From 1a380a0b0930f5a1e48618fd6ec09ea5d07d1282 Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Mon, 10 Dec 2007 07:14:08 +0000 Subject: [PATCH] reorganize features, expound on a couple more. git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@44783 91177308-0d34-0410-b5e6-96231b3b80d8 --- www/content.css | 50 +++---------- www/features.html | 186 +++++++++++++++++++++++++++++++++++++--------- www/index.html | 8 +- www/menu.css | 4 - 4 files changed, 162 insertions(+), 86 deletions(-) diff --git a/www/content.css b/www/content.css index 23244358d2..3fa7be0145 100644 --- a/www/content.css +++ b/www/content.css @@ -1,8 +1,15 @@ html, body { - margin:0px; padding:0px; } +/* Slides */ +IMG.img_slide { + display: block; + margin-left: auto; + margin-right: auto +} + + /* FIXME: This is only for the status page */ table,tr,td { border:.3ex solid black; @@ -18,46 +25,7 @@ table,tr,td { .code { font:Courier,Arial; } -.quote { - display: block; - margin: 0 5em; -} -.key_point { - color:rgb(200,0,0); -} .simple_list { /* simple lists that don't need to stand out */ margin-left:0; } - - -/* ****************** */ -/* Performance images */ -.img_container { - display:inline; - background-color:rgb(250,250,250); - width:400px; - vertical-align:top; - margin:.1em; -} -[class=img_container] { - display:inline-block; -} -.img_container img { - display:block; -} - -.img_title { - display:block; - font-weight:bold; - color:rgb(20,50,150); -} -.img_desc .img-notes { - display:block; - padding:.3em; -} -.img_notes { - font-style:italic; - color:rgb(50,50,50); - font-size:.9em; -} -/* ****************** */ \ No newline at end of file + diff --git a/www/features.html b/www/features.html index 28e98e1aa1..79439f8785 100644 --- a/www/features.html +++ b/www/features.html @@ -22,14 +22,136 @@ Clang in more detail and gives a more broad explanation about what we mean. These features are:

+

End-User Features:

+ + + +

Driving Goals and Internal Design:

+ +

End-User Features

+ + + + +

High Performance and Low Memory Use

+ + +

A major focus of our work on clang is to make it fast, light and scalable. +The library-based architecture of clang makes it straight-forward to time and +profile the cost of each layer of the stack, and the driver has a number of +options for performance analysis.

+ +

While there is still much that can be done, we find that the clang front-end +is significantly quicker than gcc and uses less memory For example, when +compiling "Carbon.h" on Mac OS/X, we see that clang is 2.5x faster than GCC:

+ + + +

Carbon.h is a monster: it transitively includes 558 files, 12.3M of code, +declares 10000 functions, has 2000 struct definitions, 8000 fields, 20000 enum +constants, etc (see slide 25+ of the clang +talk for more information). It is also #include'd into almost every C file +in a GUI app on the Mac, so its compile time is very important.

+ +

From the slide above, you can see that we can measure the time to preprocess +the file independently from the time to parse it, and independently from the +time to build the ASTs for the code. GCC doesn't provide a way to measure the +parser without AST building (it only provides -fsyntax-only). In our +measurements, we find that clang's preprocessor is consistently 40% faster than +GCCs, and the parser + AST builder is ~4x faster than GCC's. If you have +sources that do not depend as heavily on the preprocessor (or if you +use Precompiled Headers) you may see a much bigger speedup from clang. +

+ +

Compile time performance is important, but when using clang as an API, often +memory use is even moreso: the less memory the code takes the more code you can +fit into memory at a time (useful for whole program analysis tools, for +example).

+ + + +

Here we see a huge advantage of clang: its ASTs take 5x less memory +than GCC's syntax trees, despite the fact that clang's ASTs capture far more +source-level information than GCC's trees do. This feat is accomplished through +the use of carefully designed APIs and efficient representations.

+ +

In addition to being efficient when pitted head-to-head against GCC in batch +mode, clang is built with a library based +architecture that makes it relatively easy to adapt it and build new tools +with it. This means that it is often possible to apply out-of-the-box thinking +and novel techniques to improve compilation in various ways.

+ + + +

This slide shows how the clang preprocessor can be used to make "distcc" +parallelization 3x more scalable than when using the GCC preprocessor. +"distcc" quickly bottlenecks on the preprocessor running on the central driver +machine, so a fast preprocessor is very useful. Comparing the first two bars +of each group shows how a ~40% faster preprocessor can reduce preprocessing time +of these large C++ apps by about 40% (shocking!).

