On 07/30/2011 10:21 AM, Tom de Vries wrote:
> Hi,
>
> On 07/28/2011 08:20 PM, Tom de Vries wrote:
>> On 07/28/2011 06:25 PM, Richard Guenther wrote:
>>> On Thu, 28 Jul 2011, Tom de Vries wrote:
>>>
>>>> On 07/28/2011 12:22 PM, Richard Guenther wrote:
>>>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>>>
>>>>>> On 07/27/2011 05:27 PM, Richard Guenther wrote:
>>>>>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>>>>>
>>>>>>>> On 07/27/2011 02:12 PM, Richard Guenther wrote:
>>>>>>>>> On Wed, 27 Jul 2011, Tom de Vries wrote:
>>>>>>>>>
>>>>>>>>>> On 07/27/2011 01:50 PM, Tom de Vries wrote:
>>>>>>>>>>> Hi Richard,
>>>>>>>>>>>
>>>>>>>>>>> I have a patch set for bug 43513 - The stack pointer is adjusted
>>>>>>>>>>> twice.
>>>>>>>>>>>
>>>>>>>>>>> 01_pr43513.3.patch
>>>>>>>>>>> 02_pr43513.3.test.patch
>>>>>>>>>>> 03_pr43513.3.mudflap.patch
>>>>>>>>>>>
>>>>>>>>>>> The patch set has been bootstrapped and reg-tested on x86_64.
>>>>>>>>>>>
>>>>>>>>>>> I will sent out the patches individually.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> The patch replaces a vla __builtin_alloca that has a constant
>>>>>>>>>> argument with an
>>>>>>>>>> array declaration.
>>>>>>>>>>
>>>>>>>>>> OK for trunk?
>>>>>>>>>
>>>>>>>>> I don't think it is safe to try to get at the VLA type the way you do.
>>>>>>>>
>>>>>>>> I don't understand in what way it's not safe. Do you mean I don't
>>>>>>>> manage to find
>>>>>>>> the type always, or that I find the wrong type, or something else?
>>>>>>>
>>>>>>> I think you might get the wrong type,
>>>>>>
>>>>>> Ok, I'll review that code one more time.
>>>>>>
>>>>>>> you also do not transform code
>>>>>>> like
>>>>>>>
>>>>>>> int *p = alloca(4);
>>>>>>> *p = 3;
>>>>>>>
>>>>>>> as there is no array type involved here.
>>>>>>>
>>>>>>
>>>>>> I was trying to stay away from non-vla allocas. A source declared
>>>>>> alloca has
>>>>>> function livetime, so we could have a single alloca in a loop, called 10
>>>>>> times,
>>>>>> with all 10 instances live at the same time. This patch does not detect
>>>>>> such
>>>>>> cases, and thus stays away from non-vla allocas. A vla decl does not
>>>>>> have such
>>>>>> problems, the lifetime ends when it goes out of scope.
>>>>>
>>>>> Yes indeed - that probably would require more detailed analysis.
>>>>>
>>>>>>>>> In fact I would simply do sth like
>>>>>>>>>
>>>>>>>>> elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
>>>>>>>>> n_elem = size * 8 / BITS_PER_UNIT;
>>>>>>>>> array_type = build_array_type_nelts (elem_type, n_elem);
>>>>>>>>> var = create_tmp_var (array_type, NULL);
>>>>>>>>> return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));
>>>>>>>>>
>>>>>>>>
>>>>>>>> I tried this code on the example, and it works, but the newly declared
>>>>>>>> type has
>>>>>>>> an 8-bit alignment, while the vla base type has a 32 bit alignment.
>>>>>>>> This make
>>>>>>>> the memory access in the example potentially unaligned, which
>>>>>>>> prohibits an
>>>>>>>> ivopts optimization, so the resulting text size is 68 instead of the
>>>>>>>> 64 achieved
>>>>>>>> with my current patch.
