Re: [PATCH] vrp: remove rendundant has_single_use tests

[Richard Biener <richard dot guenther at gmail dot com>]

On Thu, Apr 21, 2016 at 2:47 AM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> On Wed, Apr 20, 2016 at 6:45 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
>> During assert-location discovery, if an SSA name is live according to
>> live_on_edge() on some outgoing edge E, then the SSA name definitely has
>> at least two uses: the use on the outgoing edge, and the use in some BB
>> dominating E->src from which the SSA_NAME and the potential assertion
>> was discovered.  These two uses can't be the same because the liveness
>> array is populated on-the-fly in reverse postorder so the latter use
>> which dominates BB couldn't have yet contributed to the liveness bitmap.
>>
>> So AFAICT it's not necessary to check live_on_edge() as well as
>> !has_single_use() since the former check will imply the latter.  So this
>> patch removes these redundant calls to has_single_use() (and alse
>> replaces the use of has_single_use() in find_assert_locations_1 with a
>> liveness bitmap test which should be cheaper and more accurate).
>>
>> I bootstrapped and regtested this change on x86_64-pc-linux-gnu.  I also
>> confirmed that the number of calls made to register_new_assert_for
>> before and after the patch remains the same during compilation of
>> libstdc++ and during compilation of gimple-match.c and when running the
>> tree-ssa.exp testsuite.  Does this look OK to commit?
>>
>> gcc/ChangeLog:
>>
>>         * tree-vrp.c (register_edge_assert_for_2): Remove redundant
>>         has_single_use() tests.
>>         (register_edge_assert_for_1): Likewise.
>>         (find_assert_locations_1): Check the liveness bitmap instead of
>>         calling has_single_use().
>
> By the way, would it be reasonable to cache/precompute the number of
> non-debug uses each ssa name has so that has_single_use, has_zero_uses
> etc are much cheaper?

Not sure whether that's good (think of the need to update this plus the
storage required for it).  Maybe keep the immediate use list in order
{ real uses, debug uses }?
(thus do inserting at head/tail depending on use stmt case)

Richard.