PR55124 - tentative patch for ICE in PRE
Richard Biener
richard.guenther@gmail.com
Wed Nov 28 14:38:00 GMT 2012
On Wed, Nov 28, 2012 at 3:35 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Wed, Nov 28, 2012 at 3:29 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Wed, Nov 28, 2012 at 3:05 PM, Tom de Vries <vries@codesourcery.com> wrote:
>>> On 27/11/12 13:41, Richard Biener wrote:
>>>> On Mon, Nov 19, 2012 at 3:33 AM, Tom de Vries <Tom_deVries@mentor.com> wrote:
>>>>> Richard,
>>>>>
>>>>> Consider the PR55124 example test.c:
>>>>> ...
>>>>> int a, b;
>>>>> long c;
>>>>>
>>>>> static void inline
>>>>> f2 (void)
>>>>> {
>>>>> unsigned long k = 1;
>>>>>
>>>>> foo (b ? k = 0 : 0);
>>>>>
>>>>> b = (((c = b)
>>>>> ? (k ?: (c = 0))
>>>>> : a)
>>>>> * c);
>>>>> }
>>>>>
>>>>> void
>>>>> f1 (void)
>>>>> {
>>>>> f2 ();
>>>>> a = b | c;
>>>>> }
>>>>> ...
>>>>>
>>>>> when compiling with -O2, we're running into the following assertion in pre:
>>>>> ...
>>>>> test.c:18:1: internal compiler error: in find_or_generate_expression, at
>>>>> tree-ssa-pre.c:2802
>>>>> f1 (void)
>>>>> ^
>>>>> 0xcf41d3 find_or_generate_expression
>>>>> gcc/tree-ssa-pre.c:2802
>>>>> 0xcf42f6 create_expression_by_pieces
>>>>> gcc/tree-ssa-pre.c:2861
>>>>> 0xcf4193 find_or_generate_expression
>>>>> gcc/tree-ssa-pre.c:2799
>>>>> 0xcf42f6 create_expression_by_pieces
>>>>> gcc/tree-ssa-pre.c:2861
>>>>> 0xcf4e28 insert_into_preds_of_block
>>>>> gcc/tree-ssa-pre.c:3096
>>>>> 0xcf5c7d do_regular_insertion
>>>>> gcc/tree-ssa-pre.c:3386
>>>>> ...
>>>>>
>>>>> We're hitting the assert at the end of find_or_generate_expression:
>>>>> ...
>>>>> static tree
>>>>> find_or_generate_expression (basic_block block, tree op, gimple_seq *stmts)
>>>>> {
>>>>> pre_expr expr = get_or_alloc_expr_for (op);
>>>>> unsigned int lookfor = get_expr_value_id (expr);
>>>>> pre_expr leader = bitmap_find_leader (AVAIL_OUT (block), lookfor);
>>>>> if (leader)
>>>>> {
>>>>> if (leader->kind == NAME)
>>>>> return PRE_EXPR_NAME (leader);
>>>>> else if (leader->kind == CONSTANT)
>>>>> return PRE_EXPR_CONSTANT (leader);
>>>>> }
>>>>>
>>>>> /* It must be a complex expression, so generate it recursively. */
>>>>> bitmap exprset = VEC_index (bitmap, value_expressions, lookfor);
>>>>> bitmap_iterator bi;
>>>>> unsigned int i;
>>>>> EXECUTE_IF_SET_IN_BITMAP (exprset, 0, i, bi)
>>>>> {
>>>>> pre_expr temp = expression_for_id (i);
>>>>> if (temp->kind != NAME)
>>>>> return create_expression_by_pieces (block, temp, stmts,
>>>>> get_expr_type (expr));
>>>>> }
>>>>>
>>>>> gcc_unreachable ();
>>>>> }
>>>>> ...
>>>>>
>>>>> The state in which we're asserting is the following:
>>>>> ...
