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Avoid changing PHIs in GIMPLE split_edge

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Previously I've changed gimple_split_edge to avoid PHI node
re-allocation, but this introduced swapping of PHI arguments
due to the way edge redirection works.  This is now a problem
for me and which can be solved with the following approach
reducing the overhead of split_edge even more.  We can simply
pretend there are no PHI nodes if we can make sure the
new fallthru will have the same dest_idx as the old edge
into the destination.

2020-10-20  Richard Biener  <rguenther@suse.de>

	* tree-cfg.c (reinstall_phi_args): Remove.
	(gimple_split_edge): Remove PHIs around the edge redirection
	to avoid touching them at all.
This commit is contained in:
Richard Biener 2020-10-20 12:52:31 +02:00
parent 06729598b0
commit 128f43cf67
1 changed files with 17 additions and 33 deletions
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50
gcc/tree-cfg.c
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@ -2888,35 +2888,6 @@ last_and_only_stmt (basic_block bb)
return NULL;
}
/* Reinstall those PHI arguments queued in OLD_EDGE to NEW_EDGE. */
static void
reinstall_phi_args (edge new_edge, edge old_edge)
{
edge_var_map *vm;
int i;
gphi_iterator phis;
vec<edge_var_map> *v = redirect_edge_var_map_vector (old_edge);
if (!v)
return;
for (i = 0, phis = gsi_start_phis (new_edge->dest);
v->iterate (i, &vm) && !gsi_end_p (phis);
i++, gsi_next (&phis))
{
gphi *phi = phis.phi ();
tree result = redirect_edge_var_map_result (vm);
tree arg = redirect_edge_var_map_def (vm);
gcc_assert (result == gimple_phi_result (phi));
add_phi_arg (phi, arg, new_edge, redirect_edge_var_map_location (vm));
}
redirect_edge_var_map_clear (old_edge);
}
/* Returns the basic block after which the new basic block created
by splitting edge EDGE_IN should be placed. Tries to keep the new block
near its "logical" location. This is of most help to humans looking
@ -2956,11 +2927,24 @@ gimple_split_edge (edge edge_in)
new_bb = create_empty_bb (after_bb);
new_bb->count = edge_in->count ();
e = redirect_edge_and_branch (edge_in, new_bb);
gcc_assert (e == edge_in);
/* We want to avoid re-allocating PHIs when we first
add the fallthru edge from new_bb to dest but we also
want to avoid changing PHI argument order when
first redirecting edge_in away from dest. The former
avoids changing PHI argument order by adding them
last and then the redirection swapping it back into
place by means of unordered remove.
So hack around things by temporarily removing all PHIs
from the destination during the edge redirection and then
making sure the edges stay in order. */
gimple_seq saved_phis = phi_nodes (dest);
unsigned old_dest_idx = edge_in->dest_idx;
set_phi_nodes (dest, NULL);
new_edge = make_single_succ_edge (new_bb, dest, EDGE_FALLTHRU);
reinstall_phi_args (new_edge, e);
e = redirect_edge_and_branch (edge_in, new_bb);
gcc_assert (e == edge_in && new_edge->dest_idx == old_dest_idx);
/* set_phi_nodes sets the BB of the PHI nodes, so do it manually here. */
dest->il.gimple.phi_nodes = saved_phis;
return new_bb;
}