>>>>>>>
>>>>>>> Ok, so then set DECL_ALIGN of the variable to something reasonable
>>>>>>> like MIN (size * 8, GET_MODE_PRECISION (word_mode)). Basically the
>>>>>>> alignment that the targets alloca function would guarantee.
>>>>>>>
>>>>>>
>>>>>> I tried that, but that doesn't help. It's the alignment of the type that
>>>>>> matters, not of the decl.
>>>>>
>>>>> It shouldn't. All accesses are performed with the original types and
>>>>> alignment comes from that (plus the underlying decl).
>>>>>
>>>>
>>>> I managed to get it all working by using build_aligned_type rather that
>>>> DECL_ALIGN.
>>>
>>> That's really odd, DECL_ALIGN should just work - nothing refers to the
>>> type of the decl in the IL. Can you try also setting DECL_USER_ALIGN to
>>> 1 maybe?
>>>
>>
>> This doesn't work either.
>>
>> /* Declare array. */
>> elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
>> n_elem = size * 8 / BITS_PER_UNIT;
>> align = MIN (size * 8, GET_MODE_PRECISION (word_mode));
>> array_type = build_array_type_nelts (elem_type, n_elem);
>> var = create_tmp_var (array_type, NULL);
>> DECL_ALIGN (var) = align;
>> DECL_USER_ALIGN (var) = 1;
>>
>> Maybe this clarifies it:
>>
>> Breakpoint 1, may_be_unaligned_p (ref=0xf7d9d410, step=0xf7d3d578) at
>> /home/vries/local/google/src/gcc-mainline/gcc/tree-ssa-loop-ivopts.c:1621
>> (gdb) call debug_generic_expr (ref)
>> MEM[(int[0:D.2579] *)&D.2595][0]
>> (gdb) call debug_generic_expr (step)
>> 4
>>
>> 1627 base = get_inner_reference (ref, &bitsize, &bitpos, &toffset, &mode,
>> (gdb) call debug_generic_expr (base)
>> D.2595
>>
>> 1629 base_type = TREE_TYPE (base);
>> (gdb) call debug_generic_expr (base_type)
>> <unnamed-unsigned:8>[40]
>>
>> 1630 base_align = TYPE_ALIGN (base_type);
>> (gdb) p base_align
>> $1 = 8
>>
>> So the align is 8-bits, and we return true here:
>>
>> (gdb) n
>> 1632 if (mode != BLKmode)
>> (gdb) n
>> 1634 unsigned mode_align = GET_MODE_ALIGNMENT (mode);
>> (gdb)
>> 1636 if (base_align < mode_align
>> (gdb)
>> 1639 return true;
>>
>>
>> Here we can see that the base actually has the (user) align on it:
>>
>> (gdb) call debug_tree (base)
>> <var_decl 0xf7e1b420 D.2595
>> type <array_type 0xf7e1b360
>> type <integer_type 0xf7e1b2a0 public unsigned QI
>> size <integer_cst 0xf7d3d604 constant 8>
>> unit size <integer_cst 0xf7d3d620 constant 1>
>> align 8 symtab 0 alias set -1 canonical type 0xf7e1b2a0
>> precision 8
>> min <integer_cst 0xf7dffaf0 0> max <integer_cst 0xf7dffb0c 255>
>> pointer_to_this <pointer_type 0xf7e1b3c0>>
>> BLK
>> size <integer_cst 0xf7d5d070 constant 320>
>> unit size <integer_cst 0xf7dde2a0 constant 40>
>> align 8 symtab 0 alias set -1 canonical type 0xf7e1b360
>> domain <integer_type 0xf7de12a0
>> type <integer_type 0xf7d51000 unsigned int>
>> SI
>> size <integer_cst 0xf7d3d78c constant 32>
>> unit size <integer_cst 0xf7d3d578 constant 4>
>> align 32 symtab 0 alias set -1 canonical type 0xf7de12a0
>> precision 32 min <integer_cst 0xf7d3d594 0>
>> max <integer_cst 0xf7dde284 39>>
>> pointer_to_this <pointer_type 0xf7e1b480>>
>> addressable used ignored BLK file pr43513.c line 4 col 6
>> size <integer_cst 0xf7d5d070 320> unit size <integer_cst 0xf7dde2a0 40>
>> user align 32 context <function_decl 0xf7dfd480 foo3>>
>>
>>
>>>>
>>>>>> So should we try to find the base type of the vla, and use that, or use
>>>>>> the
>>>>>> nonstandard char type?