+ +

The third bar on the slide is the interesting part: it shows how trivial +caching of file system accesses across invocations of the preprocessor allows +clang to reduce time spent in the kernel by 10x, making distcc over 3x more +scalable. This is obviously just one simple hack, doing more interesting things +(like caching tokens across preprocessed files) would yield another substantial +speedup.

+ +

The clean framework-based design of clang means that many things are possible +that would be very difficult in other systems, for example incremental +compilation, multithreading, intelligent caching, etc. We are only starting +to tap the full potential of the clang design.

+ + + +

Expressive Diagnostics

+ + +

Clang is designed to efficiently capture range information for expressions +and statements, which allows it to emit very useful and detailed diagnostic +information (e.g. warnings and errors) when a problem is detected.

+ +

For example, this slide compares the diagnostics emitted by clang (top) to +the diagnostics emitted by GCC (middle) for a simple example:

+ + + +

As you can see, clang goes beyond tracking just column number information: it +is able to highlight the subexpressions involved in a problem, making it much +easier to understand the source of the problem in many cases. For example, in +the first problem, it tells you why the operand is invalid (it +requires a pointer) and what type it really is.

+ +

In the second error, you can see how clang uses column number information to +identify exactly which "+" out of the four on that line is causing the problem. +Further, it highlights the subexpressions involved, which can be very useful +when a complex subexpression that relies on tricky precedence rules.

+ +

The example doesn't show it, but clang works very hard to retain typedef +information, ensuring that diagnostics print the user types, not the fully +expanded (and often huge) types. This is clearly important for C++ code (tell +me about "std::string", not about "std::basic_string<char, +std::char_traits<char>, std::allocator<char> >"!), but it is +also very useful in C code in some cases as well (e.g. "__m128" vs +"float __attribute__((__vector_size__(16)))").

+ + + +

Driving Goals and Internal Design

+ +

A real-world, production quality compiler

@@ -88,11 +210,36 @@ clang in "strict" mode if you desire.

C, C++'03, Objective-C 2, etc.

-

Library based architecture

+

GCC Compatibility

+ + +

GCC is currently the defacto-standard open source compiler today, and it +routinely compiles a huge volume of code. GCC supports a huge number of +extensions and features (many of which are undocumented) and a lot of +code and header files depend on these features in order to build.

+ +

While it would be nice to be able to ignore these extensions and focus on +implementing the language standards to the letter, pragmatics force us to +support the GCC extensions that see the most use. As mentioned above, all +extensions are explicitly recognized as such and marked with extension +diagnostics, which can be mapped to warnings, errors, or just ignored. +

+ + + +

Library based architecture

A major design concept for the LLVM front-end involves using a library based architecture. In this library based architecture, various parts of the front-end can be cleanly divided into separate libraries which can then be mixed up for different needs and uses. In addition, the library based approach makes it much easier for new developers to get involved and extend LLVM to do new and unique things. In the words of Chris, -
"The world needs better compiler tools, tools which are built as libraries. This design point allows reuse of the tools in new and novel ways. However, building the tools as libraries isn't enough: they must have clean APIs, be as decoupled from each other as possible, and be easy to modify/extend. This requires clean layering, decent design, and keeping the libraries independent of any specific client."
+ +
+"The world needs better compiler tools, tools which are built as libraries. +This design point allows reuse of the tools in new and novel ways. However, +building the tools as libraries isn't enough: they must have clean APIs, be as +decoupled from each other as possible, and be easy to modify/extend. This +requires clean layering, decent design, and keeping the libraries independent of +any specific client."
+ Currently, the LLVM front-end is divided into the following libraries:

Of course this is only a rough outline of the goals and features of - Clang. To get a true sense of what the new LLVM front-end is all about, - as well as why you might want to considering using it, see the Features section.

+ Clang. To get a true sense of what it is all about, see the Features section. The Features section breaks + each of these down and explains them in more detail.

Why?

@@ -58,7 +58,7 @@

The development of a new front-end was started out of a need -- a need for a compiler that allows better diagnostics, better integration with IDEs, a license that is compatible with commercial products, and a - compiler that is easier to develop and maintain. All of these were + nimble compiler that is easy to develop and maintain. All of these were motivations for starting work on a new front-end that could meet these needs.

diff --git a/www/menu.css b/www/menu.css index 46ebb1b85c..1784a9d994 100644 --- a/www/menu.css +++ b/www/menu.css @@ -23,10 +23,6 @@ /* menu style */ /**************/ -#menu { - padding-left: .3em; -} - #menu .submenu { padding-top:1em; display:block; -- 2.50.1