On Fri, Oct 14, 2011 at 1:12 AM, Tom de Vries <[email protected]> wrote:
> On 10/12/2011 02:19 PM, Richard Guenther wrote:
>> On Wed, Oct 12, 2011 at 8:35 AM, Tom de Vries <[email protected]> wrote:
>>> Richard,
>>>
>>> I have a patch for PR50672.
>>>
>>> When compiling the testcase from the PR with -ftree-tail-merge, the
>>> scenario is
>>> as follows:
>>>
>>> We start out tail_merge_optimize with blocks 14 and 20, which are alike,
>>> but not
>>> equal, since they have different successors:
>>> ...
>>> # BLOCK 14 freq:690
>>> # PRED: 25 [61.0%] (false,exec)
>>>
>>> if (wD.2197_57(D) != 0B)
>>> goto <bb 15>;
>>> else
>>> goto <bb 16>;
>>> # SUCC: 15 [78.4%] (true,exec) 16 [21.6%] (false,exec)
>>>
>>>
>>> # BLOCK 20 freq:2900
>>> # PRED: 29 [100.0%] (fallthru) 31 [100.0%] (fallthru)
>>>
>>> # .MEMD.2447_209 = PHI <.MEMD.2447_125(29), .MEMD.2447_129(31)>
>>> if (wD.2197_57(D) != 0B)
>>> goto <bb 5>;
>>> else
>>> goto <bb 6>;
>>> # SUCC: 5 [85.0%] (true,exec) 6 [15.0%] (false,exec)
>>> ...
>>>
>>> In the first iteration, we merge block 5 with block 15 and block 6 with
>>> block
>>> 16. After that, the blocks 14 and 20 are equal.
>>>
>>> In the second iteration, the blocks 14 and 20 are merged, by redirecting the
>>> incoming edges of block 20 to block 14, and removing block 20.
>>>
>>> Block 20 also contains the definition of .MEMD.2447_209. Removing the
>>> definition
>>> delinks the vuse of .MEMD.2447_209 in block 5:
>>> ...
>>> # BLOCK 5 freq:6036
>>> # PRED: 20 [85.0%] (true,exec)
>>>
>>> # PT = nonlocal escaped
>>> D.2306_58 = &thisD.2200_10(D)->D.2156;
>>> # .MEMD.2447_132 = VDEF <.MEMD.2447_209>
>>> # USE = anything
>>> # CLB = anything
>>> drawLineD.2135 (D.2306_58, wD.2197_57(D), gcD.2198_59(D));
>>> goto <bb 17>;
>>> # SUCC: 17 [100.0%] (fallthru,exec)
>>> ...
>>
>> And block 5 is retained and block 15 is discarded?
>>
>
> Indeed.
>
>>> After the pass, when executing the TODO_update_ssa_only_virtuals, we update
>>> the
>>> drawLine call in block 5 using rewrite_update_stmt, which calls
>>> maybe_replace_use for the vuse operand.
>>>
>>> However, maybe_replace_use doesn't have an effect since the old vuse and
>>> the new
>>> vuse happen to be the same (rdef == use), so SET_USE is not called and the
>>> vuse
>>> remains delinked:
>>> ...
>>> if (rdef && rdef != use)
>>> SET_USE (use_p, rdef);
>>> ...
>>>
>>> The patch fixes this by forcing SET_USE for delinked uses.
>>
>> That isn't the correct fix. Whoever unlinks the vuse (by removing its
>> definition) has to replace it with something valid, which is either the
>> bare symbol .MEM, or the VUSE associated with the removed VDEF
>> (thus, as unlink_stmt_vdef does).
>>
>
> Another try. For each deleted bb, we call unlink_stmt_vdef for the statements,
> and replace the .MEM phi uses with the bare .MEM symbol.
>
> Bootstrapped and reg-tested on x86_64.
>
> Ok for trunk?
Better. For
+
+ FOR_EACH_IMM_USE_STMT (use_stmt, iter, res)
+ {
+ FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
+ SET_USE (use_p, SSA_NAME_VAR (res));
+ }
you can use mark_virtual_phi_result_for_renaming (phi) instead.
+ for (i = gsi_last_bb (bb); !gsi_end_p (i); gsi_prev_nondebug (&i))
+ unlink_stmt_vdef (gsi_stmt (i));
is that actually necessary? That is, isn't the block that follows a
deleted block always starting with a vitual PHI? If not it should
be enough to walk to the first stmt that uses a virtual operand
and similar to the PHI case replace all its uses with the bare
symbol. But as I said, I believe handling PHIs should be sufficient?
Thanks,
Richard.
> Thanks,
> - Tom
>
>> Richard.
>>
>
>
> 2011-10-14 Tom de Vries <[email protected]>
>
> PR tree-optimization/50672
> * tree-ssa-tail-merge.c (release_vdefs): New function.
> (purge_bbs): Add update_vops parameter. Call release_vdefs for each
> deleted basic block.
> (tail_merge_optimize): Add argument to call to purge_bbs.
>
