Qt
Internal/Contributor docs for the Qt SDK. Note: These are NOT official API docs; those are found at https://doc.qt.io/
Loading...
Searching...
No Matches
finetuning.qdoc
Go to the documentation of this file.
1// Copyright (C) 2019 The Qt Company Ltd.
2// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
3/*!
4
5\page qtqml-javascript-finetuning.html
6\title Configuring the JavaScript Engine
7\brief Describes the environment variables available, to control how Javascript is run.
8
9Running JavaScript code can be influenced by a few environment variables, particularly:
10
11\table
12 \header
13 \li Environment Variable
14 \li Description
15 \row
16 \li \c{QV4_JIT_CALL_THRESHOLD}
17 \li The JavaScript engine contains a Just-In-Time compiler (JIT). The JIT will compile
18 frequently run JavaScript functions into machine code to run faster. This
19 environment variable determines how often a function needs to be run to be
20 considered for JIT compilation. The default value is 3 times.
21 \row
22 \li \c{QV4_FORCE_INTERPRETER}
23 \li Setting this environment variable runs all functions and expressions through the
24 interpreter. The JIT is never used, no matter how often a function or expression is
25 called. Functions and expressions may still be compiled ahead of time using
26 \l{qmlcachegen} or \l{qmlsc}, but only the generated byte code is used at run time. Any
27 generated C++ code and the machine code resulting from it is ignored.
28 \row
29 \li \c{QV4_FAIL_ON_INVALID_AOT}
30 \li QML code compiled to C++ is validated at runtime. If this validation fails, by default,
31 the code is then interpreted instead of running as native code. With this environment
32 variable set, a validation failure terminates the program instead. This can be useful to
33 ensure that all the code that can run as native code does.
34 \row
35 \li \c{QV4_SKIP_AOT_VALIDATION}
36 \li This environment variable skips the runtime validation of C++ code generated for
37 compiled QML code. This can be used to avoid the overhead of the validation itself.
38 Note that this only skips the validation. It will not prevent the validation code itself
39 from being generated by qmlcachegen. For that use the \c{NO_GENERATE_AOT_VALIDATION}
40 option of \l{qt_add_qml_module}.
41 \row
42 \li \c{QV4_JS_MAX_STACK_SIZE}
43 \li The JavaScript engine reserves a special memory area as a stack to run JavaScript.
44 This stack is separate from the C++ stack. Usually this area is 4MB in size. If this
45 environment variable contains a number, the JavaScript engine interprets it as the
46 size of the memory area, in bytes, to be allocated as the JavaScript stack.
47 \row
48 \li \c{QV4_GC_MAX_STACK_SIZE}
49 \li In addition to the regular JavaScript stack, the JavaScript engine keeps another stack
50 for the garbage collector, usually 2MB of memory. If the garbage collector needs to
51 handle an excessive number of objects at the same time, this stack might overrun.
52 If it contains a number, this environment variable is interpreted as the size in bytes
53 of the memory area that will be allocated as the stack for the garbage collector.
54 \row
55 \li \c{QV4_STACK_SOFT_LIMIT}
56 \li When this environment variable is set, the JavaScript engine will throw a "RangeError:
57 Maximum call stack size exceeded" exception once the call stack usage reaches the
58 defined soft limit. If the variable is not set, the engine defaults to runtime detected
59 or Qt predefined limits depending on the OS.
60
61 \row
62 \li \c{QV4_CRASH_ON_STACKOVERFLOW}
63 \li Usually the JavaScript engine tries to catch C++ stack overflows caused by
64 excessively recursive JavaScript code, and generates a non-fatal error condition.