>>>>> #5 0x0000000000cf41d4 in find_or_generate_expression (block=0x7ffff6210f08,
>>>>> op=0x7ffff62384c8, stmts=0x7fffffffdb78) at gcc/tree-ssa-pre.c:2802
>>>>> 2802 gcc_unreachable ();
>>>>> (gdb) p block.index
>>>>> $13 = 4
>>>>> (gdb) call debug_generic_expr (op)
>>>>> b.4_3
>>>>> (gdb) call debug_pre_expr (expr)
>>>>> b.4_3
>>>>> (gdb) p lookfor
>>>>> $11 = 7
>>>>> (gdb) call debug_bitmap_set (((bb_value_sets_t) ((block)->aux))->avail_out)
>>>>> debug[0] := { b.4_8 (0012), a.10_13 (0013), _14 (0014), iftmp.5_15 (0015) }
>>>>> (gdb) p leader
>>>>> $12 = (pre_expr) 0x0
>>>>> (gdb) call debug_bitmap ( exprset )
>>>>> first = 0x21fb058 current = 0x21fb058 indx = 0
>>>>> 0x21fb058 next = (nil) prev = (nil) indx = 0
>>>>> bits = { 22 25 }
>>>>> (gdb) call debug_pre_expr (expression_for_id (22))
>>>>> b.4_3
>>>>> (gdb) call debug_pre_expr (expression_for_id (25))
>>>>> b.4_31
>>>>> ...
>>>>> We're trying to find or generate an expr with value-id 0007 in block 4. We can't
>>>>> find it (there's no leader) and we can't generate it because there are no exprs
>>>>> with that value that are not names.
>>>>>
>>>>> Higher up in the call stack and skipping create_expression_by_pieces, the state
>>>>> is as follows:
>>>>> ...
>>>>> #7 0x0000000000cf4194 in find_or_generate_expression (block=0x7ffff6210f08,
>>>>> op=0x7ffff6238558, stmts=0x7fffffffdb78) at gcc/tree-ssa-pre.c:2799
>>>>> 2799 get_expr_type (expr));
>>>>> (gdb) call debug_generic_expr (op)
>>>>> c.6_5
>>>>> (gdb) call debug_pre_expr (expr)
>>>>> c.6_5
>>>>> (gdb) p lookfor
>>>>> $14 = 9
>>>>> (gdb) p leader
>>>>> $15 = (pre_expr) 0x0
>>>>> (gdb) call debug_bitmap ( exprset )
>>>>> first = 0x21fb0f8 current = 0x21fb0f8 indx = 0
>>>>> 0x21fb0f8 next = (nil) prev = (nil) indx = 0
>>>>> bits = { 23 24 26 27 }
>>>>> (gdb) call debug_pre_expr (temp)
>>>>> {nop_expr,b.4_3}
>>>>> (gdb) call debug_pre_expr (expression_for_id (23))
>>>>> c.6_5
>>>>> (gdb) call debug_pre_expr (expression_for_id (24))
>>>>> {nop_expr,b.4_3}
>>>>> (gdb) call debug_pre_expr (expression_for_id (26))
>>>>> c.6_32
>>>>> (gdb) call debug_pre_expr (expression_for_id (27))
>>>>> {mem_ref<0B>,addr_expr<&c>}@.MEM_28
>>>>> ...
>>>>> We're trying to find or generate an expr with value-id 0009 (in block 4). We
>>>>> can't find it. We're trying to generate it using {nop_expr,b.4_3}, but as we've
>>>>> seen above that won't work. The generation using expr 27 would work though.
>>>>>
>>>>> Again higher up in the call stack and skipping create_expression_by_pieces, the
>>>>> state is as follows:
>>>>> ...
>>>>> (gdb) up
>>>>> #9 0x0000000000cf4e29 in insert_into_preds_of_block (block=0x7ffff6210f70,
>>>>> exprnum=19, avail=0x22102e0) at gcc/tree-ssa-pre.c:3096
>>>>> 3096 &stmts, type);
>>>>> (gdb) l
>>>>> 3091 eprime = VEC_index (pre_expr, avail, pred->dest_idx);
>>>>> 3092
>>>>> 3093 if (eprime->kind != NAME && eprime->kind != CONSTANT)
>>>>> 3094 {
>>>>> 3095 builtexpr = create_expression_by_pieces (bprime, eprime,
>>>>> 3096 &stmts, type);
>>>>> (gdb) p block.index
>>>>> $17 = 5
>>>>> (gdb) call debug_pre_expr (expr)
>>>>> {convert_expr,c.7_16}
>>>>> (gdb) p val
>>>>> $18 = 8
>>>>> (gdb) call debug_pre_expr (eprime)
>>>>> {convert_expr,c.6_5}
>>>>> (gdb) call get_expr_value_id (eprime)
>>>>> $16 = 26
>>>>> ...
