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Re: RFC: Improving GCC8 default option settings


On Dienstag, 12. September 2017 23:27:22 CEST Michael Clark wrote:
> > On 13 Sep 2017, at 1:57 AM, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
> > 
> > Hi all,
> > 
> > At the GNU Cauldron I was inspired by several interesting talks about
> > improving GCC in various ways. While GCC has many great optimizations, a
> > common theme is that its default settings are rather conservative. As a
> > result users are required to enable several additional optimizations by
> > hand to get good code. Other compilers enable more optimizations at -O2
> > (loop unrolling in LLVM was mentioned repeatedly) which GCC could/should
> > do as well.
> 
> There are some nuances to -O2. Please consider -O2 users who wish use it
> like Clang/LLVM’s -Os (-O2 without loop vectorisation IIRC).
> 
> Clang/LLVM has an -Os that is like -O2 so adding optimisations that increase
> code size can be skipped from -Os without drastically effecting
> performance.
> 
> This is not the case with GCC where -Os is a size at all costs optimisation
> mode. GCC users option for size not at the expense of speed is to use -O2.
> 
> Clang		GCC
> -Oz		~=	-Os
> -Os		~=	-O2
> 
No. Clang's -Os is somewhat limited compared to gcc's, just like the clang -Og 
is just -O1. AFAIK -Oz is a proprietary Apple clang parameter, and not in 
clang proper.

'Allan


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