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Various updates suggested by Misha and Chris. Moved material that is
aimed more at the comiler writer than the llvmc user later in the document. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15561 91177308-0d34-0410-b5e6-96231b3b80d8
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Reid Spencer
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@ -33,6 +33,13 @@ following main goals or purposes:
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The tool itself does nothing with a user's program. It merely invokes other
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tools to get the compilation tasks done.
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The options supported by B<llvmc> generalize the compilation process and
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provide a consistent and simple interface for multiple programming languages.
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This makes it easier for developers to get their software compiled with LLVM.
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Without B<llvmc>, developers would need to understand how to invoke the
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front-end compiler, optimizer, assembler, and linker in order to compile their
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programs. B<llvmc>'s sole mission is to trivialize that process.
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=head2 Basic Operation
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B<llvmc> always takes the following basic actions:
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@ -41,23 +48,35 @@ B<llvmc> always takes the following basic actions:
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=item * Command line options and filenames are collected.
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This provides the basic instruction to B<llvmc> on what actions it should
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take. This is the I<request> the user is making of B<llvmc>.
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The command line options provide the marching orders to B<llvmc> on what actions
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it should perform. This is the I<request> the user is making of B<llvmc> and it
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is interpreted first.
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=item * Configuration files are read.
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Based on the options and the suffixes of the filenames presented, a set
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of configuration files are read to configure the actions B<llvmc>
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will take (more on this later).
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Based on the options and the suffixes of the filenames presented, a set of
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configuration files are read to configure the actions B<llvmc> will take
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(more on this later).
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=item * Determine actions to take.
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The tool chain needed to complete the task is determined. This is the
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primary work of B<llvmc>. It breaks the request specified by the
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command line options into a set of basic actions to be done:
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pre-processing, compilation, assembly, optimization, and linking.
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For each applicable action, it selects the command to be run from
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the specifications in the configuration files.
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The tool chain needed to complete the task is determined. This is the primary
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work of B<llvmc>. It breaks the request specified by the command line options
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into a set of basic actions to be done:
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=over
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=item * Pre-processing: gathering/filtering compiler input
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=item * Compilation: source language to bytecode conversion
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=item * Assembly: bytecode to native code conversion
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=item * Optimization: conversion of bytecode to something that runs faster
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=item * Linking: combining multiple bytecodes to produce executable program
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=back
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=item * Execute actions.
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@ -66,17 +85,6 @@ B<llvmc> terminates.
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=back
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=head2 Configuration Files
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B<llvmc> is highly configurable both on the command line and in configuration
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files. Configuration files specify the details of what commands to run
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for a given action. Each front end compiler must provide its own
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configuration file to tell B<llvmc> how to invoke that compiler. The LLVM
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toolset does not need to be configured as B<llvmc> just "knows" how to
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invoke those tools.
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Rest TBD.
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=head1 OPTIONS
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=head2 Control Options
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@ -160,7 +168,7 @@ considering the program as a whole.
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This is the same as B<-O4> except it employs aggressive analyses and
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aggressive inter-procedural optimization.
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=item B<-O6> (profile guided optimization - not implemented)
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=item B<-O6> (profile guided optimization: not implemented)
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This is the same as B<-O5> except that it employes profile-guided
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reoptimization of the program after it has executed. Note that this implies
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@ -168,7 +176,7 @@ a single level of reoptimization based on runtime profile analysis. Once
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the re-optimization has completed, the profiling instrumentation is
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removed and final optimizations are employed.
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=item B<-O7> (lifelong optimization - not implemented)
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=item B<-O7> (lifelong optimization: not implemented)
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This is the same as B<-O5> and similar to B<-O6> except that reoptimization
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is performed through the life of the program. That is, each run will update
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@ -225,48 +233,6 @@ options.
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=back
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=head2 Configuration Options
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=over
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=item B<--show-config> I<[suffixes...]>
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When this option is given, the only action taken by B<llvmc> is to show its
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final configuration state in the form of a configuration file. No compilation
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tasks will be conducted when this option is given; processing will stop once
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the configuration has been printed. The optional (comma separated) list of
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suffixes controls what is printed. Without any suffixes, the configuration
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for all languages is printed. With suffixes, only the languages pertaining
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to those file suffixes will be printed. The configuration information is
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printed after all command line options and configuration files have been
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read and processed. This allows the user to verify that the correct
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configuration data has been read by B<llvmc>.
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=item B<--config> :I<section>:I<name>=I<value>
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This option instructs B<llvmc> to accept I<value> as the value for configuration
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item I<name> in the section named I<section>. This is a quick way to override
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a configuration item on the command line without resorting to changing the
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configuration files.
