refactor(lora): split the activation ladder into atomic mechanisms

The per-module walk carried four mechanisms inline, each repeating the
same tail: count the layer, stamp the pair that marks it current, advance
the bar, continue. Five copies of that tail and three of the backup probe
put the deepest arm nine levels in.

Each mechanism is now a function that either takes the layer or declines
to the next, and the walk reads as the four of them in order. The pass
state they share moves onto one object built before the walk starts, with
the accept tail, the stamp and the bar tick as its methods. That takes
network_activate from 218 lines to 55, none of it deeper than the module
loop.

Two shapes are deliberately not folded into that tail: the weight path
counts weights and bias separately and tracks what the module refused,
and the factor-strip restore stamps without counting. Hosting hands a
declined delta back rather than leaving it in a flag, so a pair of Nones
still reads as assembled and no layer is calculated twice.
This commit is contained in:
CalamitousFelicitousness
2026-08-30 04:55:16 +01:00
parent 65e49a323c
commit 3f86973bc6
2 changed files with 225 additions and 146 deletions
+188 -145
View File
@@ -19,6 +19,64 @@ default_components = ['text_encoder', 'text_encoder_2', 'text_encoder_3', 'text_
deactivate_components = ['text_encoder', 'text_encoder_2', 'text_encoder_3', 'unet', 'transformer', 'llm_adapter']
class ActivationPass:
"""State of one activation walk, built before the walk so a pass that raises still has it.
`wanted_names` is built once here and reaches every layer as
`component_wanted`, either this tuple or the empty one. The factor cache
keys its pass entry on that object's identity, so an equal tuple rebuilt
per component would send every lookup back to disk.
"""
def __init__(self, fuse):
self.sd_model = getattr(shared.sd_model, "pipe", shared.sd_model)
self.fuse = fuse
self.elimit = None # the error limiter, bound for the duration of the walk
self.wanted_names = tuple((x.name, x.te_multiplier, x.unet_multiplier, x.dyn_dim) for x in l.loaded_networks) if len(l.loaded_networks) > 0 else ()
self.stack_sig = lora_stack.signature() + lora_blocks.signature() + lora_sdnq.signature() # tracked beside network_current_names so stack-setting, block-weight and mechanism changes re-apply
self.select_active = len(l.loaded_networks) > 0 and lora_stack.active_select(len(l.loaded_networks)) # restore-only walks have nothing to stack; the count warning would fire on every network-free generation
self.component_wanted = ()
self.device = None
self.group_offload = shared.opts.diffusers_offload_mode == "group"
self.group_stripped = {}
self.pbar = nullcontext()
self.task = None
self.total = 0
self.active_components = []
self.applied_weight = 0
self.applied_bias = 0
self.refused = 0
self.backup_size = 0
def stamp(self, module):
"""Mark the layer as carrying this set under these settings; the pair is the skip key."""
module.network_current_names = self.component_wanted
module.network_current_stack = self.stack_sig
def tick(self, description=None):
if self.task is None:
return
if description is None:
self.pbar.update(self.task, advance=1)
else:
self.pbar.update(self.task, advance=1, description=description)
def claim(self, module, network_layer_name, changed):
"""Accept a layer one of the mechanisms took; only a layer whose weights changed counts as applied."""
if changed and self.component_wanted:
applied_layers.append(network_layer_name)
self.applied_weight += 1
self.stamp(module)
self.tick()
def keep_selected(self, module, network_layer_name, sel_backup):
"""Hold a scheduled weight-kind layer on its pristine tensor until the schedule applies the winner."""
self.backup_size += sel_backup # counted only where this branch keeps the layer; the weight path below re-enters the shared backup call, which counts it then
network_apply_weights(module, None, None, device=self.device)
self.claim(module, network_layer_name, True)
return True
def group_will_mutate(module, network_layer_name: str, loaded) -> bool:
"""True when the pass will write to this module: a loaded network covers its layer, a
tensor backup awaits restore, or an svd factor stash awaits removal."""
@@ -116,6 +174,108 @@ def should_skip(module, network_layer_name, wanted, stack_sig):
return getattr(module, 'network_current_names', ()) == wanted and getattr(module, 'network_current_stack', 'sum') == stack_sig
def try_select(ctx, module, network_layer_name):
"""Put the layer under a selection schedule; True when it took the layer.
The three arms are mutually exclusive and their warnings are keyed, so a
layer that cannot be scheduled reports one reason and falls through.
