Re: [PATCH] Fix one part of PR42108

["H.J. Lu" <hjl.tools@gmail.com>]

On Fri, Dec 18, 2009 at 3:29 PM, Richard Guenther <rguenther@suse.de> wrote:
>
> This fixes one part of PR42108, the missed discovery of a full
> redundant load.  The issue is that the SSA SCC value-numberer
> does not visit loads and stores in a defined order.  The (or rather
> one) fix is to properly canonicalize the virtual operand SSA names
> we record in the expression hash tables.  The proper canonical
> virtual operand is the def of the first dominating may-definition
> (or a PHI node vdef, but we can as well choose a non-may-definition
> without loss of precision and generality).
>
> The patch possibly slows down SCCVN a bit for examples like
>
>   # .MEM_2 = VDEF <.MEM_1(D)>
>   may-def
>  ....
>   # VUSE <.MEM_120>
>   ... = X;
>   # VUSE <.MEM_120>
>   ... = X;
>
> where discovering the redundant load of X needs to canonicalize
> their VUSE SSA name twice (previously we entered the expression
> into the hashtable with .MEM_120 so it would be found immediately).
> Now if we had
>
>   # VUSE <.MEM_60>
>   ... = X;
>
> inbetween the may-def and the other loads we previously discovered
> the full redundancy only if we first visited the load with .MEM_60
> and only after that the loads with .MEM_120.  But nothing guarantees
> this - this is the case the patch fixes.  There are about 0.5%
> more redundant loads discovered in tramp3d with this patch.
>
> This is a regression of the alias-improvements branch merge as
> previously we had different virtual operands and thus in more
> cases the canonical vuses were automagically chosen.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu.  I have patched
> one of our SPEC / C++ testers for more testing coverage.
>

This may have caused:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42435


-- 
H.J.