From 6c787d83d3557c15994c2050246e459fdabad55f Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Thu, 20 Apr 2006 06:15:48 +0000 Subject: [PATCH] This is old, out of date, and isn't linked to by anything. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27869 91177308-0d34-0410-b5e6-96231b3b80d8 --- docs/LLVMVsTheWorld.html | 180 --------------------------------------- 1 file changed, 180 deletions(-) delete mode 100644 docs/LLVMVsTheWorld.html diff --git a/docs/LLVMVsTheWorld.html b/docs/LLVMVsTheWorld.html deleted file mode 100644 index fa804c599a0..00000000000 --- a/docs/LLVMVsTheWorld.html +++ /dev/null @@ -1,180 +0,0 @@ - - - - - LLVM vs. the World - Comparing Compilers to Compilers - - - - -
- LLVM vs. the World - Comparing Compilers to Compilers -
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  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. -
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-

Written by Brian R. Gaeke

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- Introduction -
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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 -- some of the important -ways in which we see LLVM as different from other selected compilers and -code generation systems.

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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.

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

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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 document on Lifelong -Code Optimization for more.)

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

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- Type System -
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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.) -Floating-point support may not work on all platforms (it does not appear to be -documented in the latest release).

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LLVM: Compositional type system based on C types, supporting structures, -opaque types, and C integer and floating point types. Explicit cast instructions -are required to transform a value from one type to another.

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

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

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

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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.

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- Registers -
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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.

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

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

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- Programmer Interface -
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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.

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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.

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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.

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- Machine Code Emission -
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GNU lightning: Only supports binary machine code emission to memory.

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

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

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