>>>>>
>>>>> I don't think we can reliably find the base type of the vla - well,
>>>>> in practice we may because we control how we lower VLAs during
>>>>> gimplification, but nothing in the IL constraints say that the
>>>>> resulting pointer type should be special.
>>>>>
>>>>> Using a char[] decl shouldn't be a problem IMHO.
>>>>>
>>>>>>>>> And obviously you lose the optimization we arrange with inserting
>>>>>>>>> __builtin_stack_save/restore pairs that way - stack space will no
>>>>>>>>> longer be shared for subsequent VLAs. Which means that you'd
>>>>>>>>> better limit the size you allow this promotion.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Right, I could introduce a parameter for this.
>>>>>>>
>>>>>>> I would think you could use PARAM_LARGE_STACK_FRAME for now and say,
>>>>>>> allow a size of PARAM_LARGE_STACK_FRAME / 10?
>>>>>>>
>>>>>>
>>>>>> That unfortunately is too small for the example from bug report. The
>>>>>> default
>>>>>> value of the param is 250, so that would be a threshold of 25, and the
>>>>>> alloca
>>>>>> size of the example is 40. Perhaps we can try a threshold of
>>>>>> PARAM_LARGE_STACK_FRAME - estimated_stack_size or some such?
>>>>>
>>>>> Hm. estimated_stack_size is not O(1), so no. I think we need to
>>>>> find a sensible way of allowing stack sharing. Eventually Michas
>>>>> patch for introducing points-of-death would help here, if we'd
>>>>> go for folding this during stack-save/restore optimization.
>>>>>
>>>>
>>>> I changed the heuristics to this:
>>>>
>>>> + /* Heuristic: don't fold large vlas. */
>>>> + threshold = (unsigned HOST_WIDE_INT)PARAM_VALUE
>>>> (PARAM_LARGE_STACK_FRAME);
>>>> + /* In case a vla is declared at function scope, it has the same
>>>> lifetime as a
>>>> + declared array, so we allow a larger size. */
>>>> + block = gimple_block (stmt);
>>>> + if (!(cfun->after_inlining
>>>> + && TREE_CODE (BLOCK_SUPERCONTEXT (block)) == FUNCTION_DECL))
>>>> + threshold /= 10;
>>>> + if (size > threshold)
>>>> + return NULL_TREE;
>>>>
>>>> The heuristics distinguishes between before and after inlining.
>>>>
>>>> After inlining, vla's declared at function scope have the same lifetimes as
>>>> declared arrays, and don't share their space. There should be no negative
>>>> effects from folding an alloca in this case, but for safety we set a
>>>> threshold
>>>> of PARAM_LARGE_STACK_FRAME.
>>>>
>>>> Before inlining, such a vla might be inlined and share its space with
>>>> another
>>>> vla, so we stick with the normal threshold before inlining.
>>>
>>> That sounds reasonable, though the block check should probably use the
>>> original VLA decl block, not that of the basic-block of the allocation,
>>> but unfortunately we don't have access to that. So I suppose using
>>> the allocation basic-block BLOCK is good enough (still we don't
>>> really care about BLOCK boundaries when doing CFG manipulations, so
>>> the allocation bbs block may be not the outermost scope in more cases
>>> than necessary).
>>>
>>>> However, using this heuristic we still don't generate optimal code.
>>>>
>>>> During the first pass_ccp, the folding is not done, because the size (40)
>>>> is
>>>> larger than the threshold 25. The threshold is 25, because inlining is not
>>>> yet done.
