149 lines
7 KiB
HTML
149 lines
7 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Stack Layout (Guile Reference Manual)</title>
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<meta name="description" content="Stack Layout (Guile Reference Manual)">
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<meta name="keywords" content="Stack Layout (Guile Reference Manual)">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="A-Virtual-Machine-for-Guile.html" rel="up" title="A Virtual Machine for Guile">
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<link href="Variables-and-the-VM.html" rel="next" title="Variables and the VM">
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<link href="VM-Concepts.html" rel="prev" title="VM Concepts">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Stack-Layout">
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<div class="nav-panel">
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<p>
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Next: <a href="Variables-and-the-VM.html" accesskey="n" rel="next">Variables and the VM</a>, Previous: <a href="VM-Concepts.html" accesskey="p" rel="prev">VM Concepts</a>, Up: <a href="A-Virtual-Machine-for-Guile.html" accesskey="u" rel="up">A Virtual Machine for Guile</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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</div>
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<hr>
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<h4 class="subsection" id="Stack-Layout-1"><span>9.3.3 Stack Layout<a class="copiable-link" href="#Stack-Layout-1"> ¶</a></span></h4>
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<p>The stack of Guile’s virtual machine is composed of <em class="dfn">frames</em>. Each
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frame corresponds to the application of one compiled procedure, and
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contains storage space for arguments, local variables, and some
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bookkeeping information (such as what to do after the frame is
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finished).
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</p>
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<p>While the compiler is free to do whatever it wants to, as long as the
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semantics of a computation are preserved, in practice every time you
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call a function, a new frame is created. (The notable exception of
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course is the tail call case, see <a class="pxref" href="Tail-Calls.html">Tail calls</a>.)
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</p>
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<p>The structure of the top stack frame is as follows:
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</p>
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<div class="example">
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<pre class="example-preformatted"> | ...previous frame locals... |
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+==============================+ <- fp + 3
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| Dynamic link |
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+------------------------------+
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| Virtual return address (vRA) |
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+------------------------------+
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| Machine return address (mRA) |
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+==============================+ <- fp
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| Local 0 |
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+------------------------------+
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| Local 1 |
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+------------------------------+
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| ... |
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+------------------------------+
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| Local N-1 |
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\------------------------------/ <- sp
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</pre></div>
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<p>In the above drawing, the stack grows downward. At the beginning of a
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function call, the procedure being applied is in local 0, followed by
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the arguments from local 1. After the procedure checks that it is being
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passed a compatible set of arguments, the procedure allocates some
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additional space in the frame to hold variables local to the function.
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</p>
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<p>Note that once a value in a local variable slot is no longer needed,
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Guile is free to re-use that slot. This applies to the slots that were
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initially used for the callee and arguments, too. For this reason,
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backtraces in Guile aren’t always able to show all of the arguments: it
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could be that the slot corresponding to that argument was re-used by
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some other variable.
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</p>
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<p>The <em class="dfn">virtual return address</em> is the <code class="code">ip</code> that was in effect
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before this program was applied. When we return from this activation
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frame, we will jump back to this <code class="code">ip</code>. Likewise, the <em class="dfn">dynamic
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link</em> is the offset of the <code class="code">fp</code> that was in effect before this
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program was applied, relative to the current <code class="code">fp</code>.
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</p>
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<p>There are two return addresses: the virtual return address (vRA), and
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the machine return address (mRA). The vRA is always present and
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indicates a bytecode address. The mRA is only present when a call is
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made from a function with machine code (e.g. a function that has been
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JIT-compiled).
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</p>
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<p>To prepare for a non-tail application, Guile’s VM will emit code that
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shuffles the function to apply and its arguments into appropriate stack
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slots, with three free slots below them. The call then initializes
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those free slots to hold the machine return address (or NULL), the
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virtual return address, and the offset to the previous frame pointer
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(<code class="code">fp</code>). It then gets the <code class="code">ip</code> for the function being called
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and adjusts <code class="code">fp</code> to point to the new call frame.
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</p>
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<p>In this way, the dynamic link links the current frame to the previous
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frame. Computing a stack trace involves traversing these frames.
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</p>
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<p>Each stack local in Guile is 64 bits wide, even on 32-bit architectures.
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This allows Guile to preserve its uniform treatment of stack locals
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while allowing for unboxed arithmetic on 64-bit integers and
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floating-point numbers. See <a class="xref" href="Instruction-Set.html">Instruction Set</a>, for more on unboxed
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arithmetic.
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</p>
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<p>As an implementation detail, we actually store the dynamic link as an
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offset and not an absolute value because the stack can move at runtime
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as it expands or during partial continuation calls. If it were an
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absolute value, we would have to walk the frames, relocating frame
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pointers.
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</p>
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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="Variables-and-the-VM.html">Variables and the VM</a>, Previous: <a href="VM-Concepts.html">VM Concepts</a>, Up: <a href="A-Virtual-Machine-for-Guile.html">A Virtual Machine for Guile</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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</div>
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</body>
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</html>
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