"""
if not ctx.select_active or not ctx.component_wanted or network_layer_name.startswith('lora_te'):
return False
if lora_sdnq.select_candidate(module, network_layer_name, ctx.component_wanted): # SDNQ pairs ride the channel as separate segments at any bit width; weight rewrites cannot flip a quantized layer
restore_pristine(module, ctx.device)
applied = lora_sdnq.apply_select_cached(module, network_layer_name, ctx.component_wanted) # a stored score record and factor pair serve before the deltas are assembled
if applied is None:
per_net, sel_bias = network_calc_weights(module, network_layer_name, elimit=ctx.elimit, per_net=True)
if sel_bias is None:
applied = lora_sdnq.apply_select(module, network_layer_name, per_net, ctx.component_wanted)
if applied is not None:
ctx.claim(module, network_layer_name, applied)
return True
lora_stack.warn_once('select-unridable', f'Network stack: mode={lora_stack.mode()} layer="{network_layer_name}" fallback=sum') # a pair the channel cannot carry (bias delta or malformed member) sums like any unsupported set
elif getattr(module, 'sdnq_dequantizer', None) is not None: # hosting disabled: quantized layers have no side-channel to carry segments and packed backups cannot flip, so the sum paths below take the layer
if any(net.modules.get(network_layer_name, None) is not None for net in l.loaded_networks):
lora_stack.warn_once('select-host-disabled', f'Network stack: mode={lora_stack.mode()} quant=sdnq host=disabled fallback=sum')
else: # other layers select by recomputing the winner from the pristine backup at schedule time
sel_backup = network_backup_weights(module, network_layer_name, ctx.component_wanted, ctx.fuse)
if tensor_backup(module) is not None: # a flip recomputes the winner from the pristine tensor, which fuse mode does not keep
if lora_stack.register_weight_pair_cached(network_layer_name, module, ctx.component_wanted): # a stored score record registers without assembling the pair
return ctx.keep_selected(module, network_layer_name, sel_backup)
per_net, sel_bias = network_calc_weights(module, network_layer_name, elimit=ctx.elimit, per_net=True)
if sel_bias is None and lora_stack.register_weight_pair(network_layer_name, module, per_net, ctx.component_wanted):
return ctx.keep_selected(module, network_layer_name, sel_backup)
return False
def try_factors(ctx, module, network_layer_name):
"""Attach the set to the quantized side channel as exact factors; True when it took the layer."""
if not lora_sdnq.factor_candidate(module, network_layer_name, ctx.component_wanted):
return False
restore_pristine(module, ctx.device) # an earlier non-factorable set may have requantized this layer
applied = lora_sdnq.apply_factors(module, network_layer_name, ctx.component_wanted)
if applied is None: # the exact path declined; hosting or the weight path takes the layer
return False
ctx.claim(module, network_layer_name, applied)
return True
def try_hosted(ctx, module, network_layer_name):
"""Host the combined delta on the side channel as truncated factors.
Returns whether it took the layer and, when it declined after assembling
the delta, that delta, so the weight path applies it without a second
calc. A returned pair of Nones still counts as assembled.
"""
if not lora_sdnq.host_candidate(module, network_layer_name, ctx.component_wanted):
return False, None
restore_pristine(module, ctx.device) # the hosted delta is measured against the pristine base
batch = None
hosted = lora_sdnq.apply_cached(module, network_layer_name, ctx.component_wanted) # a stored entry serves the layer before the delta is assembled
if hosted is None:
batch_updown, batch_ex_bias = network_calc_weights(module, network_layer_name, elimit=ctx.elimit)
batch = (batch_updown, batch_ex_bias)
if batch_ex_bias is None: # bias deltas need the plain path; weight-only sets ride the side-channel without a weight backup
hosted = lora_sdnq.apply_hosted(module, network_layer_name, batch_updown, ctx.component_wanted)
if hosted is not None:
batch = None # hosting took the delta
if hosted is None:
return False, batch
ctx.claim(module, network_layer_name, hosted)
return True, None
def apply_generic(ctx, module, network_layer_name, batch):
"""The weight path, which takes any layer the mechanisms above declined."""
