Re: Overwhelmed by GCC frustration

[Eric Gallager <egall at gwmail dot gwu dot edu>]

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.