llvm dev - Aug 2007 - [LLVMdev] Documentation error in http://llvm.org/docs/LinkTimeOptimization.html

The web page "LLVM Link Time Optimization: Design and Implementation"
contains errors.

In the gray block below "Example of link time optimization" there are
some source files and then a list of commands to execute:

$ llvm-gcc4 --emit-llvm -c a.c -o a.o
$ llvm-gcc4 -c main.c -o main.o
$ llvm-gcc4 a.o main.o -o main

1) in current LLVM (e.g. llvm itself, cfe, llvm-gcc-4.0) no binary with
the name "llvm-gcc4" get's created


Now, let's assume that the gcc compiler from llvm-gcc-4.0 has been meant 
and let us substitute therefore "llvm-gcc4" with "llvm-gcc".
This gives
us:

$ llvm-gcc --emit-llvm -c a.c -o a.o
$ llvm-gcc -c main.c -o main.o
$ llvm-gcc a.o main.o -o main
a.o: file not recognized: File format not recognized
collect2: ld returned 1 exit status

Okay, that did not really work :-/

On Aug 24, 2007, at 1:45 PM, Holger Schurig wrote:
> $ llvm-gcc --emit-llvm -c a.c -o a.o
> $ llvm-gcc -c main.c -o main.o
> $ llvm-gcc a.o main.o -o main
> a.o: file not recognized: File format not recognized
> collect2: ld returned 1 exit status
>
> Okay, that did not really work :-/
This document describes interface between llvm and linker. You need  
system linker that implements this interface. LLVM tool kit does not  
supply system linker.

-
Devang
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On Aug 24, 2007, at 1:45 PM, Holger Schurig wrote:
> In the gray block below "Example of link time optimization" there
are
> some source files and then a list of commands to execute:
>
> $ llvm-gcc4 --emit-llvm -c a.c -o a.o
> $ llvm-gcc4 -c main.c -o main.o
> $ llvm-gcc4 a.o main.o -o main
>
> 1) in current LLVM (e.g. llvm itself, cfe, llvm-gcc-4.0) no binary  
> with
> the name "llvm-gcc4" get's created
Well, it depends on the configure parameter used while building llvm- 
gcc. The point is, these examples are not verified with gcc-3.x based  
llvm-gcc.

-
Devang
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> Well, it depends on the configure parameter used while building llvm- 
> gcc.
Hmm, there are other pages on the the website that tell you how you
should configure gcc, e.g. you're led from "How to build the C/C++
Frontend" at http://llvm.org/docs/CFEBuildInstrs.html to Subversion
HEAD (by virtue of an "svn co"). Then you're supposed to look at
README.LLVM. And there you're told to configure like this:

../llvm-gcc4-x.y.source/configure --prefix=`pwd`/../install \
    --enable-llvm=$LLVMOBJDIR --enable-languages=c,c++$EXTRALANGS $TARGETOPTIONS

(You can check this at
http://llvm.org/svn/llvm-project/llvm-gcc-4.0/trunk/README.LLVM)

And if I follow this part of the documentation, then I don't get an
"llvm-gcc4". So either this documentation or the other one should be
fixed, because they're out-of-sync.



> The point is, these examples are not verified with gcc-3.x based  
> llvm-gcc.
This has nothing to do with a 3.x compiler. AFAIK llvm SVN HEAD
doesn't contain any gcc 3.x frontend for LLVM. In fact, I have
4.0.1:

$ llvm-gcc --version
llvm-gcc (GCC) 4.0.1 (Apple Computer, Inc. build 5449)(LLVM build )


Also, the new llvm-top project ALSO creates an "llvm-gcc" binary, that
out of llvm-gcc-4.0


So, neither llvm-top nor llvm-gcc-4.0/trunk/README.LLVM will create
you a "llvm-gcc4" compiler, so the documentation is STILL wrong.

> > $ llvm-gcc --emit-llvm -c a.c -o a.o
> > $ llvm-gcc -c main.c -o main.o
> > $ llvm-gcc a.o main.o -o main
> > a.o: file not recognized: File format not recognized
> > collect2: ld returned 1 exit status
> >
> > Okay, that did not really work :-/
> 
> This document describes interface between llvm and linker. You need  
> system linker that implements this interface. LLVM tool kit does not  
> supply system linker.
I'm not sure this is right. I still think that this is a documentation
error.

