Re: Question on documentation about RTL PRE in gccint

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

Bin. Cheng wrote:
> Quoting from GCCINT, section "9.5 RTL passes":
> "When optimizing for size, GCSE is done using Morel-Renvoise Partial
> Redundancy Elimination, with the exception that it does not try to
> move invariants out of loops—that is left to the loop optimization
> pass. If MR PRE GCSE is done, code hoisting (aka unification) is also
> done, as well as load motion."
>
> While the pass gate function is as below:
> static bool
> gate_rtl_pre (void)
> {
>   return optimize > 0 && flag_gcse
>     && !cfun->calls_setjmp
>     && optimize_function_for_speed_p (cfun)
>     && dbg_cnt (pre);
> }
>
> It seems the PRE pass is disabled when not optimizing for speed.
> Doesn't this conflict with the documentation, which says
> Morel-Renvoise PRE will be used when optimizing for size.

The documentation should say "When *not* optimizing for size..." .

But this piece of documentation seems to be in need of some TLC anyway:
* hoisting is not enabled (or used to be not enabled, not sure what
it's like now) when not optimizing for size. hoisting is enabled with
-Os, PRE is disabled.
* MR PRE was replaced with edge-based lazy code motion even before GCC 3.0
* loop code motion is now done on GIMPLE way before RTL PRE
* ... (probably half a dozen more issues) ...

If you file a PR, I'll update the documentation for the old gcse.c
passes (HOIST, PRE, CPROP).

Ciao!
Steven