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