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diff --git a/share/doc/gccint/Changing-Multiple-RTL-SSA-Instructions.html b/share/doc/gccint/Changing-Multiple-RTL-SSA-Instructions.html new file mode 100644 index 0000000..c9b65bb --- /dev/null +++ b/share/doc/gccint/Changing-Multiple-RTL-SSA-Instructions.html @@ -0,0 +1,211 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>GNU Compiler Collection (GCC) Internals: Changing Multiple RTL SSA Instructions</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Changing Multiple RTL SSA Instructions"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Changing Multiple RTL SSA Instructions"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Changing-RTL-Instructions.html#Changing-RTL-Instructions" rel="up" title="Changing RTL Instructions"> +<link href="Sharing.html#Sharing" rel="next" title="Sharing"> +<link href="Changing-One-RTL-SSA-Instruction.html#Changing-One-RTL-SSA-Instruction" rel="previous" title="Changing One RTL SSA Instruction"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Changing-Multiple-RTL-SSA-Instructions"></a> +<div class="header"> +<p> +Previous: <a href="Changing-One-RTL-SSA-Instruction.html#Changing-One-RTL-SSA-Instruction" accesskey="p" rel="previous">Changing One RTL SSA Instruction</a>, Up: <a href="Changing-RTL-Instructions.html#Changing-RTL-Instructions" accesskey="u" rel="up">Changing RTL Instructions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Changing-Multiple-RTL-SSA-Instructions-1"></a> +<h4 class="subsubsection">14.21.8.2 Changing Multiple RTL SSA Instructions</h4> + +<p>The process for changing multiple instructions is similar +to the process for changing single instructions +(see <a href="Changing-One-RTL-SSA-Instruction.html#Changing-One-RTL-SSA-Instruction">Changing One RTL SSA Instruction</a>). The pass should +again start the change attempt with: +</p> +<div class="smallexample"> +<pre class="smallexample">auto attempt = crtl->ssa->new_change_attempt (); +</pre></div> + +<p>and keep <code>attempt</code> in scope for the duration of the change +attempt. It should then construct an <code>rtl_ssa::insn_change</code> +for each change that it wants to make. +</p> +<p>After this, it should combine the changes into a sequence of +<code>rtl_ssa::insn_change</code> pointers. This sequence must be in +reverse postorder; the instructions will remain strictly in the +order that the sequence specifies. +</p> +<p>For example, if a pass is changing exactly two instructions, +it might do: +</p> +<div class="smallexample"> +<pre class="smallexample">rtl_ssa::insn_change *changes[] = { &change1, change2 }; +</pre></div> + +<p>where <code>change1</code>’s instruction must come before <code>change2</code>’s. +Alternatively, if the pass is changing a variable number of +instructions, it might build up the sequence in a +<code>vec<rtl_ssa::insn_change *></code>. +</p> +<p>By default, <code>rtl_ssa::restrict_movement</code> assumes that all +instructions other than the one passed to it will remain in their +current positions and will retain their current uses and definitions. +When changing multiple instructions, it is usually more effective +to ignore the other instructions that are changing. The sequencing +described above ensures that the changing instructions remain +in the correct order with respect to each other. +The way to do this is: +</p> +<div class="smallexample"> +<pre class="smallexample">if (!rtl_ssa::restrict_movement (change, insn_is_changing (changes))) + return false; +</pre></div> + +<p>Similarly, when <code>rtl_ssa::restrict_movement</code> is detecting +whether a register can be clobbered, it by default assumes that +all other instructions will remain in their current positions and +retain their current form. It is again more effective to ignore +changing instructions (which might, for example, no longer need +to clobber the flags register). The way to do this is: +</p> +<div class="smallexample"> +<pre class="smallexample">if (!rtl_ssa::recog (change, insn_is_changing (changes))) + return false; +</pre></div> + +<p>When changing multiple instructions, the important question is usually +not whether each individual change is worthwhile, but whether the changes +as a whole are worthwhile. The way to test this is: +</p> +<div class="smallexample"> +<pre class="smallexample">if (!rtl_ssa::changes_are_worthwhile (changes)) + return false; +</pre></div> + +<p>The process for changing single instructions makes sure that one +<code>rtl_ssa::insn_change</code> in isolation is valid. But when changing +multiple instructions, it is also necessary to test whether the +sequence as a whole is valid. For example, it might be impossible +to satisfy all of the <code>move_range</code>s at once. +</p> +<p>Therefore, once the pass has a sequence of changes that are +individually correct, it should use: +</p> +<div class="smallexample"> +<pre class="smallexample">if (!crtl->ssa->verify_insn_changes (changes)) + return false; +</pre></div> + +<p>to check whether the sequence as a whole is valid. If all checks pass, +the final step is: +</p> +<div class="smallexample"> +<pre class="smallexample">confirm_change_group (); +crtl->ssa->change_insns (changes); +</pre></div> + +<p>Putting all this together, the process for a two-instruction change is: +</p> +<div class="smallexample"> +<pre class="smallexample">auto attempt = crtl->ssa->new_change_attempt (); + +rtl_ssa::insn_change change (insn1); +change1.new_defs = …; +change1.new_uses = …; +change1.move_range = …; + +rtl_ssa::insn_change change (insn2); +change2.new_defs = …; +change2.new_uses = …; +change2.move_range = …; + +rtl_ssa::insn_change *changes[] = { &change1, change2 }; + +auto is_changing = insn_is_changing (changes); +if (!rtl_ssa::restrict_movement (change1, is_changing) + || !rtl_ssa::restrict_movement (change2, is_changing)) + return false; + +insn_change_watermark watermark; +// Use validate_change etc. to change INSN1's and INSN2's patterns. +… +if (!rtl_ssa::recog (change1, is_changing) + || !rtl_ssa::recog (change2, is_changing) + || !rtl_ssa::changes_are_worthwhile (changes) + || !crtl->ssa->verify_insn_changes (changes)) + return false; + +confirm_change_group (); +crtl->ssa->change_insns (changes); +</pre></div> + +<hr> +<div class="header"> +<p> +Previous: <a href="Changing-One-RTL-SSA-Instruction.html#Changing-One-RTL-SSA-Instruction" accesskey="p" rel="previous">Changing One RTL SSA Instruction</a>, Up: <a href="Changing-RTL-Instructions.html#Changing-RTL-Instructions" accesskey="u" rel="up">Changing RTL Instructions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |