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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34192 91177308-0d34-0410-b5e6-96231b3b80d8
432 lines
13 KiB
Plaintext
432 lines
13 KiB
Plaintext
=pod
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=head1 NAME
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llvmc - The LLVM Compiler Driver (experimental)
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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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B<llvmc> is a configurable driver for invoking other LLVM (and non-LLVM) tools
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in order to compile, optimize and link software for multiple languages. For
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those familiar with FSF's B<gcc> tool, it is very similar. Please note that
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B<llvmc> is considered an experimental tool. B<llvmc> has the following goals:
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=over
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=item * provide 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, and
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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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=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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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 of
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configuration files are read to configure the actions B<llvmc> will take.
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Configuration files are provided by either LLVM or the front end compiler tools
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that B<llvmc> invokes. Users generally don't need to be concerned with the
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contents of the configuration files.
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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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=over
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=item * Pre-processing: gathering/filtering compiler input (optional).
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=item * Translation: 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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=back
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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>
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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>
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This option specifies that no compilation or linking should be
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performed. Only pre-processing, if applicable 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 elimination, 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 optimizations 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> (link time 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 link time 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 employs profile-guided
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re-optimization of the program after it has executed. Note that this implies
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a single level of re-optimization 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 re-optimization
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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 re-optimizations 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<--no-op>
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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<-T,pre>=I<options>
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Pass an arbitrary option to the pre-processor.
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=item B<-T,opt>=I<options>
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Pass an arbitrary option to the optimizer.
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=item B<-T,lnk>=I<options>
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Pass an arbitrary option to the linker.
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=item B<-T,asm>=I<options>
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Pass an arbitrary option to the code generator.
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=back
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=head2 C/C++ Specific Options
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=over
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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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=item B<-D>F<symbol>
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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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to define a symbol.
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=back
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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<--version>
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This option will cause B<llvmc> to print out its version number 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<--config-dir> 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 configuration 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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=back
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=head2 Unimplemented Options
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The options below are not currently implemented in B<llvmc> but will be
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eventually. They are documented here as "future design".
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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-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 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 DEFICIENCIES
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B<llvmc> is considered an experimental LLVM tool because it has these
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deficiencies:
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=over
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=item Insufficient support for native linking
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Because B<llvm-ld> doesn't handle native linking, neither can B<llvmc>
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=item Poor configuration support
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The support for configuring new languages, etc. is weak. There are many
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command line configurations that cannot be achieved with the current
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support. Furthermore the grammar is cumbersome for configuration files.
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Please see L<http://llvm.org/PR686> for further details.
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=item Does not handle target specific configurations
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This is one of the major deficiencies, also addressed in
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L<http://llvm.org/PR686>
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=back
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=head1 SEE ALSO
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L<llvm-as|llvm-as>, L<llvm-dis|llvm-dis>, L<llc|llc>, L<llvm-link|llvm-link>
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=head1 AUTHORS
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Maintained by the LLVM Team (L<http://llvm.org>).
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=cut
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