CODEWITHSUNDEEP
clinuxcompiler-constructionclangllvm
Suo
Suo

Reputation: 89

What does "llvm/libexec/clang-cc" do?

Using command clang -### a.c can output the commands that clang a.c calls.

For me, it outputs followings on my screen(focus on last three lines):

clang version 1.0 (https://llvm.org/svn/llvm-project/cfe/branches/release_26 exported)
Target: x86_64-unknown-linux-gnu
Thread model: posix
 "/usr/local/bin/llvm+clang-2.6-x86_64-linux/bin/../libexec/clang-cc" "-triple" "x86_64-unknown-linux-gnu" "-S" "-disable-free" "-main-file-name" "a.c" "--relocation-model" "static" "--disable-fp-elim" "--unwind-tables=1" "--mcpu=x86-64" "--fmath-errno=1" "-fdiagnostics-show-option" "-o" "/tmp/cc-yffqSv.s" "-x" "c" "a.c"
 "/usr/bin/gcc" "-c" "-m64" "-o" "/tmp/cc-pa2Qo4.o" "-x" "assembler" "/tmp/cc-yffqSv.s"
 "/usr/bin/gcc" "-m64" "-o" "a.out" "/tmp/cc-pa2Qo4.o"

In the line: /usr/local/bin/llvm+clang-2.6-x86_64-linux/bin/../libexec/clang-cc" "-triple" "x86 ......, it shows that the command clang called this: /libexec/clang-cc.

What does it(file "libexec/clang-cc") do?

And there is another question related I desired to ask:

Whether or not: command clang uses the front end of the project "clang", and the back end of "gcc"?

Because I find the last two line of code above called "/usr/bin/gcc".

I have search this for hours, can you help me?

Thanks in advance.

Upvotes: 0

Views: 290

Answers (2)

rici
rici

Reputation: 241671

You seem to be using quite an old version of Clang. You might want to try a newer one.

clang-cc is the clang C compiler, which converts a C program to assembly language. [Note 1]. Depending on version and target, it may also be able to directly produce an object file. See the -integrated-as command-line flag.

If your version of Clang does not have an assembler for your target architecture, the clang driver will try to use the system assembler (and linker). On some systems, these will be part of Gcc, although there are other options.

Notes

  1. Clang actually first compiles C (or other C-like languages) into platform-neutral LLVM Intermediate Representation (IR), and then uses the LLVM library to convert that into optimised platform-specific assembler code. These two parts are what are generally referred to as the Clang "front-end" (C⇒LLVM) and "back-end" (LLVM⇒Assembler). These are not separate programs

Upvotes: 2

Maxim Egorushkin
Maxim Egorushkin

Reputation: 136208

What does it(file "libexec/clang-cc") do?

Its output "-o" "/tmp/cc-yffqSv.s" suggests it outputs assembly.

and the back end of "gcc"?

The last two lines with gcc invocations are:

  1. Generate an object file from the assembly.
  2. Link object files into a.out.

Upvotes: 2

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