[LLVMdev] What llvm tool will llvm-gcc call in each phase?

[Devang Patel]

On Feb 27, 2008, at 5:57 AM, Guang-Yuan.Wang wrote:

> Dear all,
>
>  I have installed llvm-gcc and llvm-2.2 successfully, now I have been
> puzzled.
>
>  Suppose I have a file test.c, If i use
>
>  llvm-gcc -o test.out test.c
>
>  then what llvm tool will llvm-gcc call in each step?
>
>  if I use llvm-gcc -O2 -o test.out test.c
>
>  then the argument '-O2' will be pass to opt? that is all the
> optimization is done by opt or llvm will benefit some optimization  
> from
> gcc?

llvm-gcc is a stand alone compiler like gcc. It accepts all standard  
gcc command line options and llvm-gcc produces assembly code, object  
file, executable or preprocessed source file depending upon the  
command line options.

llvm-gcc does not invoke any llvm tool directly.  llvm-gcc compiler  
uses llvm optimizer and code generator instead of gcc optimizer and  
code generator. These llvm optimizer and code generators are directly  
linked into llvm-gcc compiler.

llvm-backend.cpp is the source file where llvm-gcc selects various  
optimization passes and code generator features based on command line  
options.

llvm-gcc supports two additional command line options, --emit-llvm and  
-O4, which are not supported by standard GCC. These options instructs  
llvm-gcc to emit llvm bitcode instead of target specific assembly file  
or object file. You can use various llvm tools to operate on llvm  
bitcode. llvm-gcc is one way to generate llvm bitcode, but it is not  
necessarily only way to generate llvm bitcode. llvm IR is a well  
described language.

'opt' is a stand alone tool that optimizes llvm bitcode using  
specified optimization passes. Use opt --help for more info.

'llc' is a stand alone tool that generates target specific assembly  
code from llvm bitcode. You can feed 'llc' output to an assembler  
(e.g. gas) to generate object file for your target.

-
Devang