Re: Some benchmark comparison of gcc4.5 and dragonegg (was dragonegg in FSF gcc?)

[Duncan Sands <baldrick at free dot fr>]

Hi Vladimir,

> Dragonegg does not work with -flto. It generates assembler code on which
> gas complaints (a lot of non-assembler code like target data-layout
> which are not in comments).

actually it does work with -flto, in an awkward way.  When you use -flto
it spits out LLVM IR.  You need to use -S, otherwise the system assembler
tries (and fails) to compile this.  You need to then use llvm-as to turn
this into LLVM bitcode.  You can then link and optimize the bitcode either
by hand (using llvm-ld) or using the gold plugin, as described in
  http://llvm.org/docs/GoldPlugin.html

It is annoying that gcc insists on running the system assembler when passed
-c.  Not running the assembler isn't only good for avoiding the -S + llvm-as
rigmarole mentioned above.  LLVM is now capable of writing out object files
directly, i.e. without having to pass via an assembler at all.  It would be
neat if I could have the plugin immediately write out the final object file
if -c is passed.  I didn't work out how to do this yet.  It probably requires
some gcc modifications, so maybe something can be done for gcc-4.6.

For transparent LTO another possibility is to encode LLVM bitcode in the
assembler in the same way as gcc does for gimple when passed -flto.  I didn't
investigate this yet.

Ciao,

Duncan.