The "a.o" file is really a LLVM bitcode file:

$ file *.o
a.o:    data
main.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped

libmagic doesn't know about LLVM bitcode, but rest assured that a.o
*IS* indeed an llvm bitcode file. E.g. "llvm-dis a.o -o -" works.

However, when you call llvm-gcc without --emit-llvm it behaves like a plain
gcc. So it calls the system linker, ld, with this file.

$ strace -oa -efile -ff llvm-gcc a.o main.o -o main
$ grep -h execve a.*
execve("/usr/src/llvm/dist/bin/llvm-gcc", ["llvm-gcc",
"a.o", "main.o", "-o", "main"], [/* 38
vars */]) = 0
execve("/usr/src/llvm/dist/libexec/gcc/i686-pc-linux-gnu/4.0.1/collect2",
["/usr/src/llvm/dist/libexec/gcc/i"..., "--eh-frame-hdr",
"-m", "elf_i386", "-dynamic-linker",
"/lib/ld-linux.so.2", "-o", "main",
"/usr/lib/crt1.o", "/usr/lib/crti.o",
"/usr/src/llvm/dist/lib/gcc/i686-"...,
"-L/usr/src/llvm/dist/lib/gcc/i68"...,
"-L/usr/src/llvm/dist/lib/gcc/i68"..., "a.o",
"main.o", "-lgcc", ...], [/* 42 vars */]) = 0
execve("/usr/bin/ld", ["/usr/bin/ld",
"--eh-frame-hdr", "-m", "elf_i386",
"-dynamic-linker", "/lib/ld-linux.so.2", "-o",
"main", "/usr/lib/crt1.o", "/usr/lib/crti.o",
"/usr/src/llvm/dist/lib/gcc/i686-"...,
"-L/usr/src/llvm/dist/lib/gcc/i68"...,
"-L/usr/src/llvm/dist/lib/gcc/i68"..., "a.o",
"main.o", "-lgcc", ...], [/* 43 vars */]) = 0

See the last line, llvm-gcc calls /usr/bin/ld with "a.o" ...

And it's obvious that the system linker cannot handle LLVM bitcode files,
this linker is for ELF files. Can you tell me ONE system linker that
can load and handle LLVM files?  If not, then the documentation still
describes something that doesn't work in real life.

If llvm-gcc in this case was NOT supposed to call the system-linker
with an bitcode a.o file, then llvm-gcc-4.0 in SVN trunk contains a bug.


However, if I'm supposed to run not "llvm-gcc" in step 3 to get a
main
program but, say, llvm-ld, then the documentation is wrong again.

> The documentation could be updated to say llvm-gcc 
> but unless you specify the prefix "llvm-" during configure, it
wont be
> named that either :)
llvm-top did this for you, see the "--program-prefix=llvm-" in
http://llvm.org/svn/llvm-project/llvm-gcc-4.0/trunk/build-for-llvm-top.sh


(I know that llvm-top hasn't even been announced, but at least when
Reid does this announcement, the page should be changed anyway).

> In the gray block below "Example of link time optimization" there
are
> some source files and then a list of commands to execute:
>
> $ llvm-gcc4 --emit-llvm -c a.c -o a.o
> $ llvm-gcc4 -c main.c -o main.o
> $ llvm-gcc4 a.o main.o -o main
>
> 1) in current LLVM (e.g. llvm itself, cfe, llvm-gcc-4.0) no binary with
> the name "llvm-gcc4" get's created
When you build llvm-gcc you can give it any program prefix/postfix name 
you wish, so its possible to name it llvm-gcc-4.0. However, the main 
reason behind this was to distinguish from llvm-gcc3 which we supported 
along with llvm-gcc4. The documentation could be updated to say llvm-gcc 
but unless you specify the prefix "llvm-" during configure, it wont be
named that either :)

Devang already answered the second part.

-Tanya
>
>
> Now, let's assume that the gcc compiler from llvm-gcc-4.0 has been
meant
> and let us substitute therefore "llvm-gcc4" with
"llvm-gcc". This gives
> us:
>
> $ llvm-gcc --emit-llvm -c a.c -o a.o
> $ llvm-gcc -c main.c -o main.o
> $ llvm-gcc a.o main.o -o main
> a.o: file not recognized: File format not recognized
> collect2: ld returned 1 exit status
>
> Okay, that did not really work :-/
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