[m-rev.] preliminary review: Removing dead code at link time.

[Peter Wang]

On Fri, 8 Aug 2014 11:37:09 +1000 (EST), Julien Fischer <jfischer at opturion.com> wrote:
> 
> Hi Peter,
> 
> On Fri, 8 Aug 2014, Peter Wang wrote:
> 
> > After making this patch I remembered to check what to do on OS X.
> > It seems there you only need to pass -dead_strip to the linker,
> > so the --split-sections flag is specific to Linux or ELF targets.
> >
> > We could drop most of this patch and just build the standard library
> > and runtime with `-ffunction-sections -fdata-sections' where it helps,
> > and leave it to the user to pass the correct linker flag if they care.
> >
> > ---
> >
> > This patch makes it more convenient to reduce executable sizes,
> > using the gcc and clang options `-ffunction-sections -fdata-sections'
> > and the linker option `--gc-sections'.  The former places functions and
> > data into their own sections in object files so they can be removed
> > individually; the latter performs the removal.
> >
> > We introduce a Mercury compiler and mgnuc flag `--split-sections'
> > which passes `-ffunction-sections -fdata-sections' to the C compiler.
> > It is off by default because it makes the object files larger and most
> > user code would hopefully not see much benefit from it.  There are some
> > warnings about problems with gprof or debugging in the gcc manual.
> 
> Perhaps it should be disabled by default for everything in the .ll_debug
> grades.

I'll disable it for any sort of debugging or profiling grade.

> > We introduce a Mercury compiler and ml flag `--gc-sections' which passes
> 
> Given that the documentation you added below has to have sentence
> mentioning that this has nothing to do with the garbage collector, a
> better name for this option might be somthing like
> 
>     --strip-dead-sections
> 
> (Indeed, since the option for dead procedure elimination in Mercury is
> --optimize-dead-procs, perhaps --optimize-dead-sections?)

--strip-dead-sections sounds good.

> > `-Wl,--gc-sections' through to the linker.  It is off by default, as it
> > makes linking noticeably slower.
> 
> Perhaps it should be implied by --opt-space?
> 

Not sure, probably not.

> > We enable `--split-sections' when building the runtime and standard
> > library.
> >
> > On Linux/x86-64, `hello' goes from 4.1 MiB to 663 KiB in size
> > and `mercury_compile' goes from 16 MiB to 13 MiB.
> > (both statically linked to the Mercury libraries and stripped)
> >
> > configure.ac:
> > scripts/Mercury.config.in:
> > 	Set CFLAGS_FOR_SPLIT_SECTIONS for gcc or clang.  gcc has
> > 	supported `-ffunction-sections -fdata-sections' since at least
> > 	2.95.3, and clang as of 2010, so probably long enough.
> >
> > 	Set LD_GC_SECTIONS_FLAG after testing if "-Wl,--gc-sections"
> > 	works.
> > 	XXX -dead_strip on Darwin?
> 
> I'll look into the Darwin side of things after you commit.

Can you check if -ffunction-sections -fdata-sections
does anything at all?

Peter