2004-08-06 16:58:48 +00:00
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=pod
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=head1 NAME
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llvmc - The LLVM Compiler Driver
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=head1 SYNOPSIS
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B<llvmc> [I<options>] [I<filenames>...]
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=head1 DESCRIPTION
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The B<llvmc> command is a configurable driver for invoking other
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LLVM (and non-LLVM) tools in order to compile, optimize and link software
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for multiple languages. For those familiar with the GNU Compiler
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Collection's B<gcc> tool, it is very similar. This tool has the
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following main goals or purposes:
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=over
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=item * A Single point of access to the LLVM tool set.
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=item * Hide the complexities of the LLVM tools through a single interface.
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=item * Make integration of existing non-LLVM tools simple.
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=item * Extend the capabilities of minimal front ends.
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=item * Make the interface for compiling consistent for all languages.
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=back
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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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2004-08-06 16:58:48 +00:00
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=head2 Basic Operation
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B<llvmc> always takes the following basic actions:
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=over
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=item * Command line options and filenames are collected.
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2004-08-06 22:28:47 +00:00
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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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2004-08-06 22:28:47 +00:00
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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 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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2004-08-06 22:28:47 +00:00
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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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The actions determined previously are executed sequentially and then
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B<llvmc> terminates.
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2004-08-06 22:28:47 +00:00
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=back
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2004-08-06 16:58:48 +00:00
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=head1 OPTIONS
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=head2 Control Options
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Control options tell B<llvmc> what to do at a high level. The
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following control options are defined:
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=over
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=item B<-c> or B<--compile>
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This option specifies that the linking phase is not to be run. All
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previous phases, if applicable will run. This is generally how a given
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bytecode file is compiled and optimized for a source language module.
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=item B<-k> or B<--link> or default
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This option (or the lack of any control option) specifies that all stages
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of compilation, optimization, and linking should be attempted. Source files
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specified on the command line will be compiled and linked with objects and
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libraries also specified.
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=item B<-S> or B<--assemble>
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This option specifies that compilation should end in the creation of
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an LLVM assembly file that can be later converted to an LLVM object
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file.
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=item B<-E> or B<--preprocess>
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This option specifies that no compilation or linking should be
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performed. Only pre-processing, if applicabe to the language being
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compiled, is performed. For languages that support it, this will
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result in the output containing the raw input to the compiler.
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=back
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=head2 Optimization Options
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Optimization with B<llvmc> is based on goals and specified with
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the following -O options. The specific details of which
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optimizations run is controlled by the configuration files because
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each source language will have different needs.
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=over
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=item B<-O1> or B<-O0> (default, fast compilation)
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Only those optimizations that will hasten the compilation (mostly by reducing
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the output) are applied. In general these are extremely fast and simple
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optimizations that reduce emitted code size. The goal here is not to make the
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resulting program fast but to make the compilation fast. If not specified,
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this is the default level of optimization.
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=item B<-O2> (basic optimization)
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This level of optimization specifies a balance between generating good code
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that will execute reasonably quickly and not spending too much time optimizing
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the code to get there. For example, this level of optimization may include
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things like global common subexpression elimintation, aggressive dead code
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elimination, and scalar replication.
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=item B<-O3> (aggressive optimization)
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This level of optimization aggressively optimizes each set of files compiled
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together. However, no link-time inter-procedural optimization is performed.
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This level implies all the optimizations of the B<-O1> and B<-O2> optimization
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levels, and should also provide loop optimizatiosn and compile time
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inter-procedural optimizations. Essentially, this level tries to do as much
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as it can with the input it is given but doesn't do any link time IPO.
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=item B<-O4> (linktime optimization)
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In addition to the previous three levels of optimization, this level of
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optimization aggressively optimizes each program at link time. It employs
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basic analysis and basic link-time inter-procedural optimizations,
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considering the program as a whole.
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=item B<-O5> (aggressive linktime optimization)
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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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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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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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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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the profile by which future reoptimizations are directed.
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=back
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=head2 Input Options
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=over
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=item B<-l> I<LIBRARY>
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This option instructs B<llvmc> to locate a library named I<LIBRARY> and search
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it for unresolved symbols when linking the program.
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=item B<-L> F<path>
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This option instructs B<llvmc> to add F<path> to the list of places in which
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the linker will
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=item B<-x> I<LANGUAGE>
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This option instructs B<llvmc> to regard the following input files as
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containing programs in the language I<LANGUAGE>. Normally, input file languages
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are identified by their suffix but this option will override that default
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behavior. The B<-x> option stays in effect until the end of the options or
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a new B<-x> option is encountered.
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=back
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=head2 Output Options
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=over
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=item B<-m>I<arch>
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This option selects the back end code generator to use. The I<arch> portion
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of the option names the back end to use.
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=item B<--native>
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Normally, B<llvmc> produces bytecode files at most stages of compilation.
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With this option, B<llvmc> will arrange for native object files to be
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generated with the B<-c> option, native assembly files to be generated
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with the B<-S> option, and native executables to be generated with the
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B<--link> option. In the case of the B<-E> option, the output will not
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differ as there is no I<native> version of pre-processed output.
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=item B<-o> F<filename>
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Specify the output file name. The contents of the file depend on other
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options.
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=back
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=head2 Information Options
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=over
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=item B<-n> or B<--noop>
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This option tells B<llvmc> to do everything but actually execute the
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resulting tools. In combination with the B<-v> option, this causes B<llvmc>
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to merely print out what it would have done.
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=item B<-v> or B<--verbose>
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This option will cause B<llvmc> to print out (on standard output) each of the
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actions it takes to accomplish the objective. The output will immediately
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precede the invocation of other tools.
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=item B<--stats>
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Print all statistics gathered during the compilation to the standard error.
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Note that this option is merely passed through to the sub-tools to do with
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as they please.
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=item B<--time-passes>
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Record the amount of time needed for each optimization pass and print it
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to standard error. Like B<--stats> this option is just passed through to
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the sub-tools to do with as they please.
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=item B<--time-programs>
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Record the amount of time each program (compilation tool) takes and print
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it to the standard error.
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=back
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=head2 Language Specific Options
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=over
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=item B<-Tool,opt>=I<options>
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Pass an arbitrary option to the optimizer.
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=item B<-Tool,link>=I<options>
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Pass an arbitrary option to the linker.
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=item B<-Tool,asm>=I<options>
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Pass an arbitrary optionsto the code generator.
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=back
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=head3 C/C++ Specific Options
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=over
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2004-08-06 22:28:47 +00:00
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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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2004-08-06 22:28:47 +00:00
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=head2 Miscellaneous Options
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=over
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=item B<--help>
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Print a summary of command line options.
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=item B<-V> or B<--version>
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This option will cause B<llvmc> to print out its version number
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and terminate.
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=back
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2004-08-06 16:58:48 +00:00
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=head2 Advanced Options
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2004-08-06 22:28:47 +00:00
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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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2004-08-06 16:58:48 +00:00
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=over
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2004-08-06 22:28:47 +00:00
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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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2004-08-06 22:28:47 +00:00
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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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2004-08-06 16:58:48 +00:00
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=over
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2004-08-06 22:28:47 +00:00
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=item * Pre-processing (filter and combine source files)
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2004-08-06 22:28:47 +00:00
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=item * Translation (translate source language to LLVM assembly or bytecode)
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2004-08-06 22:28:47 +00:00
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=item * Optimization (make bytecode execute quickly)
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2004-08-06 22:28:47 +00:00
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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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2004-08-06 16:58:48 +00:00
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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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occurs, it will exit with a non-zero value and no compilation actions
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will be taken. If one of the compilation tools returns a non-zero
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status, pending actions will be discarded and B<llvmc> will return the
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same result code as the failing compilation tool.
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=head1 SEE ALSO
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L<gccas|gccas>, L<gccld|gccld>, L<llvm-as|llvm-as>, L<llvm-dis|llvm-dis>,
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L<llc|llc>, L<llvm-link|llvm-link>
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=head1 AUTHORS
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Reid Spencer
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=cut
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