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Re: Overwhelmed by GCC frustration
- From: James Greenhalgh <james dot greenhalgh at arm dot com>
- To: Eric Gallager <egall at gwmail dot gwu dot edu>
- Cc: Jakub Jelinek <jakub at redhat dot com>, Richard Biener <richard dot guenther at gmail dot com>, Andrew Haley <aph at redhat dot com>, Oleg Endo <oleg dot endo at t-online dot de>, Georg-Johann Lay <avr at gjlay dot de>, GCC Development <gcc at gcc dot gnu dot org>, <nd at arm dot com>
- Date: Tue, 1 Aug 2017 17:00:22 +0100
- Subject: Re: Overwhelmed by GCC frustration
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On Tue, Aug 01, 2017 at 11:12:12AM -0400, Eric Gallager wrote:
> On 8/1/17, Jakub Jelinek <email@example.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"
> (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:
> 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
> 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
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