Improvements to code hoisting

[Maxim Kuvyrkov]

The following series of patches improves code hoisting and PRE RTL-level 
optimizations.  The two threads of the patches correspond to 
target-independent changes to gcse.c and to changes to ARM backend to 
make it emit RTL that is better suited for optimizers.

Motivating examples for this work are ARM PRs
http://gcc.gnu.org/PR42495
http://gcc.gnu.org/PR42574
With the patches applied GCC produces perfect code for these examples.

The general effect of the patches is that they allow more expressions to 
be hoisted or PRE'd, thus making the optimizations more aggressive.  The 
negative effect of this are extended live ranges and increased register 
pressure.  Luckily, IRA and reload do good job in dealing with excessive 
register pressure.

Still, while investigating size regressions I came upon a problem in 
IRA's estimation of cost of putting a pseudo to memory when optimizing 
for size.  It appears that [ARM's] rtx_cost() model overestimates cost 
of assigning a constant to memory, and, thus, makes IRA allocate a 
register for something it shouldn't have.  This results in a spill of 
another variable that didn't get a register.  I believe, this should not 
block inclusion of the patches as they still provide significant 
improvement on average.

The cumulative patch was tested on x86_64-linux-gnu (bootstrap, default 
languages) and arm-linux-gnu (no bootstrap, c and c++ only).

There's a new FAIL on x86_64-linux-gnu gfortran testsuite: an ICE in 
output_die, at dwarf2out.c:10875.  Although I didn't look in the details 
of the problem, it seems to be a latent bug uncovered by the patches.

I benchmarked an earlier version of these patches on a Cortex-A9 board 
and got 0.1-0.5% size decrease on SPEC2K at -Os for both thumb1 and 
thumb2 modes when compiled with and without -fpic.

I'm now working on getting SPEC2K speed numbers.  I will post detailed 
benchmarking results before committing the patches that aren't obvious 
improvements.  I'll appreciate if someone posts benchmark numbers for 
other architectures.

Each patch will be posted in a subthread.

Thank you,

-- 
Maxim Kuvyrkov
CodeSourcery
maxim@codesourcery.com
(650) 331-3385 x724