[Patch] Fix regex multiple consecutive quantifiers bug.

[Marc Glisse]

On Mon, 20 Jan 2014, Tim Shen wrote:

> ...even the regex input is nothing to do with quantifiers at all (say
> regex re(" ")), g++ -O3 generates slower code than -O2:
>
> ~ # g++ -O2 perf.cc && time ./a.out
> ./a.out  0.46s user 0.00s system 99% cpu 0.461 total
> ~ # g++ -O3 perf.cc && time ./a.out
> ./a.out  0.56s user 0.00s system 99% cpu 0.569 total
>
> perf.cc is almost the same as testsuite/performance/28_regex/split.cc.
>
> Following the man page, I found that g++ claims that the difference
> between -O3 and -O2 are:

It is a FAQ that you can't get the effects of -Oy with -Ox and a bunch of 
-f flags. Some things depend directly on the optimization level. Note that 
you can also try the reverse: start from -O3 and use -fno-* flags.

I would first add -march=native to make sure this isn't the result of 
optimizing for a different platform.

I would suggest you use -fdump-tree-optimized and look at the generated 
files at -O2 and -O3 and see (they are in a vaguely C-like dialect) if you 
can find a difference that might explain the result. If so, then you can 
use -fdump-tree-all and find out where exactly gcc is going wrong. If not, 
there is -da, but the files will be much harder to read.

In any case, reducing the testcase can only make it easier to understand 
the issue.

-- 
Marc Glisse