160 lines
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160 lines
10 KiB
HTML
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Non-Local Control Flow Instructions (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="Non_002dLocal-Control-Flow-Instructions">
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Next: <a href="Instrumentation-Instructions.html" accesskey="n" rel="next">Instrumentation Instructions</a>, Previous: <a href="Trampoline-Instructions.html" accesskey="p" rel="prev">Trampoline Instructions</a>, Up: <a href="Instruction-Set.html" accesskey="u" rel="up">Instruction Set</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
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<h4 class="subsubsection" id="Non_002dLocal-Control-Flow-Instructions-1"><span>9.3.7.5 Non-Local Control Flow Instructions<a class="copiable-link" href="#Non_002dLocal-Control-Flow-Instructions-1"> ¶</a></span></h4>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-capture_002dcontinuation"><span class="category-def">Instruction: </span><span><strong class="def-name">capture-continuation</strong> <code class="def-code-arguments">s24:<var class="var">dst</var></code><a class="copiable-link" href="#index-capture_002dcontinuation"> ¶</a></span></dt>
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<dd><p>Capture the current continuation, and write it to <var class="var">dst</var>. Part of
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the implementation of <code class="code">call/cc</code>.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-continuation_002dcall"><span class="category-def">Instruction: </span><span><strong class="def-name">continuation-call</strong> <code class="def-code-arguments">c24:<var class="var">contregs</var></code><a class="copiable-link" href="#index-continuation_002dcall"> ¶</a></span></dt>
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<dd><p>Return to a continuation, nonlocally. The arguments to the continuation
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are taken from the stack. <var class="var">contregs</var> is a free variable containing
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the reified continuation.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-abort-1"><span class="category-def">Instruction: </span><span><strong class="def-name">abort</strong> <code class="def-code-arguments">x24:<var class="var">_</var></code><a class="copiable-link" href="#index-abort-1"> ¶</a></span></dt>
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<dd><p>Abort to a prompt handler. The tag is expected in slot 1, and the rest
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of the values in the frame are returned to the prompt handler. This
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corresponds to a tail application of <code class="code">abort-to-prompt</code>.
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</p>
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<p>If no prompt can be found in the dynamic environment with the given tag,
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an error is signaled. Otherwise all arguments are passed to the
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prompt’s handler, along with the captured continuation, if necessary.
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</p>
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<p>If the prompt’s handler can be proven to not reference the captured
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continuation, no continuation is allocated. This decision happens
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dynamically, at run-time; the general case is that the continuation may
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be captured, and thus resumed. A reinstated continuation will have its
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arguments pushed on the stack from slot 0, as if from a multiple-value
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return, and control resumes in the caller. Thus to the calling
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function, a call to <code class="code">abort-to-prompt</code> looks like any other function
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call.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-compose_002dcontinuation"><span class="category-def">Instruction: </span><span><strong class="def-name">compose-continuation</strong> <code class="def-code-arguments">c24:<var class="var">cont</var></code><a class="copiable-link" href="#index-compose_002dcontinuation"> ¶</a></span></dt>
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<dd><p>Compose a partial continuation with the current continuation. The
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arguments to the continuation are taken from the stack. <var class="var">cont</var> is a
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free variable containing the reified continuation.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-prompt"><span class="category-def">Instruction: </span><span><strong class="def-name">prompt</strong> <code class="def-code-arguments">s24:<var class="var">tag</var> b1:<var class="var">escape-only?</var> x7:<var class="var">_</var> f24:<var class="var">proc-slot</var> x8:<var class="var">_</var> l24:<var class="var">handler-offset</var></code><a class="copiable-link" href="#index-prompt"> ¶</a></span></dt>
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<dd><p>Push a new prompt on the dynamic stack, with a tag from <var class="var">tag</var> and a
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handler at <var class="var">handler-offset</var> words from the current <var class="var">ip</var>.
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</p>
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<p>If an abort is made to this prompt, control will jump to the handler.
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The handler will expect a multiple-value return as if from a call with
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the procedure at <var class="var">proc-slot</var>, with the reified partial continuation
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as the first argument, followed by the values returned to the handler.
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If control returns to the handler, the prompt is already popped off by
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the abort mechanism. (Guile’s <code class="code">prompt</code> implements Felleisen’s
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<em class="dfn">–F–</em> operator.)
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</p>
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<p>If <var class="var">escape-only?</var> is nonzero, the prompt will be marked as
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escape-only, which allows an abort to this prompt to avoid reifying the
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continuation.
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</p>
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<p>See <a class="xref" href="Prompts.html">Prompts</a>, for more information on prompts.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-throw-1"><span class="category-def">Instruction: </span><span><strong class="def-name">throw</strong> <code class="def-code-arguments">s12:<var class="var">key</var> s12:<var class="var">args</var></code><a class="copiable-link" href="#index-throw-1"> ¶</a></span></dt>
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<dd><p>Raise an error by throwing to <var class="var">key</var> and <var class="var">args</var>. <var class="var">args</var>
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should be a list.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-throw_002fvalue"><span class="category-def">Instruction: </span><span><strong class="def-name">throw/value</strong> <code class="def-code-arguments">s24:<var class="var">value</var> n32:<var class="var">key-subr-and-message</var></code><a class="copiable-link" href="#index-throw_002fvalue"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-throw_002fvalue_002bdata"><span class="category-def">Instruction: </span><span><strong class="def-name">throw/value+data</strong> <code class="def-code-arguments">s24:<var class="var">value</var> n32:<var class="var">key-subr-and-message</var></code><a class="copiable-link" href="#index-throw_002fvalue_002bdata"> ¶</a></span></dt>
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<dd><p>Raise an error, indicating <var class="var">val</var> as the bad value.
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<var class="var">key-subr-and-message</var> should be a vector, where the first element
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is the symbol to which to throw, the second is the procedure in which to
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signal the error (a string) or <code class="code">#f</code>, and the third is a format
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string for the message, with one template. These instructions do not
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fall through.
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</p>
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<p>Both of these instructions throw to a key with four arguments: the
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procedure that indicates the error (or <code class="code">#f</code>, the format string, a
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list with <var class="var">value</var>, and either <code class="code">#f</code> or the list with <var class="var">value</var>
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as the last argument respectively.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-unreachable"><span class="category-def">Instruction: </span><span><strong class="def-name">unreachable</strong> <code class="def-code-arguments">x24:<var class="var">_</var></code><a class="copiable-link" href="#index-unreachable"> ¶</a></span></dt>
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<dd><p>Abort the process. This instruction should never be reached and must
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not continue. You would think this is useless but that’s not the case:
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it is inserted after a primcall to <code class="code">raise-exception</code>, and allows
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compilers to know that this branch of control flow does not rejoin the
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graph.
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</p></dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Instrumentation-Instructions.html">Instrumentation Instructions</a>, Previous: <a href="Trampoline-Instructions.html">Trampoline Instructions</a>, Up: <a href="Instruction-Set.html">Instruction Set</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</body>
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