scm":336

In 0.9.13, you'll get this:

gosh$ (cxr a a a a r '(1 2 3 4))
*** ERROR: pair required, but got 1
Stack Trace:
_______________________________________
  0  (car (cxr a r '(1 2 3 4)))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":15
        expanded from (cxr a a r '(1 2 3 4))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":15
        expanded from (cxr a a a r '(1 2 3 4))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":15
        expanded from (cxr a a a a r '(1 2 3 4))
        at "(standard input)":34
  1  (eval expr env)
        at "/home/shiro/src/Gauche/src/../lib/gauche/interactive.scm":354

This works with ER-macro, too. Suppose we have another macro, c*r, in which you can give a's and d's in a single symbol. That is, (c*r aada obj) is (caadar obj). We also let the code print the given symbol, just for the sake of making things complicated.

;; (c*r aa obj) == print 'aa' and return (caar obj)
;; (c*r addar obj) == print 'addar' and return (caadr obj)
;; etc.
(define-syntax c*r
  (er-macro-transformer
   (^[form rename cmp]
     (match form
       [(_ xs obj)
        (let1 cs (map ($ string->symbol $ string $)
                      (string->list (symbol->string xs)))
          (quasirename rename
            `(begin
               (print ',xs)
               (cxr ,@cs r ,obj))))]))))

Here's 0.9.12:

gosh$ (c*r aad '(1 2 3 4))
aad
*** ERROR: pair required, but got 2
Stack Trace:
_______________________________________
  0  (car (cxr a d r '(1 2 3 4)))
        [unknown location]
  1  (eval expr env)
        at "/usr/share/gauche-0.98/0.9.12/lib/gauche/interactive.scm":336

And HEAD:

gosh$ (c*r aad '(1 2 3 4))
aad
*** ERROR: pair required, but got 2
Stack Trace:
_______________________________________
  0  (car (cxr a d r '(1 2 3 4)))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":15
        expanded from (cxr a a d r '(1 2 3 4))
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":60
        expanded from (quasirename rename `(begin (print ',xs) (cxr ,@cs r  ...
        at "/home/shiro/src/Gauche/test/macro-source-info.scm":57
  1  (eval expr env)
        at "/home/shiro/src/Gauche/src/../lib/gauche/interactive.scm":354

Now, if you're user of syntax-case or syntax-rules, there's no wonder how it can be done. Macro output is constructed as syntactic objects, which can carry any sideband information. But with ER-macro, you construct the output as a simple S-expression, so it's not obviouhs where those information comes from.

Gauche has an extended pair that can carry extra information other than car and cdr. Those sideband data isn't visible as far as you're treating it as a pair, nor it affects equal?-ity of the pairs. Source code information is stored there by read procedure and its families.

gosh$ (read-from-string "(a b c d)")
(a b c d)
gosh$ (pair-attributes *1)
((source-info "(input string port)" 1))

If you consturct lists with cons or list, those information won't be attached. However, quasirename does the trick. It extracts the original source info from the input, and re-attaches it to the constructed form.

Note that, for a macro expander, we need to consider two kinds of source information: One is of the macro definition, and another is of the macro input. The source info of the macro definition is available through the argument of quasirename. But how can it get the macro input information? The macro input is already deconstructed by the time quasirename is called.

We use another sideband mechanism, procedure tags. Srfi-229 defines a general mechanism to attach an arbitrary Scheme object to a procedure. Gauche has more general mechanism (although not documented yet) that a procedure can have multiple tags, and the macro input is attached to the rename procedure as one of such tags. Then quasirename extracts that information from the rename procedure and applies it to the output.

In the definition of quasirename, the output construction code looks like this:

;; in src/libmacro.scm

   (if-let1 si (pair-attribute-get objs 'source-info #f)
     (let1 orig (assoc-ref ((with-module gauche.internal %procedure-tags-alist) r)
                           'macro-input)
       `(,extended-cons. ,xx ,yys '((source-info ,@si)
                                    ,@(cond-list
                                       [orig `(original . ,orig)]))))
     `(,cons. ,xx ,yys)))))

The pair attribute source-info holds the source info of macro definition, and original holds the macro input form.