>>>>> So we're trying to insert expr {convert_expr,c.6_5} with value-id 0026 into
>>>>> block 4. The expr is the phi-translation of expr {convert_expr,c.7_16} with
>>>>> value-id 0008 in block 5.
>>>>>
>>>>> One of the reasons why we're inserting the phi-translation of expr
>>>>> {convert_expr,c.7_16} in block 4 is because it's a member of ANTIC_IN[5]:
>>>>> ...
>>>>> ANTIC_IN[5] := { iftmp.5_18 (0018), {mem_ref<0B>,addr_expr<&c>}@.MEM_23 (0016),
>>>>> {nop_expr,c.7_16} (0017), {mult_expr,_17,iftmp.5_18} (0019),
>>>>> {nop_expr,_19} (0020), {convert_expr,c.7_16} (0008),
>>>>> {bit_ior_expr,_4,b.11_20} (0010) }
>>>>> ...
>>>>> A requirement for an expr to be in ANTIC_IN is that that it's either 'a live
>>>>> temporary or a non-simple expression whose operands are represented in the
>>>>> anti-leader set'. The operand is c.7_16, which has value id 0016, as we can see
>>>>> here:
>>>>> ...
>>>>> tmp_gen[5] := { c.7_16 (0016), _17 (0017), _19 (0019), b.11_20 (0020), _4
>>>>> (0008), a.2_6 (0010) }
>>>>> ...
>>>>> And it has this representation in ANTIC_IN[5] in expr
>>>>> {mem_ref<0B>,addr_expr<&c>}@.MEM_23. So that looks ok.
>>>>>
>>>>> The order in which we traverse ANTIC_IN[5] in do_regular_insertion is this:
>>>>> ...
>>>>> (gdb) call debug_pre_expr ( exprs.vec_[0] )
>>>>> {convert_expr,c.7_16}
>>>>> (gdb) call debug_pre_expr ( exprs.vec_[1] )
>>>>> {bit_ior_expr,_4,b.11_20}
>>>>> (gdb) call debug_pre_expr ( exprs.vec_[2] )
>>>>> {mem_ref<0B>,addr_expr<&c>}@.MEM_23
>>>>> (gdb) call debug_pre_expr ( exprs.vec_[3] )
>>>>> {nop_expr,c.7_16}
>>>>> (gdb) call debug_pre_expr ( exprs.vec_[4] )
>>>>> iftmp.5_18
>>>>> (gdb) call debug_pre_expr ( exprs.vec_[5] )
>>>>> {mult_expr,_17,iftmp.5_18}
>>>>> (gdb) call debug_pre_expr ( exprs.vec_[6] )
>>>>> {nop_expr,_19}
>>>>> ...
>>>>>
>>>>> The order is indeed in increasing value-id order:
>>>>> ...
>>>>> (gdb) call get_expr_value_id ( exprs.vec_[0] )
>>>>> $11 = 8
>>>>> (gdb) call get_expr_value_id ( exprs.vec_[1] )
>>>>> $12 = 10
>>>>> (gdb) call get_expr_value_id ( exprs.vec_[2] )
>>>>> $13 = 16
>>>>> (gdb) call get_expr_value_id ( exprs.vec_[3] )
>>>>> $14 = 17
>>>>> (gdb) call get_expr_value_id ( exprs.vec_[4] )
>>>>> $15 = 18
>>>>> (gdb) call get_expr_value_id ( exprs.vec_[5] )
>>>>> $16 = 19
>>>>> (gdb) call get_expr_value_id ( exprs.vec_[6] )
>>>>> $17 = 20
>>>>> ...
>>>>>
>>>>> But the operand of the first expr {convert_expr,c.7_16} has value-id 0016, which
>>>>> corresponds to the 3rd expr {mem_ref<0B>,addr_expr<&c>}@.MEM_23. So if I
>>>>> understand the intended topological sort correctly, this is in the wrong order,
>>>>> we should be processing the 3rd element before the first element. I'm not quite
>>>>> sure this is the root cause of the problem though.
>>>>>
>>>>> Assuming for the moment that the order is correct, I've written a tentative
>>>>> patch that fixes the assert, simply by predicting whether
>>>>> create_expression_by_pieces will succeed or not, and to skip those calls that
>>>>> will fail in find_or_generate_expression. The patch has only been tested with a
>>>>> tree-ssa.exp testrun, but no issues found there.
