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annotate lwcc/README.txt @ 557:b2ee86298516

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Reduce default optimization level to -O2 It seems -O3 isn't as reliable as it maybe should be. There is plenty of information out there indicating that it is more likely to trigger compiler bugs so setting the default to -O2 instead.
author William Astle <lost@l-w.ca>
date Fri, 08 Sep 2023 00:35:11 -0600
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1 This is the lwcc C compiler for lwtools. It was written using various other
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2 C compilers as guides. Special thanks to the developers of the PCC compiler.
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3 While none of the actual code from PCC was actually used, much of compiler
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4 itself served as a template for creating lwcc.
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7 LIMITATIONS AND DESIGN CHOICES
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8 ==============================
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10 The direct interface to both the compiler proper and the preprocessor is
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11 specifically undefined. Indeed, the preprocessor may, in fact, not be a
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12 separate program at all. Relying on the specific format of the output of the
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13 preprocessor is specifically forbidden even though it is possible to obtain
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14 preprocessed output from the compiler driver. This is provided for debugging
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15 purposes only.
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17 The preprocessor supports variadic macros. It also supports stringification,
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18 and token concatenation but only within a macro expansion. There are
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19 examples online that use the construct "#__LINE__" to get a string version
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20 of the current line number.
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22 The preprocessor defaults to ignoring trigraphs because they are basically a
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23 stupid idea on any current system. They have their place for systems where
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24 creating the nine characters specified by the trigraphs is very difficult or
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25 impossible. It is possible, however, to instruct the preprocessor to decode
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26 trigraph sequences.
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27
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28 The nonstandard "#pragma once" feature is not supported at all. The effect
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29 is easily accomplished using standard macros and conditionals. It is,
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30 therefore, unneeded complexity.
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32 The nonstandard idea of preprocessor assertions is also completely
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33 unsupported. It is just as easy to test predefined macros and such tests are
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34 much more portable.
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36 The preprocessor supports __LINE__, __FILE__, __DATE__, and __TIME__. The
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37 compiler itself supports __func__ as a predefined string constant if
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38 encountered because there is no way for the preprocessor to determine what
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39 function it occurs within. The preprocessor does not define __STDC__,
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40 __STDC_VERSION__, or __STDC_HOSTED__. I have seen no truly useful purpose
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41 for these and since lwcc does not, at this time, conform to any known C
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42 standard, it would be incorrect to define the first two.
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43
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44 The compiler driver may define additional macros depending on its idea of
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45 the context.
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48 RUNTIME INFORMATION
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49 ===================
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50
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51 The compiler driver has a built in base directory where it searches for its
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52 various components as needed. In the discussion below, BASEDIR stands for
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53 that directory.
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55 BASEDIR may be specified by the -B option to the driver. Care must be taken
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56 when doing so, however, because specifying an invalid -B will cause the
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57 compiler to fail completely. It will completely override the built in search
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58 paths for the compiler provided files and programs.
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60 Because BASEDIR is part of the actual compiler, it is not affected by
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61 --sysroot or -isysroot options.
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63 If BASEDIR does not exist, compiler component programs will be searched for
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64 in the standard execution paths. This may lead to incorrect results so it is
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65 important to make certain that the specified BASEDIR exists.
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66
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67 If -B is not specified, the default BASEDIR is
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68 $(PREFIX)/lib/lwcc/$(VERSION)/ where PREFIX is the build prefix from the
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69 Makefile and VERSION is the lwtools version.
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70
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71 The contents of BASEDIR are as follows:
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73 BASEDIR/bin
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75 Various binaries for the parts of the compiler system. Notably, this
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76 includes the preprocessor and compiler proper. The specific names and
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77 contents of this directory cannot be relied upon and these programs should
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78 not be called directly. Ever. Don't do it.
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81 BASEDIR/lib
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83 This directory contains various libraries that provide support for any
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84 portion of the compiler's output. The driver will arrange to pass the
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85 appropriate arguments to the linker to include these as required.
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86
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87 The most notable file in this directory is liblwcc.a wich contains the
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88 support routines for the compiler's code generation. Depending on ABI and
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89 code generation options supported, there may be multiple versions of
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90 liblwcc.a. The driver will arrange for the correct one to be referenced.
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93 BASEDIR/include
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94
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95 This directory contains any C header files that the compiler provides.
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96 Notably, this includes stdint.h, stdarg.h, and setjmp.h as these are
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97 specific to the compiler. The driver will arrange for this directory to be
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98 searched prior to the standard system directories so that these files will
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99 override any present in those directories.
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100