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=item B<--config-file> F<dirname>
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This option tells B<llvmc> to read configuration data from the I<directory>
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named F<dirname>. Data from such directories will be read in the order
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specified on the command line after all other standard config files have
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been read. This allows users or groups of users to conveniently create
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their own configuration directories in addition to the standard ones to which
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they may not have write access.
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=item B<--config-only-from> F<dirname>
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This option tells B<llvmc> to skip the normal processing of configuration
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files and only configure from the contents of the F<dirname> directory. Multiple
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B<--config-only-from> options may be given in which case the directories are
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read in the order given on the command line.
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=back
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=head2 Information Options
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=over
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@ -325,29 +291,13 @@ Pass an arbitrary optionsto the code generator.
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=over
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=item B<-I>F<path> (C/C++ Only)
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=item B<-I>F<path>
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This option is just passed through to a C or C++ front end compiler to tell it
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where include files can be found.
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=back
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=head2 Advanced Options
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=over
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=item B<--emit-raw-code>
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No optimization is done whatsoever. The compilers invoked by B<llvmc> with
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this option given will be instructed to produce raw, unoptimized code. This
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option is useful only to front end language developers and therefore does not
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participate in the list of B<-O> options. This is distinctly different from
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the B<-O0> option (a synonym for B<-O1>) because those optimizations will
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reduce code size to make compilation faster. With B<--emit-raw-code>, only
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the full raw code produced by the compiler will be generated.
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=back
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=head2 Miscellaneous Options
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=over
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@ -363,6 +313,205 @@ and terminate.
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=back
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=head2 Advanced Options
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You better know what you're doing if you use these options. Improper use
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of these options can produce drastically wrong results.
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=over
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=item B<--show-config> I<[suffixes...]>
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When this option is given, the only action taken by B<llvmc> is to show its
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final configuration state in the form of a configuration file. No compilation
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tasks will be conducted when this option is given; processing will stop once
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the configuration has been printed. The optional (comma separated) list of
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suffixes controls what is printed. Without any suffixes, the configuration
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for all languages is printed. With suffixes, only the languages pertaining
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to those file suffixes will be printed. The configuration information is
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printed after all command line options and configuration files have been
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read and processed. This allows the user to verify that the correct
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configuration data has been read by B<llvmc>.
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=item B<--config> :I<section>:I<name>=I<value>
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This option instructs B<llvmc> to accept I<value> as the value for configuration
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item I<name> in the section named I<section>. This is a quick way to override
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a configuration item on the command line without resorting to changing the
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configuration files.
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=item B<--config-file> F<dirname>
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This option tells B<llvmc> to read configuration data from the I<directory>
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named F<dirname>. Data from such directories will be read in the order
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specified on the command line after all other standard config files have
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been read. This allows users or groups of users to conveniently create
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their own configuration directories in addition to the standard ones to which
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they may not have write access.
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=item B<--config-only-from> F<dirname>
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This option tells B<llvmc> to skip the normal processing of configuration
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files and only configure from the contents of the F<dirname> directory. Multiple
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B<--config-only-from> options may be given in which case the directories are
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read in the order given on the command line.
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=item B<--emit-raw-code>
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No optimization is done whatsoever. The compilers invoked by B<llvmc> with
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this option given will be instructed to produce raw, unoptimized code. This
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option is useful only to front end language developers and therefore does not
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participate in the list of B<-O> options. This is distinctly different from
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the B<-O0> option (a synonym for B<-O1>) because those optimizations will
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reduce code size to make compilation faster. With B<--emit-raw-code>, only
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the full raw code produced by the compiler will be generated.
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=back
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=head1 CONFIGURATION
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=head2 Warning
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Configuration information is relatively static for a given release of LLVM and
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a front end compiler. However, the details may change from release to release.
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Users are encouraged to simply use the various options of the B<llvmc> command
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and ignore the configuration of the tool. These configuration files are for
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compiler writers and LLVM developers. Those wishing to simply use B<llvmc>
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don't need to understand this section but it may be instructive on what the tool
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does.
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=head2 Introduction
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B<llvmc> is highly configurable both on the command line and in configuration
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files. The options it understands are generic, consistent and simple by design.
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Furthermore, the B<llvmc> options apply to the compilation of any LLVM enabled
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programming language. To be enabled as a supported source language compiler, a
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compiler writer must provide a configuration file that tells B<llvmc> how to
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invoke the compiler and what its capabilities are. The purpose of the
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configuration files then is to allow compiler writers to specify to B<llvmc> how
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the compiler should be invoked. Users may but are not advised to alter the
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compiler's B<llvmc> configuration.