The disadvantage of having source info in the sideband data of pairs is, obviously, that you can't attach source info to other objects than pairs. I find it not a big issue in practice, for most expressions that need attention are function calls, macro calls or special forms.

On the other hand, it has an advantage that quoted literal lists can have source code information. It can't be done with syntax objects, for quote strips any syntax wrappings. It is handy when you put a literal nested structure as DSL and let its walker signals an error with the location of the literal structure.

This is a desired feature and I'm happy to have it. However, I'm feeling a bit of ambivalence, too.

The reason I prefer ER-macro to syntax-case is that ER-macro is explicit---input and output are raw S-expression which you can direclty touch and rearrange. With syntax-case, things are wrapped in opaque syntax object, and even though you can unwrap and rewrap the objects, that opaqueness bothers me.

However, with this quasirename modification, I did introduce an implicit operation; even though the output of quasirename can be treated as an ordinary S-expression, it does more to it than just consing.

If I feel comfortable with this, maybe I can also feel comfortable with syntax-case, too. I don't know yet. Let's see.

Tags: Macro,, 0.9.13,, quasirename

2022/06/17

Using Gauche in GitHub Actions

I created a GitHub action to install Gauche in the runner, so that you can use Gauche in subsequent steps: setup-gauche. Currently the action works on Linux and OSX.

To use the action, simply say uses: shirok/setup-gauche@v3 in your job steps (check the latest version number in the setup-gauche page). The following is an excerpt of .github/workflow/main.yml of Gauche-mecab:

jobs:
  build-and-test-linux:
    runs-on: ubuntu-latest
    timeout-minutes: 10
    steps:
    - uses: actions/checkout@v3
    - uses: shirok/setup-gauche@v3
    - name: Install dependencies
      run: |
        sudo apt install -y libmecab-dev mecab-ipadic-utf8
    - name: Build and check
      run: |
        ./configure
        make
        make -s check
        make -s check-dep

Gauche is installed in standard path (/usr on Linux, /usr/local on OSX) so that you can build Gauche extensions or run Gauche applications without any extra settings.

By default, it installs the latest release. You can choose a specific version of Gauche to install via gauche-version input parameter; specifically, saying 'snapshot' installs the latest snapshot (prerelease) build, if there's any newer than the latest release.

Tags: github, CI

2022/04/02

Running gosh without installing

Recently I wrote some test scripts in Gauche for a project that didn't use Gauche in particular. I could've kicked get-gauche script during make check to install Gauche locally as needed, but that seemed a bit of overkill, especially it was just for small test scripts.

Then I thought, well, I already have a Docker image. If I can feed a local script to it...

So here it is. I included in a Docker image a small script gosh-script, which chdirs into /home/app and run gosh. If you mount local cwd on /home/app, the scripts, libraries and data in it are all visible to gosh in the Docker:

docker run --rm -ti -v `pwd`:/home/app practicalscheme/gauche gosh-script TEST-SCRIPT

Or, you can use run-gosh-in-docker.sh script.

You can't acceses local resources other than the filesystem below the current directory, and you can't use extra libraries. But for the simple tasks this is enough.

See README in Gauche-docker-image for the details.

Tag: Docker

2021/10/24

Is this an Undefined Behavior?

Automated tests of Gauche HEAD on Windows platform started failing since several days ago. The log showed SHA1 digest result didn't match. It's weird, for I haven't touched that part of code for long time.

I isolated the reproducible condition. It happens with the fairly new gcc (11.2.0) with -O2. It doesn't exhibit without optimization, nor with gcc 10.2.0 or other previous versions of gcc I have.