>>>>
>>>> During pass_fold_builtins, the folding is done because it's after
>>>> inlining, but
>>>> it's later than pass_iv_optimize, so that still doesn't yield the optimal
>>>> size
>>>> of 64.
>>>>
>>>> The folding is not done during any of the other invocations or pass_ccp,
>>>> because
>>>> the argument has already become constant in the earlier invocation.
>>>
>>> Yeah, that's the issue with relying on folding to do this transformation.
>>>
>>>> Using this change, I manage to trigger folding during the second
>>>> invocation of
>>>> pass_ccp, before iv_optimize so we generate optimal code.
>>>>
>>>> Index: gcc/tree-ssa-ccp.c
>>>> ===================================================================
>>>> --- gcc/tree-ssa-ccp.c (revision 173734)
>>>> +++ gcc/tree-ssa-ccp.c (working copy)
>>>> @@ -1727,6 +1727,13 @@ ccp_fold_stmt (gimple_stmt_iterator *gsi
>>>> if (gimple_call_internal_p (stmt))
>>>> return false;
>>>>
>>>> + /* The heuristic of fold_builtin_alloca differs before and after
>>>> + inlining, so we don't require the arg to be changed into a
>>>> constant
>>>> + for folding, but just to be constant. */
>>>> + if (gimple_call_alloca_for_var_p (stmt)
>>>> + && get_constant_value (gimple_call_arg (stmt, 0)))
>>>
>>> Probably reverse the get_constant_value check and the transformation
>>
>> Done.
>>
>>> (gimple_call_alloca_for_var_p isn't a predicate as it has side-effects,
>>> so its name should be changed).
>>>
>>>> + return true;
>>>> +
>>>> /* Propagate into the call arguments. Compared to replace_uses_in
>>>> this can use the argument slot types for type verification
>>>> instead of the current argument type. We also can safely
>>>>
>>>> But, to me it feels like a hack. Do you have any ideas how to do this
>>>> better?
>>>
>>> It's somewhat of a hack, but at least it is more of a defined place
>>> for this transformation - which then suggests to remove it from
>>> generic folding and only keep calling it from CCP this way.
>>>
>>
>> Done.
>>
>
> This is an updated version of the patch. I have 2 new patches and an updated
> testcase which I will sent out individually.
>
Previously stores to vla's were marked as alive in dce because
is_hidden_global_store triggered. With them converted to stores to a normal
array, that doesn't happen anymore.
The stores where then sometimes removed, while there was an aliasing load, a
function argument. But since ref_may_be_aliased was called with WITH_SIZE_EXPR
as toplevel expr, ref_may_be_aliased returned false for the aliasing load.
This patch fixes this.
OK for trunk?
Thanks,
- Tom
2011-07-30 Tom de Vries <[email protected]>
PR middle-end/43513
* tree-ssa-dce.c (ref_may_be_aliased): Add assert.
(propagate_necessity): Handle WITH_SIZE_EXPR call arg.
Index: gcc/tree-ssa-dce.c
===================================================================
--- gcc/tree-ssa-dce.c (revision 173734)
+++ gcc/tree-ssa-dce.c (working copy)
@@ -489,6 +489,7 @@ find_obviously_necessary_stmts (struct e
static bool
ref_may_be_aliased (tree ref)
{
+ gcc_assert (TREE_CODE (ref) != WITH_SIZE_EXPR);
while (handled_component_p (ref))
ref = TREE_OPERAND (ref, 0);
if (TREE_CODE (ref) == MEM_REF
@@ -839,6 +840,8 @@ propagate_necessity (struct edge_list *e
if (TREE_CODE (arg) == SSA_NAME
|| is_gimple_min_invariant (arg))
continue;
+ if (TREE_CODE (arg) == WITH_SIZE_EXPR)
+ arg = TREE_OPERAND (arg, 0);
if (!ref_may_be_aliased (arg))
mark_aliased_reaching_defs_necessary (stmt, arg);
}
