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[block_pass] Relink successors after block becomes empty #20909

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[block_pass] Relink successors after block becomes empty #20909

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16 changes: 15 additions & 1 deletion Zend/Optimizer/block_pass.c
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Original file line number Diff line number Diff line change
Expand Up @@ -546,7 +546,7 @@ static void zend_optimize_block(zend_basic_block *block, zend_op_array *op_array
}
}
break;

case ZEND_BOOL:
case ZEND_BOOL_NOT:
optimize_bool:
Expand Down Expand Up @@ -1800,6 +1800,7 @@ void zend_optimize_cfg(zend_op_array *op_array, zend_optimizer_ctx *ctx)
}

/* Eliminate NOPs */
bool recompute_successors = false;
for (b = blocks; b < end; b++) {
if (b->flags & ZEND_BB_UNREACHABLE_FREE) {
/* In unreachable_free blocks only preserve loop var frees. */
Expand All @@ -1813,6 +1814,19 @@ void zend_optimize_cfg(zend_op_array *op_array, zend_optimizer_ctx *ctx)
if (b->flags & (ZEND_BB_REACHABLE|ZEND_BB_UNREACHABLE_FREE)) {
strip_nops(op_array, b);
}
if (b->len == 0 && (b->flags & ZEND_BB_TARGET)) {
recompute_successors = true;
}
}
if (recompute_successors) {
for (b = blocks; b < end; b++) {
for (int s = 0; s < b->successors_count; s++) {
get_target_block(&cfg, b, s, &opt_count);
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@arnaud-lb arnaud-lb Jan 12, 2026

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It's possible that get_target_block() will update successors to a non-target block. I'm wondering if this could become an issue. Should we propagate the ZEND_BB_TARGET flag to the new successor?

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@iluuu1994 iluuu1994 Jan 12, 2026 •
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Oh, good catch. get_target_block() should probably take care of that. Edit: Ah, I misunderstood. Yeah you're correct, this needs fixing.

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@dstogov dstogov Jan 12, 2026

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Note, that your previous fix relays on ZEND_BB_TARGET flag, that may be not accurately set for empty blocks and their followers. Updating it lazily in get_target_block() won't work for all cases.

}
if (b->len == 0 && (b->flags & ZEND_BB_TARGET)) {
b->flags &= ~ZEND_BB_TARGET;
}
}
}

opt_count = 0;
Expand Down
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