[llvm-dev] (Thin)LTO llvm build

Carsten Mattner via llvm-dev
Fri, 30 Sep 2016 20:39:22 -0000

On Fri, Sep 30, 2016 at 10:19 PM, Teresa Johnson <tejohnson at google.com>
wrote:
>
>
> On Fri, Sep 30, 2016 at 1:11 PM, Mehdi Amini <mehdi.amini at
apple.com> wrote:
> >
> >
> > > On Sep 30, 2016, at 12:57 PM, Carsten Mattner <carstenmattner
at gmail.com>
> > > wrote:
> > >
> > > On Fri, Sep 30, 2016 at 6:35 PM, Teresa Johnson <tejohnson at
google.com>
> > > wrote:
> > > > I just built a stage-1 compiler from the 3.9 release bits
and
> > > > built the lldb from head sources which worked fine. Let me
try
> > > > again using 3.9 build compiler to build 3.9 bits.
> > >
> > > I had also started a build a few hours ago, what a coincidence.
> > > I had to ditch both build and source trees because of cmake
> > > failing to reconfigure due to some file it surprisingly had
> > > generated in the source (not build) tree.
> > >
> > > So, with -DBUILD_SHARED_LIBS=OFF, and a fresh checkout of the
> > > 3.9 release branch, I've run into the same error, this time
only
> > > 39 (not a typo, and not 3.9) ninja targets left.
> > >
> > > Some parts finished, so here's a surprising size difference
for
> > > clang-3.9.
> > >
> > > LTO=OFF  stripped clang-3.9 is 44MB
> > > LTO=Thin stripped clang-3.9 is 50MB
> >
> > That’s expected: there’s more inlining and other optimizations
happening.

Right, though usually the chance to do complete whole program optimization
provides enough opportunities for it to go the other way. Of course, it can
also increase.

> > Also we rely more on linker dead stripping with ThinLTO. On MacOS
> > it works well with the atom model, on ELF I’d expect the LTO
> > equivalent of -ffunction-sections to be used (I don’t know if Gold
> > and the gold-plugin have such an option though).
>
>
> It does, but I found you have to pass the options to the plugin to get them
> to kick in for any *LTO compile. See D24644 for my proposed fix. To
> workaround, pass them to the plugin-opt:
> -Wl,--gc-sections -Wl,-plugin-opt,-function-sections
> -Wl,-plugin-opt,-data-sections
> Not sure if there is any benefit to passing them also on the command line
> (-ffunction-sections -fdata-sections), but probably a good idea.

I'll extend LDFLAGS with that before running cmake the next time.
Thank you.

> > Finally, there are few other things that we need to do in ThinLTO
> > to gain a few more percents on this aspect.
> >
> > > I would have expected more aggressive pruning or at least not an
> > > increase, though I don't know if it can be caused by
> > > -DBUILD_SHARED_LIBS=OFF.
> >
> > BUILD_SHARED_LIBS is a non-sense from a performance point of view.
> > It is really not advised to use this.
>
>
> I actually think Carsten was not getting BUILD_SHARED_LIBS=ON as I
> had originally thought given the DSO error: His cmake command didn't
> contain it, and from his email earlier this week, the __morestack
> reference is coming from lib/liblldb.so, not an llvm library built
> shared as I had assumed. Which also explains why he is still getting
> it when configuring with -DBUILD_SHARED_LIBS=OFF.

Correct.

> I am having a bunch of trouble getting a release-built 3.9 compiler,
> lots of warnings when building with clang that are being turned into
> errors from -Werror. Need to try again with -w...

Did it ever build warning-free, or why have you enabled -Werror? I
mean, I see warnings here, so it would fail if promoted to errors.