Re: Overwhelmed by GCC frustration

[James Greenhalgh <james dot greenhalgh at arm dot com>]

On Tue, Aug 01, 2017 at 11:12:12AM -0400, Eric Gallager wrote:
> On 8/1/17, Jakub Jelinek <jakub@redhat.com> wrote:
> > On Tue, Aug 01, 2017 at 07:08:41AM -0400, Eric Gallager wrote:
> >> > Heh.  I suspect -Os would benefit from a separate compilation pipeline
> >> > such as -Og.  Nowadays the early optimization pipeline is what you
> >> > want (mostly simple CSE & jump optimizations, focused on code
> >> > size improvements).  That doesn't get you any loop optimizations but
> >> > loop optimizations always have the chance to increase code size
> >> > or register pressure.
> >> >
> >>
> >> Maybe in addition to the -Os optimization level, GCC mainline could
> >> also add the -Oz optimization level like Apple's GCC had, and clang
> >> still has? Basically -Os is -O2 with additional code size focus,
> >> whereas -Oz is -O0 with the same code size focus. Adding it to the
> >> FSF's GCC, too, could help reduce code size even further than -Os
> >> currently does.
> >
> > No, lack of optimizations certainly doesn't reduce the code size.
> > For small code, you need lots of optimizations, but preferrably code-size
> > aware ones.  For RTL that is usually easier, because you can often compare
> > the sizes of the old and new sequences and choose smaller, for GIMPLE
> > optimizations it is often just a wild guess on what optimizations generally
> > result in smaller and what optimizations generally result in larger code.
> > There are too many following passes to know for sure, and finding the right
> > heuristics is hard.
> >
> > 	Jakub
> >
> 
> Upon rereading of the relevant docs, I guess it was a mistake to
> compare -Oz to -O0. Let me quote from the apple-gcc "Optimize Options"
> page:
> 
> -Oz
>         (APPLE ONLY) Optimize for size, regardless of performance. -Oz
>         enables the same optimization flags that -Os uses, but -Oz also
>         enables other optimizations intended solely to reduce code size.
>         In particular, instructions that encode into fewer bytes are
>         preferred over longer instructions that execute in fewer cycles.
>         -Oz on Darwin is very similar to -Os in FSF distributions of GCC.
>         -Oz employs the same inlining limits and avoids string instructions
>         just like -Os.
> 
> Meanwhile, their description of -Os as contrasted to -Oz reads:
> 
> -Os
>         Optimize for size, but not at the expense of speed. -Os enables all
>         -O2 optimizations that do not typically increase code size.
>         However, instructions are chosen for best performance, regardless
>         of size. To optimize solely for size on Darwin, use -Oz (APPLE
>         ONLY).
> 
> And the clang docs for -Oz say:
> 
> -Oz Like -Os (and thus -O2), but reduces code size further.
> 
> So -Oz does actually still optimize, so it's more like -O2 than -O0
> after all, just even more size-focused than -Os.

The relationship between -Os and -Oz is like the relationship between -O2
and -O3.

If -O3 says, try everything you can to increase performance even at the
expense of code-size and compile time, then -Oz says, try everything you
can to reduce the code size, even at the expense of performance and
compile time.

Thanks,
James