Why llvm-gcc? Another beginner's question.

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#1

What is the reason for llvm-gcc? Will regular gcc work (except for the
releases that are broken, of course)? Does llvm in any way depend on
features of llvm-gcc instead of gcc?

Or is it optional? Was it conceived to give llvm itself a through
shakedown? Is it there just in case you really want llvm code in some
form from a C program? Is such llvm code necessary for some kind of use
-- might it be that that a compiler that uses llvm can only link with
ofher llvm-generated code?

-- hendrik

#2

Hendrik Boom wrote:

What is the reason for llvm-gcc? Will regular gcc work (except for the
releases that are broken, of course)? Does llvm in any way depend on
features of llvm-gcc instead of gcc?
  

llvm-gcc has a two-fold purpose. First, they are the front-end compilers for C/C++ for LLVM. In other words, they take C/C++ source code and translate it to LLVM assembly code which can then be assembled using llvm-as and transformed/analysis using the LLVM tools. This functionality is enabled using the -emit-llvm option.

Second, llvm-gcc is a compiler built using LLVM that compiles C/C++ code to native code. It does the same thing as regular gcc except that it uses the LLVM optimization and code generation libraries instead of the original ones found in gcc. This is the default way of using llvm-gcc; it basically makes it easier for people to use LLVM as a drop-in replacement for GCC.

Or is it optional? Was it conceived to give llvm itself a through
shakedown? Is it there just in case you really want llvm code in some
form from a C program? Is such llvm code necessary for some kind of use
-- might it be that that a compiler that uses llvm can only link with
ofher llvm-generated code?
  

It depends on what you want to do. If you want to use LLVM as a replacement for GCC, then yes. If you want to compiler C/C++ code to LLVM so that you can analyze/transform it using your own LLVM passes, yes. If you're writing your own language front-end for LLVM, then no.

Hope that helps.

-- John T.

#3

That's exactly what I needed to know. Thanks.

-- hendrik

#4

Hi Hendrik,

What is the reason for llvm-gcc? Will regular gcc work (except for the
releases that are broken, of course)? Does llvm in any way depend on
features of llvm-gcc instead of gcc?

llvm-gcc is a frontend: It can compile various languages (C, C++ and probably
more) into llvm assembly / bitcode. Regular gcc can only compile into native
machine code, and is not really needed or useful with llvm.

LLVM can work fine without llvm-gcc, but you will have to have some other way
of creating llvm assembly / bitcode, such as writing assembly directly,
writing your own frontend, or using the clang frontend (also developed within
the llvm project).

LLVM does require binutils for assembling (and perhaps also linking?)
executables at the end of the chain. However, for the middle part (optimizing,
codegeneration and I think also linking) llvm is selfsufficient.

-- might it be that that a compiler that uses llvm can only link with
ofher llvm-generated code?

llvm can codegen into normal object files, which can be linked against normal
libraries, also by other linkers. However, any llvm code that is still in
bitcode (LLVM IR) form can only be processed by LLVM of course.

Gr.

Matthijs

#5

What is the reason for llvm-gcc? Will regular gcc work
(except for the releases that are broken, of course)?

No one can tell the answer to this question ... because the
answer depends on what you're up to.

LLVM itself is frontend agnostic. It itself starts from a
low-level, assembly like, but SSA-formed language. See
LLVM Language Reference Manual — LLVM 16.0.0git documentation.

Mangling objects in this language are a bunch of analyzers and
optimizers, http://www.llvm.org/docs/Passes.html. And finally,
there are a bunch of various backends that generate
assembly-language ... to JIT or to link as a standalong
applicant.

Now, how do you get this Low-Level-Language? For one, you can
write it by hand. Many test-case in the LLVM framework have been
written by hand, see all the *.ll file at
http://www.llvm.org/viewvc/llvm-project/llvm/trunk/test/

Or you can use a compiler. llvm-gcc is just one of the possible
compilers. It uses gcc to parse C, C++, produces gimple, like
gcc normally does. But then the gimple is converted to the LLVM
low-level language and handed over to LLVM.

There are other frontends, e.g. "clang", which aims to be a
faster and less-resource-hungry compiler (or, compiler library,
e.g. easy to include/link into an IDE). Or I have heard about a
compiler for the "D" language to LLVM, see
ldc - D Programming Language - Trac. There's an examply
forth-link front page ... and so on.

Does llvm in any way depend on features of llvm-gcc instead of
gcc?

Now you might realize that this question doesn't really make
sense.

Or is it optional?

Again, this depends. If you want to compile/optimize C++ via
llvm, you need llvm-g++, so it's not optional. But for some C
programs, clang migth already be usable (yet it's highly
experimental). If it's needed for your needs ... who knows,
but you?

Greetings, Holger