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? > > >> 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 (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. Richard. > Attaching untested patch for reference (will test overnight). > > Thanks, > - Tom > > 2011-07-28 Tom de Vries <t...@codesourcery.com> > > PR middle-end/43513 > * gimple-fold.c (params.h): Include. > (fold_builtin_alloca): New function. > (gimple_fold_builtin): Use fold_builtin_alloca. > * tree-ssa-ccp.c (ccp_fold_stmt): Force folding of vla-related alloca. > * Makefile.in (gimple-fold.o): Add $(PARAMS_H) to rule.