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Because B<llvmc> just invokes other programs, it must deal with the
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available command line options for those programs regardless of whether they
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were written for LLVM or not. Furthermore, not all compilation front ends will
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have the same capabilities. Some front ends will simply generate LLVM assembly
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code, others will be able to generate fully optimized byte code. In general,
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B<llvmc> doesn't make any assumptions about the capabilities or command line
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options of a sub-tool. It simply uses the details found in the configuration
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files and leaves it to the compiler writer to specify the configuration
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correctly.
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Ths approach means that new compiler front ends can be up and working very
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quickly. As a first cut, a front end can simply compile its source to raw
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(unoptimized) bytecode or LLVM assembly and B<llvmc> can be configured to pick
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up the slack (translate LLVm assembly to bytecode, optimize the bytecode,
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generate native assembly, link, etc.). In fact, the front end need not use
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any LLVM libraries, and it could be written in any language (instead of C++).
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The configuration data will allow the full range of optimization, assembly,
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and linking capabilities that LLVM provides to be added to these kinds of tools.
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Enabling the rapid development of front-ends is one of the primary goals of
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B<llvmc>.
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As a compiler front end matures, it may utilize the LLVM libraries and tools to
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more efficiently produce optimized bytecode directly in a single compilation and
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optimization program. In these cases, multiple tools would not be needed and
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the configuration data for the compiler would change.
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Configuring B<llvmc> to the needs and capabilities of a source language compiler
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is relatively straight forward. The compilation process is broken down into five
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phases:
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=over
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=item * Pre-processing (filter and combine source files)
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=item * Translation (translate source language to LLVM assembly or bytecode)
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=item * Optimization (make bytecode execute quickly)
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=item * Assembly (converting bytecode to object code)
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=item * Linking (converting translated code to an executable)
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=back
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A compiler writer must provide a definition of what to do for each of these five
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phases for each of the optimization levels. The specification consists simply of
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prototypical command lines into which B<llvmc> can substitute command line
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arguments and file names. Note that any given phase can be completely blank if
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the source language's compiler combines multiple phases into a single program.
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For example, quite often pre-processng, translation, and optimization are
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combined into a single program. The specification for such a compiler would have
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blank entries for pre-processing and translation but a full command line for
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optimization.
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=head2 Configuration File Types
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There are two types of configuration files: the master configuration file
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and the language specific configuration file.
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The master configuration file contains the general configuration of B<llvmc>
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itself. This includes things like the mapping between file extensions and
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source languages. This mapping is needed in order to quickly read only the
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applicable language-specific configuration files (avoiding reading every config
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file for every compilation task).
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Language specific configuration files tell B<llvmc> how to invoke the language's
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compiler for a variety of different tasks and what other tools are needed to
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I<backfill> the compiler's missing features (e.g. optimization).
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Language specific configuration files are placed in directories and given
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specific names to foster faster lookup. The name of a given configuration file
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is the name of the source language.
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=head2 Default Directory Locations
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B<llvmc> will look for configuration files in two standard locations: the
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LLVM installation directory (typically C</usr/local/llvm/etc>) and the user's
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home directory (typically C</home/user/.llvm>). In these directories a file named
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C<master> provides the master configuration for B<llvmc>. Language specific
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files will have a language specific name (e.g. C++, Stacker, Scheme, FORTRAN).
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When reading the configuration files, the master files are always read first in
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the following order:
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=over
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=item 1 C<master> in LLVM installation directory
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=item 2 C<master> in the user's home directory.
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=back
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Then, based on the command line options and the suffixes of the file names
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provided on B<llvmc>'s command line, one or more language specific configuration
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files are read. Only the language specific configuration files actually needed
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to complete B<llvmc>'s task are read. Other language specific files will be
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ignored.
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Note that the user can affect this process in several ways using the various
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B<--config-*> options and with the B<--x LANGUAGE> option.
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Although a user I<can> override the master configuration file, this is not
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advised. The capability is retained so that compiler writers can affect the
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master configuration (such as adding new file suffixes) while developing a new
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compiler front end since they might not have write access to the installed
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master configuration.
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=head2 Syntax
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The syntax of the configuration files is yet to be determined. There are three
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viable options:
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=over
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=item XML
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=item Windows .ini
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=item specific to B<llvmc>
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=back
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=head1 EXIT STATUS
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If B<llvmc> succeeds, it will exit with 0. Otherwise, if an error
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Block a user