>>>>>
>>>>> Do you think this patch is the way to fix this ICE, or is it the order of
>>>>> generation that needs fixing, or is the problem yet something else?
>>>>
>>>> This looks like an ordering issue. But rather in what value-numbers were
>>>> assigned to the expressions, not the sorting itself.
>>>
>>> The sorting done by sorted_array_from_bitmap_set assumes that value_id order is
>>> in topological order:
>>> ...
>>> FOR_EACH_VALUE_ID_IN_SET (set, i, bi)
>>> {
>>> /* The number of expressions having a given value is usually
>>> relatively small. Thus, rather than making a vector of all
>>> the expressions and sorting it by value-id, we walk the values
>>> and check in the reverse mapping that tells us what expressions
>>> have a given value, to filter those in our set. As a result,
>>> the expressions are inserted in value-id order, which means
>>> topological order.
>>>
>>> If this is somehow a significant lose for some cases, we can
>>> choose which set to walk based on the set size. */
>>> bitmap exprset = VEC_index (bitmap, value_expressions, i);
>>> EXECUTE_IF_SET_IN_BITMAP (exprset, 0, j, bj)
>>> {
>>> if (bitmap_bit_p (&set->expressions, j))
>>> VEC_safe_push (pre_expr, heap, result, expression_for_id (j));
>>> }
>>> }
>>> ...
>>>
>>> The relevant ssa-names are _4 and _16:
>>> ...
>>> # VUSE <.MEM_23>
>>> c.7_16 = cD.1716;
>>> _4 = (intD.6) c.7_16;
>>> ...
>>>
>>> which have the following value ids, which means that they're not in topological
>>> order:
>>> ...
>>> _4 = _4 value_id 8
>>> c.7_16 = c.7_16 value_id 16
>>> ...
>>>
>>> If I revert patch r189321, the compiler doesn't assert anymore. But if I look at
>>> the relevant ssa-names, the value numbers are still not in topological order:
>>> ...
>>> _4 = _4 value_id 5
>>> c.7_16 = c.7_16 value_id 13
>>> ...
>>>
>>> Assigning these value_ids is done in run_scc_vn. I don't find any evidence there
>>> that an effort is done to number values in topological order, so my conclusion
>>> is that the premise in sorted_array_from_bitmap_set about value-id order meaning
>>> topological order is invalid. I suspect that value_ids introduced after value
>>> numbering, by pre itself, are in topological order though.
>>
>> Ick.
>>
>>>> I suppose it may
>>>> result from your vitrual operand numbering changes and compute_avail
>>>> doing
>>>>
>>>> case VN_REFERENCE:
>>>> {
>>>> vn_reference_t ref;
>>>> vn_reference_lookup (gimple_assign_rhs1 (stmt),
>>>> gimple_vuse (stmt),
>>>> VN_WALK, &ref);
>>>>
>>>> which valueizes the VUSE here?
>>>>
>>>
>>> The value numbers are out of order, with and without the patch, so I don't see
>>> the connection with the patch or with virtual operand numbering changes.
>>>
>>>
>>> I can think of a few ways to fix this:
>>> - add assignment of value_id during value numbering rather than after
>>> value numbering
>>> - try to add a topo_id <-> value_id mapping during building up the pre_exprs,
>>> and use that in sorted_array_from_bitmap_set
>>> - do actual topological sorting in sorted_array_from_bitmap_set (FWIW, patch
>>> attached, passes tree-ssa.exp)
>>>
>>> In what way do you think should this be fixed?
>>
>> I think that now that FRE (and thus PRE eliminate ()) no longer needs
>> value-id's we should move value-id's back to PRE and assign them at
>> compute_avail time (which walks in proper oder).
>>
>> Something like the attached, testing in progress.
>
> Ok, that doesn't seem to work because vn_set_hashtable_value_ids is called
> too late. I suppose we cannot do without an extra topological walk over
> SSA name defs :/
But maybe it can be made to work if also assigning value-ids in compute_avail
when we lookup from the hashtable (and that value-id assigning doing the
propagation bit itself). Eventually the whole ->value_id stuff should be
moved to the PRE IL instead of being in the SCCVN IL.
Richard.
>
>> Richard.
>>
>>> Thanks,
>>> - Tom
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