RFC: Improving GCC8 default option settings

[Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>]

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.

Here are a few concrete proposals to improve GCC's option settings which will
enable better code generation for most targets:

* Make -fno-math-errno the default - this mostly affects the code generated for
  sqrt, which should be treated just like floating point division and not set
  errno by default (unless you explicitly select C89 mode).

* Make -fno-trapping-math the default - another obvious one. From the docs:
  "Compile code assuming that floating-point operations cannot generate 
   user-visible traps."
  There isn't a lot of code that actually uses user-visible traps (if any -
  many CPUs don't even support user traps as it's an optional IEEE feature). 
  So assuming trapping math by default is way too conservative since there is
  no obvious benefit to users. 

* Make -fno-common the default - this was originally needed for pre-ANSI C, but
  is optional in C (not sure whether it is still in C99/C11). This can
  significantly improve code generation on targets that use anchors for globals
  (note the linker could report a more helpful message when ancient code that
  requires -fcommon fails to link).

* Make -fomit-frame-pointer the default - various targets already do this at
  higher optimization levels, but this could easily be done for all targets.
  Frame pointers haven't been needed for debugging for decades, however if there
  are still good reasons to keep it enabled with -O0 or -O1 (I can't think of any
  unless it is for last-resort backtrace when there is no unwind info at a crash),
  we could just disable the frame pointer from -O2 onwards.

These are just a few ideas to start. What do people think? I'd welcome discussion
and other proposals for similar improvements.

Wilco