On Fri, Jul 28, 2023 at 2:57 PM Jan Hubicka via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi,
> This patch extends tree-ssa-loop-split to understand test of the form
> if (i==0)
> and
> if (i!=0)
> which triggers only during the first iteration. Naturally we should
> also be able to trigger last iteration or split into 3 cases if
> the test indeed can fire in the middle of the loop.
>
> Last iteration is bit trickier pattern matching so I want to do it
> incrementally, but I implemented easy case using value range that handled
> loops with constant iterations.
>
> The testcase gets misupdated profile, I will also fix that incrementally.
>
> Bootstrapped/regtested x86_64-linux, OK?
OK, though I think we can handle more loops by simply conservatively peeling
one iteration at the beginning/end with such conditions and would be not subject
to all other limitations the loop splitting pass has?
Richard.
> gcc/ChangeLog:
>
> PR middle-end/77689
> * tree-ssa-loop-split.cc: Include value-query.h.
> (split_at_bb_p): Analyze cases where EQ/NE can be turned
> into LT/LE/GT/GE; return updated guard code.
> (split_loop): Use guard code.
>
> gcc/testsuite/ChangeLog:
>
> PR middle-end/77689
> * g++.dg/tree-ssa/loop-split-1.C: New test.
>
> diff --git a/gcc/testsuite/g++.dg/tree-ssa/loop-split-1.C
> b/gcc/testsuite/g++.dg/tree-ssa/loop-split-1.C
> new file mode 100644
> index 00000000000..9581438b536
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/tree-ssa/loop-split-1.C
> @@ -0,0 +1,23 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O3 -fdump-tree-lsplit-details -std=c++11" } */
> +#include <vector>
> +#include <cmath>
> +
> +constexpr unsigned s = 100000000;
> +
> +int main()
> +{
> + std::vector<float> a, b, c;
> + a.reserve(s);
> + b.reserve(s);
> + c.reserve(s);
> +
> + for(unsigned i = 0; i < s; ++i)
> + {
> + if(i == 0)
> + a[i] = b[i] * c[i];
> + else
> + a[i] = (b[i] + c[i]) * c[i-1] * std::log(i);
> + }
> +}
> +/* { dg-final { scan-tree-dump-times "loop split" 1 "lsplit" } } */
> diff --git a/gcc/tree-ssa-loop-split.cc b/gcc/tree-ssa-loop-split.cc
> index 70cd0aaefa7..641346cba70 100644
> --- a/gcc/tree-ssa-loop-split.cc
> +++ b/gcc/tree-ssa-loop-split.cc
> @@ -42,6 +42,7 @@ along with GCC; see the file COPYING3. If not see
> #include "gimple-fold.h"
> #include "gimplify-me.h"
> #include "print-tree.h"
> +#include "value-query.h"
>
> /* This file implements two kinds of loop splitting.
>
> @@ -75,7 +76,8 @@ along with GCC; see the file COPYING3. If not see
> point in *BORDER and the comparison induction variable in IV. */
>
> static tree
> -split_at_bb_p (class loop *loop, basic_block bb, tree *border, affine_iv *iv)
> +split_at_bb_p (class loop *loop, basic_block bb, tree *border, affine_iv *iv,
> + enum tree_code *guard_code)
> {
> gcond *stmt;
> affine_iv iv2;
> @@ -87,19 +89,6 @@ split_at_bb_p (class loop *loop, basic_block bb, tree
> *border, affine_iv *iv)
>
> enum tree_code code = gimple_cond_code (stmt);
>
> - /* Only handle relational comparisons, for equality and non-equality
> - we'd have to split the loop into two loops and a middle statement. */
> - switch (code)
> - {
> - case LT_EXPR:
> - case LE_EXPR:
> - case GT_EXPR:
> - case GE_EXPR:
> - break;
> - default:
> - return NULL_TREE;
> - }
> -
> if (loop_exits_from_bb_p (loop, bb))
> return NULL_TREE;
>
> @@ -129,6 +118,56 @@ split_at_bb_p (class loop *loop, basic_block bb, tree
> *border, affine_iv *iv)
> if (!iv->no_overflow)
> return NULL_TREE;
>
> + /* Only handle relational comparisons, for equality and non-equality
> + we'd have to split the loop into two loops and a middle statement. */
> + switch (code)
> + {
> + case LT_EXPR:
> + case LE_EXPR:
> + case GT_EXPR:
> + case GE_EXPR:
> + break;
> + case NE_EXPR:
> + case EQ_EXPR:
> + /* If the test check for first iteration, we can handle NE/EQ
> + with only one split loop. */
> + if (operand_equal_p (iv->base, iv2.base, 0))
> + {
> + if (code == EQ_EXPR)
> + code = !tree_int_cst_sign_bit (iv->step) ? LE_EXPR : GE_EXPR;
> + else
> + code = !tree_int_cst_sign_bit (iv->step) ? GT_EXPR : LT_EXPR;
> + break;
> + }
> + /* Similarly when the test checks for minimal or maximal
> + value range. */
> + else
> + {
> + int_range<2> r;
> + get_global_range_query ()->range_of_expr (r, op0, stmt);
> + if (!r.varying_p () && !r.undefined_p ()
> + && TREE_CODE (op1) == INTEGER_CST)
> + {
> + wide_int val = wi::to_wide (op1);
> + if (known_eq (val, r.lower_bound ()))
> + {
> + code = (code == EQ_EXPR) ? LE_EXPR : GT_EXPR;
> + break;
> + }
> + else if (known_eq (val, r.upper_bound ()))
> + {
> + code = (code == EQ_EXPR) ? GE_EXPR : LT_EXPR;
> + break;
> + }
> + }
> + }
> + /* TODO: We can compare with exit condition; it seems that testing for
> + last iteration is common case. */
> + return NULL_TREE;
> + default:
> + return NULL_TREE;
> + }
> +
> if (dump_file && (dump_flags & TDF_DETAILS))
> {
> fprintf (dump_file, "Found potential split point: ");
> @@ -143,6 +182,7 @@ split_at_bb_p (class loop *loop, basic_block bb, tree
> *border, affine_iv *iv)
> }
>
> *border = iv2.base;
> + *guard_code = code;
> return op0;
> }
>
> @@ -566,8 +606,9 @@ split_loop (class loop *loop1)
> }
>
> /* Find a splitting opportunity. */
> + enum tree_code guard_code;
> for (i = 0; i < loop1->num_nodes; i++)
> - if ((guard_iv = split_at_bb_p (loop1, bbs[i], &border, &iv)))
> + if ((guard_iv = split_at_bb_p (loop1, bbs[i], &border, &iv,
> &guard_code)))
> {
> profile_count entry_count = loop_preheader_edge (loop1)->count ();
> /* Handling opposite steps is not implemented yet. Neither
> @@ -585,7 +626,6 @@ split_loop (class loop *loop1)
> gcond *guard_stmt = as_a<gcond *> (*gsi_last_bb (bbs[i]));
> tree guard_init = PHI_ARG_DEF_FROM_EDGE (phi,
> loop_preheader_edge (loop1));
> - enum tree_code guard_code = gimple_cond_code (guard_stmt);
>
> /* Loop splitting is implemented by versioning the loop, placing
> the new loop after the old loop, make the first loop iterate