65 There are separate recursion checks for compiling JavaScript and running JavaScript. A
66 stack overflow when compiling JavaScript indicates that the code contains deeply nested
67 objects and functions. A stack overflow at run-time indicates that the code results in
68 a deeply recursive program. The check for this is only indirectly related to the
69 JavaScript stack size mentioned above, as each JavaScript function call consumes stack
70 space on both, the C++ and the JavaScript stack. The code that checks for excessive
71 recursion is necessarily conservative, as the available stack size depends on many
72 factors and can often be customized by the user. With this environment variable set, the
73 JavaScript engine does not check for stack overflows when compiling or running
74 JavaScript and will not generate exceptions for them. Instead, when the stack overflows
75 the program attempts an invalid memory access. This most likely terminates the
76 program. In turn, the program gets to use up all the stack space the operating system
77 can provide.
78 \warning malicious code may be able to evade the termination and access unexpected
79 memory locations this way.
80 \row
81 \li \c{QV4_MAX_CALL_DEPTH}
82 \li Stack overflows when running (as opposed to compiling) JavaScript are prevented by
83 controlling the call depth: the number of nested function invocations. By
84 default, an exception is generated if the call depth exceeds a maximum number tuned
85 to the platform's default stack size. If the \c{QV4_MAX_CALL_DEPTH} environment
86 variable contains a number, this number is used as maximum call depth. Beware that
87 the recursion limit when compiling JavaScript is not affected. The default maximum
88 call depth is 1234 on most platforms. On QNX it is 640 because on QNX the default
89 stack size is smaller than on most platforms.
90 \row
91 \li \target{QV4_GC_TIMELIMIT}
92 \c{QV4_GC_TIMELIMIT}
93 \li This value is used to tell the engine how much time it should spend in each
94 incremental garbage collection step. It can either be a positive number,
95 specifying the timelimit in milliseconds, or 0. If the value is 0,
96 garbage collection becomes non-incremental.
97 \row
98 \li \c{QV4_MM_AGGRESSIVE_GC}
99 \li Setting this environment variable runs the garbage collector before each memory
100 allocation. This is very expensive at run-time, but it quickly uncovers many memory
101 management errors, for example the manual deletion of an object belonging to the QML
102 engine from C++.
103 \row
104 \li \c{QV4_MM_CROSS_VALIDATE_INCREMENTAL_GC}
105 \li Setting this environment variable runs additional debug steps to
106 recognize cases where the incremental garbage collector is
107 misbehaving by comparing its behavior to that of the non-incremetal
108 garbage collector.
109 \row
110 \li \c{QV4_PROFILE_WRITE_PERF_MAP}
111 \li On Linux, the \c perf utility can be used to profile programs. To analyze JIT-compiled
112 JavaScript functions, it needs to know about their names and locations in memory. To
113 provide this information, there's a convention to create a special file called
114 \c{perf-<pid>.map} in \e{/tmp} which perf then reads. This environment variable, if
115 set, causes the JIT to generate this file.
116 \row
117 \li \c{QV4_SHOW_BYTECODE}
118 \li Outputs the IR bytecode generated by Qt to the console.
119 Has to be combined with \c{QML_DISABLE_DISK_CACHE} or already cached bytecode will not
120 be shown.
121 \row
122 \li \c{QV4_DUMP_BASIC_BLOCKS}
123 \li Outputs the basic blocks of each function compiled ahead of time. The details of the
124 blocks are printed to the console. Additionally, control flow graphs with the byte code
125 for each block are generated in the DOT format for each compiled function. The value of
126 \c {QV4_DUMP_BASIC_BLOCKS} is used as the path to the folder where the DOT files should
127 be generated. If the path is any of ["-", "1", "true"] or if files can't be opened,
128 the graphs are dumped to stdout instead.
129 \row
130 \li \c{QV4_VALIDATE_BASIC_BLOCKS}
131 \li Performs checks on the basic blocks of a function compiled ahead of time to validate
132 its structure and coherence. If the validation fails, an error message is printed to
133 the console.
134\endtable
135
136\l{The QML Disk Cache} accepts further environment variables that allow fine tuning its behavior.
137In particular \c{QML_DISABLE_DISK_CACHE} may be useful for debugging.
138
139*/