The problematic code is Aaron D. Gifford's SHA implementation sha2.c ( http://www.aarongifford.com/computers/sha.html ). It was last updated in January 2004, so it's pretty old, but I think it's still widely used.

I narrowed down the problem to around here:

        /* Set the bit count: */
#if BYTE_ORDER == LITTLE_ENDIAN
        /* Convert FROM host byte order */
        REVERSE64(context->s1.bitcount,context->s1.bitcount);
#endif
        void *buf56 = &context->s1.buffer[56];
        *(sha_word64*)buf56 = context->s1.bitcount;

        /* Final transform: */
        SHA1_Internal_Transform(context, (sha_word32*)context->s1.buffer);

In our case, BYTE_ORDER is LITTLE_ENDIAN. REVERSE64 is a macro to swap the byte order of a 64bit word. context->s1.bitcount is uint64_t, and context->s1.buffer is an array of unsigned chars. What it does is to store 64bit-word of bitcount into the buffer from 56th octet in the network byte order, and calls SHA1_Internal_Transform.

It compiles to this code with optimization:

   25ca75e9d:   48 8b 53 18             mov    0x18(%rbx),%rdx
   25ca75ea1:   48 0f ca                bswap  %rdx
   25ca75ea4:   48 89 53 18             mov    %rdx,0x18(%rbx)
   25ca75ea8:   48 89 d9                mov    %rbx,%rcx
   25ca75eab:   4c 89 e2                mov    %r12,%rdx
   25ca75eae:   e8 8d fa ff ff          call   25ca75940 <SHA1_Internal_Transform>

Here, %rbx contains the pointer to context, and %r12 to context->s1.buffer. The first three instructions swap the 64bit word. (By the way, REVERSE64 macro is written with shifts and bitmasks. Gcc cleverly figures out its intent and replaces the whole expression by a bswap instrucion.) The next three instruction is the calling sequence of SHA1_Internal_Transform.

Wait. There're no instructions emitted to store bitcount into *buf56. I checked the assembly after this but there're no instructions for the store either.

If I insert a dummy external function call before SHA1_Internal_Transform like this:

        /* Set the bit count: */
#if BYTE_ORDER == LITTLE_ENDIAN
        /* Convert FROM host byte order */
        REVERSE64(context->s1.bitcount,context->s1.bitcount);
#endif
        void *buf56 = &context->s1.buffer[56];
        *(sha_word64*)buf56 = context->s1.bitcount;

        puts("foo");

        /* Final transform: */
        SHA1_Internal_Transform(context, (sha_word32*)context->s1.buffer);

Then the storing to *buf56 appears (mov %rdx, 0x58(%rbx)):

   25ca75e9d:   48 8b 53 18             mov    0x18(%rbx),%rdx
   25ca75ea1:   48 0f ca                bswap  %rdx
   25ca75ea4:   48 89 53 18             mov    %rdx,0x18(%rbx)
   25ca75ea8:   48 8d 0d 8f c7 00 00    lea    0xc78f(%rip),%rcx        # 25ca8263e <.rdata+0x9e>
   25ca75eaf:   48 89 53 58             mov    %rdx,0x58(%rbx)
   25ca75eb3:   e8 60 2e 00 00          call   25ca78d18 <puts>
   25ca75eb8:   4c 89 e2                mov    %r12,%rdx
   25ca75ebb:   48 89 d9                mov    %rbx,%rcx
   25ca75ebe:   e8 7d fa ff ff          call   25ca75940 <SHA1_Internal_Transform>

Now, accessing type punned pointer can break the strict aliasing rule. The gcc might have figured the storing into *buf56 had nothing to do with SHA1_Internal_Transform. But I feel there still needs to be a leap that it completely eliminates the store instruction.

Does *(sha_word64*)buf56 = context->s1.bitcount triggers Undefined Behavior? That's why gcc is entitled to remove that code?

Tags: gcc, Optimization, UndefinedBehavior