From: Brian Gaeke Date: Wed, 12 Nov 2003 20:19:40 +0000 (+0000) Subject: First draft of LLVM-to-anything comparison document. X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=e65a8e47fbc9bbe5f1e2deaed64785fff1068466;p=oota-llvm.git First draft of LLVM-to-anything comparison document. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@9926 91177308-0d34-0410-b5e6-96231b3b80d8 --- diff --git a/docs/LLVMVsTheWorld.html b/docs/LLVMVsTheWorld.html new file mode 100644 index 00000000000..955fbabcd15 --- /dev/null +++ b/docs/LLVMVsTheWorld.html @@ -0,0 +1,178 @@ + + + + + LLVM vs. the World - Comparing Compilers to Compilers + + + + +
+ LLVM vs. the World - Comparing Compilers to Compilers +
+ +
    +
  1. Introduction
    2. +
  2. General Applicability
    3. +
  3. Type System
    4. +
  4. Control-flow and Data-flow Information
    5. +
  5. Registers
    6. +
  6. Programmer Interface
    7. +
  7. Machine Code Emission
    8. +
+ +
+

Written by Brian R. Gaeke

+
+ + +
+ Introduction +
+ + +
+

Whether you are a stranger to LLVM or not, and whether you are considering +using it for your projects or not, you may find it useful to understand how we +compare ourselves to other well-known compilers. The following list of points +should help you understand -- from our point of view -- the way we see LLVM as +being better than and/or different from a few other selected compilers and code +generation systems.

+ +

At the moment, we only compare ourselves below to GCC and GNU lightning, but we will try +to revise and expand it as our knowledge and experience permit. Contributions are +welcome.

+
+ + +
+ General Applicability +
+ + +
+

GNU lightning: Only currently usable for dynamic runtime emission of binary +machine code to memory. Supports one backend at a time.

+ +

LLVM: Supports compilation of C and C++ (with more languages coming soon), +strong SSA-based optimization at compile-time, link-time, run-time, and +off-line, and multiple platform backends with Just-in-Time and ahead-of-time +compilation frameworks. (See our tech report on Lifelong +Code Optimization for more.)

+
+ +

GCC: Many relatively mature platform backends support assembly-language code +generation from many source languages. No run-time compilation +support. Relatively weak optimization support.

+ + +
+ Type System +
+ + +
+

GNU lightning: C integer types and "void *" are supported. No type checking +is performed. Explicit type casts are not typically necessary unless the +underlying machine-specific types are distinct (e.g., sign- or zero-extension is +apparently necessary, but casting "int" to "void *" would not be.) No floating +point (?!)

+ +

LLVM: Compositional type system based on C types, supporting structures, +opaque types, and C integer and floating point types.

+ +

GCC: Union of high-level types including those used in Pascal, C, C++, Ada, +Java, and FORTRAN.

+
+ + +
+ Control-flow and Data-flow Information +
+ + +
+

GNU lightning: No data-flow information encoded in the generated program. No +support for calculating CFG or def-use chains over generated programs.

+ +

LLVM: Scalar values in Static Single-Assignment form; def-use chains and CFG +always implicitly available and automatically kept up to date.

+ +

GCC: Trees and RTL do not directly encode data-flow info; but def-use chains +and CFGs can be calculated on the side. They are not automatically kept up to +date.

+
+ + +
+ Registers +
+ + +
+

GNU lightning: Very small fixed register set -- it takes the least common +denominator of supported platforms; basically it inherits its tiny register set +from IA-32, unnecessarily crippling targets like PowerPC with a large register +set.

+ +

LLVM: An infinite register set, reduced to a particular platform's finite +register set by register allocator.

+ +

GCC: Trees and RTL provide an arbitrarily large set of values. Reduced to a +particular platform's finite register set by register allocator.

+
+ + +
+ Programmer Interface +
+ + +
+

GNU lightning: Library interface based on C preprocessor macros that emit +binary code for a particular instruction to memory. No support for manipulating +code before emission.

+ +

LLVM: Library interface based on classes representing platform-independent +intermediate code (Instruction) and platform-dependent code (MachineInstr) which +can be manipulated arbitrarily and then emitted to memory.

+ +

GCC: Internal header file interface (tree.h) to abstract syntax trees, +representing roughly the union of all possible supported source-language +constructs; also, an internal header file interface (rtl.h, rtl.def) to a +low-level IR called RTL which represents roughly the union of all possible +target machine instructions.

+
+ + +
+ Machine Code Emission +
+ + +
+

GNU lightning: Only supports binary machine code emission to memory.

+ +

LLVM: Supports writing out assembly language to a file, and binary machine +code to memory, from the same back-end.

+ +

GCC: Supports writing out assembly language to a file. No support for +emitting machine code to memory.

+
+ + + +
+
+
Brian R. Gaeke
+ The LLVM Compiler Infrastructure +
+ Last modified: $Date$ +
+ + +