Thanks for the comments. As David wrote, the intent of the patch is
not to do a general purpose scheduling, but to compensate for the
possible live range lengthening introduced by reassociation.
On Thu, Oct 11, 2012 at 6:16 AM, Richard Biener
<richard.guent...@gmail.com> wrote:
> On Thu, Oct 11, 2012 at 3:52 AM, Easwaran Raman <era...@google.com> wrote:
>>
>> +/* Move STMT up within its BB until it can not be moved any further. */
>> +
>> +static void move_stmt_upwards (gimple stmt)
>> +{
>> + gimple_stmt_iterator gsi, gsistmt;
>> + tree rhs1, rhs2;
>> + gimple rhs1def = NULL, rhs2def = NULL;
>> + rhs1 = gimple_assign_rhs1 (stmt);
>> + rhs2 = gimple_assign_rhs2 (stmt);
>> + gcc_assert (rhs1);
>
> Please no such senseless asserts. The following line will segfault anyway
> if rhs1 is NULL and with a debugger an assert doesn't add any useful
> information.
Ok.
>
>> + if (TREE_CODE (rhs1) == SSA_NAME)
>> + rhs1def = SSA_NAME_DEF_STMT (rhs1);
>> + else if (TREE_CODE (rhs1) != REAL_CST
>> + && TREE_CODE (rhs1) != INTEGER_CST)
>> + return;
>> + if (rhs2)
>
> You may not access gimple_assign_rhs2 if it is not there. So you have
> to check whether you have an unary, binary or ternary (yes) operation.
gimple_assign_rhs2 returns NULL_TREE if it the RHS of an assignment
has less than two operands. Regarding the check for ternary
operation, I believe it is not necessary. A statement is considered
for reassociation only if the RHS class is GIMPLE_BINARY_RHS.
Subsequently, for rhs1 and rhs2, it checks if the def statements also
have the same code and so it seems to me that a statement with a
ternary operator in the RHS will never be considered in
rewrite_expr_tree.
>
>> + {
>> + if (TREE_CODE (rhs2) == SSA_NAME)
>> + rhs2def = SSA_NAME_DEF_STMT (rhs2);
>> + else if (TREE_CODE (rhs1) != REAL_CST
>> + && TREE_CODE (rhs1) != INTEGER_CST)
>> + return;
>> + }
>> + gsi = gsi_for_stmt (stmt);
>> + gsistmt = gsi;
>> + gsi_prev (&gsi);
>> + for (; !gsi_end_p (gsi); gsi_prev (&gsi))
>
> ????
>
> This doesn't make much sense. You can move a stmt to the nearest
> common post-dominator. Assuming you only want to handle placing
> it after rhs1def or after rhs2def(?) you don't need any loop, just
> two dominator queries and an insertion after one of the definition
> stmts.
Within a BB isn't that still O(size of BB)?
> But this code should consider BBs.
For reassociation to look across BBs, the code should look something like this:
L1 :
a_2 = a_1 + 10
jc L3
L2:
a_3 = a_2 + 20
- L1 should dominate L2 (otherwise there will be a phi node at L2 and
the reassociation of a_3 will not consider the definition of a_2)
- There are no other uses of a_2 other than the one in L3.
After reassociation, the stmt defining a_2 would be moved to L2. In
that case, the downward code motion of a_2 = a_1 + 10 to L2 is
beneficial (one less instruction if the branch is taken). It is not
obvious to me that moving it to L1 (or whereever a_1 is defined) is
beneficial.
> And I don't see why more optimal
> placement cannot be done during rewrite_expr_tree itself.
I started with that idea, but my current approach looks more simpler.
Thanks,
Easwaran
>
> NB, the whole reassoc code needs a re-write to avoid the excessive
> stmt modifications when it does nothing. So I'd very much rather avoid
> adding anything to reassoc until that rewrite happened.
>
> Richard.
>
>> + {
>> + gimple curr_stmt = gsi_stmt (gsi);
>> + if (curr_stmt == rhs1def || curr_stmt == rhs2def) {
>> + gsi_move_after (&gsistmt, &gsi);
>> + return;
>> + }
>> + }
>> +
>> +}
>> +
>> /* Recursively rewrite our linearized statements so that the operators
>> match those in OPS[OPINDEX], putting the computation in rank
>> order. */
>>
>> static void
>> rewrite_expr_tree (gimple stmt, unsigned int opindex,
>> - VEC(operand_entry_t, heap) * ops, bool moved)
>> + VEC(operand_entry_t, heap) * ops, bool moved,
>> + VEC(gimple, heap) **stmts_to_move)
>> {
>> tree rhs1 = gimple_assign_rhs1 (stmt);
>> tree rhs2 = gimple_assign_rhs2 (stmt);
>> @@ -2299,6 +2337,7 @@ rewrite_expr_tree (gimple stmt, unsigned int opind
>> print_gimple_stmt (dump_file, stmt, 0, 0);
>> }
>> }
>> + VEC_safe_push (gimple, heap, *stmts_to_move, stmt);
>> return;
>> }
>>
>> @@ -2346,7 +2385,9 @@ rewrite_expr_tree (gimple stmt, unsigned int opind
>> }
>> /* Recurse on the LHS of the binary operator, which is guaranteed to
>> be the non-leaf side. */
>> - rewrite_expr_tree (SSA_NAME_DEF_STMT (rhs1), opindex + 1, ops, moved);
>> + rewrite_expr_tree (SSA_NAME_DEF_STMT (rhs1), opindex + 1, ops, moved,
>> + stmts_to_move);
>> + VEC_safe_push (gimple, heap, *stmts_to_move, stmt);
>> }
>>
>> /* Find out how many cycles we need to compute statements chain.
>> @@ -3427,6 +3468,9 @@ reassociate_bb (basic_block bb)
>> {
>> gimple_stmt_iterator gsi;
>> basic_block son;
>> + VEC(gimple, heap) *stmts_to_move = NULL;
>> + gimple stmt;
>> + int i;
>>
>> for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi_prev (&gsi))
>> {
>> @@ -3542,7 +3586,7 @@ reassociate_bb (basic_block bb)
>> && VEC_length (operand_entry_t, ops) > 3)
>> rewrite_expr_tree_parallel (stmt, width, ops);
>> else
>> - rewrite_expr_tree (stmt, 0, ops, false);
>> + rewrite_expr_tree (stmt, 0, ops, false, &stmts_to_move);
>>
>> /* If we combined some repeated factors into a
>> __builtin_powi call, multiply that result by the
>> @@ -3560,6 +3604,7 @@ reassociate_bb (basic_block bb)
>> target_ssa);
>> gimple_set_location (mul_stmt, gimple_location (stmt));
>> gsi_insert_after (&gsi, mul_stmt, GSI_NEW_STMT);
>> + VEC_safe_push (gimple, heap, stmts_to_move, mul_stmt);
>> }
>> }
>>
>> @@ -3567,6 +3612,11 @@ reassociate_bb (basic_block bb)
>> }
>> }
>> }
>> +
>> + FOR_EACH_VEC_ELT (gimple, stmts_to_move, i, stmt)
>> + move_stmt_upwards (stmt);
>> + VEC_free (gimple, heap, stmts_to_move);
>> +
>> for (son = first_dom_son (CDI_POST_DOMINATORS, bb);
>> son;
>> son = next_dom_son (CDI_POST_DOMINATORS, son))
