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Take pi defs into account when propagating defs

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Previously pi placement happened after initial phi placement.
Afterwards a second phi placement pass was performed, however it
incorrectly only placed phis on the dominance frontier, rather
than the iterated dominance frontier.

This is fixed by moving pi placement before the propagation of
defs on the iterated DFs, and adding a def for each added pi.

While this ensures that we generate correct conservative SSA, there
is still one remaining case in which we may generate non-minimal
SSA form. Consider:

   |1|
    |pi
    v
   |2|<--\
    |    |
    \----/

The pi is semanically located along the edge 1->2, however we place
it (and its def point) in 2, thus leading to the generation of an
additional (trivial) phi in 2.

Conflicts:
	ext/opcache/Optimizer/zend_ssa.c
This commit is contained in:
Nikita Popov
2016-02-02 17:18:19 +01:00
parent 721be3e0c1
commit 434e0fb3a5
1 changed files with 17 additions and 64 deletions
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ext/opcache/Optimizer/zend_ssa.c
+17 -64
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@@ -90,6 +90,11 @@ static zend_ssa_phi *add_pi(
phi->next = ssa->blocks[to].phis;
ssa->blocks[to].phis = phi;
/* Block "to" now defines "var" via the pi statement, so add it to the "def" set. Note that
* this is not entirely accurate, because the pi is actually placed along the edge from->to.
* If there is a back-edge to "to" this may result in non-minimal SSA form. */
DFG_SET(dfg->def, dfg->size, to, var);
return phi;
}
/* }}} */
@@ -830,6 +835,10 @@ int zend_build_ssa(zend_arena **arena, const zend_op_array *op_array, uint32_t b
def = dfg.def;
in = dfg.in;
/* Place e-SSA pis. This will add additional "def" points, so it must
* happen before def propagation. */
place_essa_pis(arena, op_array, build_flags, ssa, &dfg);
/* SSA construction, Step 1: Propagate "def" sets in merge points */
do {
changed = 0;
@@ -897,73 +906,17 @@ int zend_build_ssa(zend_arena **arena, const zend_op_array *op_array, uint32_t b
phi->pi = -1;
phi->var = i;
phi->ssa_var = -1;
phi->next = ssa_blocks[j].phis;
ssa_blocks[j].phis = phi;
} ZEND_BITSET_FOREACH_END();
}
}
}
place_essa_pis(arena, op_array, build_flags, ssa, &dfg);
/* SSA construction, Step ?: Phi after Pi placement based on Dominance Frontiers */
for (j = 0; j < blocks_count; j++) {
if ((blocks[j].flags & ZEND_BB_REACHABLE) == 0) {
continue;
}
if (blocks[j].predecessors_count > 1) {
zend_bitset_clear(tmp, set_size);
if (blocks[j].flags & ZEND_BB_IRREDUCIBLE_LOOP) {
/* Prevent any values from flowing into irreducible loops by
replacing all incoming values with explicit phis. The
register allocator depends on this property. */
zend_bitset_copy(tmp, in + (j * set_size), set_size);
} else {
for (k = 0; k < blocks[j].predecessors_count; k++) {
i = ssa->cfg.predecessors[blocks[j].predecessor_offset + k];
while (i != -1 && i != blocks[j].idom) {
zend_ssa_phi *p = ssa_blocks[i].phis;
while (p) {
if (p) {
if (p->pi >= 0) {
if (zend_bitset_in(in + (j * set_size), p->var) &&
!zend_bitset_in(def + (i * set_size), p->var)) {
zend_bitset_incl(tmp, p->var);
}
} else {
zend_bitset_excl(tmp, p->var);
}
/* Place phis after pis */
{
zend_ssa_phi **pp = &ssa_blocks[j].phis;
while (*pp) {
if ((*pp)->pi < 0) {
break;
}
p = p->next;
pp = &(*pp)->next;
}
i = blocks[i].idom;
}
}
}
if (!zend_bitset_empty(tmp, set_size)) {
ZEND_BITSET_REVERSE_FOREACH(tmp, set_size, i) {
zend_ssa_phi **pp = &ssa_blocks[j].phis;
while (*pp) {
if ((*pp)->pi < 0 && (*pp)->var == i) {
break;
}
pp = &(*pp)->next;
}
if (*pp == NULL) {
zend_ssa_phi *phi = zend_arena_calloc(arena, 1,
sizeof(zend_ssa_phi) +
sizeof(int) * blocks[j].predecessors_count +
sizeof(void*) * blocks[j].predecessors_count);
phi->sources = (int*)(((char*)phi) + sizeof(zend_ssa_phi));
memset(phi->sources, 0xff, sizeof(int) * blocks[j].predecessors_count);
phi->use_chains = (zend_ssa_phi**)(((char*)phi->sources) + sizeof(int) * ssa->cfg.blocks[j].predecessors_count);
phi->pi = -1;
phi->var = i;
phi->ssa_var = -1;
phi->next = NULL;
phi->next = *pp;
*pp = phi;
}
} ZEND_BITSET_FOREACH_END();