[PR/middle-end 81897] make tree-ssa-uninit.c handle longer sequences

[Jeff Law]

On 01/10/2018 10:45 AM, Aldy Hernandez wrote:
>>> @@ -671,11 +668,9 @@ convert_control_dep_chain_into_preds (vec<edge> *dep_chains,
>>>         e = one_cd_chain[j];
>>>         guard_bb = e->src;
>>>         gsi = gsi_last_bb (guard_bb);
>>> +       /* Ignore empty BBs as they're basically forwarder blocks.  */
>>>         if (gsi_end_p (gsi))
>>> -         {
>>> -           has_valid_pred = false;
>>> -           break;
>>> -         }
>>> +         continue;
>>>         cond_stmt = gsi_stmt (gsi);
>>>         if (is_gimple_call (cond_stmt) && EDGE_COUNT (e->src->succs) >= 2)
>>>           /* Ignore EH edge.  Can add assertion on the other edge's flag.  */
>> ISTM that you want to use empty_block_p (bb) && single_succ_p (bb) to
>> detect the forwarder block.  Otherwise ISTM that labels and debug
>> statements would affect the uninit analysis.
> We still need to check for gsi_end_p() because guard_bb can have no
> statements but be considered non empty according to empty_block_p().
> This is the case with a seemingly empty basic block that actually has
> an incoming PHI.
> 
> Jakub suggested the following patch which fixes the new ICE in the PR.
> I've adjusted the comments accordingly.
> 
> OK?
> 
> 
> curr.patch
> 
> 
> gcc/
> 
> 	PR middle-end/81897
> 	* tree-ssa-uninit.c (convert_control_dep_chain_into_preds): Skip
> 	empty blocks.
OK.

Sorry for mucking things up and making more work :(

jeff