stripped = lora_sdnq.remove_factors(module) # the mechanism gate can decline a layer still carrying attached factors; the weight path must start from the pristine channel
if stripped and not ctx.component_wanted: # factor-mode layers have no tensor backup, dropping the factors is the whole restore
ctx.stamp(module)
ctx.tick()
return
ctx.backup_size += network_backup_weights(module, network_layer_name, ctx.component_wanted, ctx.fuse)
if not ctx.component_wanted:
lora_stack.drop(network_layer_name) # a restored layer must leave the selection schedule
if tensor_backup(module) is None: # fuse mode has no tensor backup, restore stays with network_deactivate
ctx.tick()
return
batch_updown, batch_ex_bias = None, None # restore-only pass, apply with no weights reverts to backup
else:
batch_updown, batch_ex_bias = batch if batch is not None else network_calc_weights(module, network_layer_name, elimit=ctx.elimit)
if batch_updown is not None:
lora_sdnq.note_fallback(module, network_layer_name) # only layers whose quantized weight actually takes a delta
if ctx.fuse:
weight_written, bias_written = network_apply_direct(module, batch_updown, batch_ex_bias, device=ctx.device)
else:
weight_written, bias_written = network_apply_weights(module, batch_updown, batch_ex_bias, device=ctx.device)
if batch_updown is not None or batch_ex_bias is not None:
applied_layers.append(network_layer_name)
ctx.applied_weight += 1 if weight_written else 0
ctx.applied_bias += 1 if bias_written else 0
ctx.refused += (batch_updown is not None and not weight_written) + (batch_ex_bias is not None and not bias_written) # a delta the module would not take leaves that layer on its base value
ctx.stamp(module)
bs = round(ctx.backup_size/1024/1024/1024, 2) if ctx.backup_size > 0 else None
ctx.tick(f'networks={len(l.loaded_networks)} modules={ctx.active_components} layers={ctx.total} weights={ctx.applied_weight} bias={ctx.applied_bias} backup={bs} device={ctx.device}')
def network_activate(include=None, exclude=None):
if exclude is None:
exclude = []
@@ -123,169 +283,52 @@ def network_activate(include=None, exclude=None):
include = []
promote_pending()
t0 = time.time()
fuse = lora_overrides.fuse_native() # resolve once: backup and apply passes must agree
ctx = ActivationPass(lora_overrides.fuse_native()) # fuse resolved once: the backup, apply and restore paths must agree
with limit_errors("network_activate") as elimit:
sd_model = prepare_model_for_write(getattr(shared.sd_model, "pipe", shared.sd_model))
group_offload = shared.opts.diffusers_offload_mode == "group"
group_stripped = {}
device = None
modules, components, active_components, total = collect_components(sd_model, include, exclude, default_components, restore_filtered=True)
pbar, task = pass_progress('activate', total, len(l.loaded_networks) > 0)
applied_weight = 0
applied_bias = 0
refused = 0
with devices.inference_context(), pbar:
wanted_names = tuple((x.name, x.te_multiplier, x.unet_multiplier, x.dyn_dim) for x in l.loaded_networks) if len(l.loaded_networks) > 0 else ()
stack_sig = lora_stack.signature() + lora_blocks.signature() + lora_sdnq.signature() # tracked beside network_current_names so stack-setting, block-weight and mechanism changes re-apply
select_active = len(l.loaded_networks) > 0 and lora_stack.active_select(len(l.loaded_networks)) # restore-only walks have nothing to stack; the count warning would fire on every network-free generation
ctx.elimit = elimit
ctx.sd_model = prepare_model_for_write(ctx.sd_model)
modules, components, ctx.active_components, ctx.total = collect_components(ctx.sd_model, include, exclude, default_components, restore_filtered=True)
ctx.pbar, ctx.task = pass_progress('activate', ctx.total, len(l.loaded_networks) > 0)
with devices.inference_context(), ctx.pbar:
applied_layers.clear()
lora_sdnq.fallback_layers.clear() # a raise mid-pass leaves stale entries behind
lora_sdnq.hosted_layers.clear()
lora_sdnq.factor_layers.clear()
lora_sdnq.select_layers.clear()
backup_size = 0
for component in modules.keys():
component_wanted = wanted_names if component in components else ()
device = getattr(sd_model, component, None).device
ctx.component_wanted = ctx.wanted_names if component in components else () # the pass tuple itself, never a copy
ctx.device = getattr(ctx.sd_model, component, None).device
for _, module in modules[component]:
network_layer_name = getattr(module, 'network_layer_name', None)
if should_skip(module, network_layer_name, component_wanted, stack_sig):
if task is not None:
pbar.update(task, advance=1)
if should_skip(module, network_layer_name, ctx.component_wanted, ctx.stack_sig):
ctx.tick()
continue
lora_stack.drop(network_layer_name) # re-application invalidates any live selection schedule; the select branch re-registers
if group_offload and component not in group_stripped and group_will_mutate(module, network_layer_name, l.loaded_networks):
device = group_offload_strip(sd_model, component, group_stripped)
calced = False # tracks whether this iteration assembled the delta, so the fallthrough reuses it instead of recomputing
if select_active and component_wanted and not network_layer_name.startswith('lora_te'):
if lora_sdnq.select_candidate(module, network_layer_name, component_wanted): # SDNQ pairs ride the channel as separate segments at any bit width; weight rewrites cannot flip a quantized layer
restore_pristine(module, device)
applied = lora_sdnq.apply_select_cached(module, network_layer_name, component_wanted) # a stored score record and factor pair serve before the deltas are assembled
if applied is None:
per_net, sel_bias = network_calc_weights(module, network_layer_name, elimit=elimit, per_net=True)
if sel_bias is None:
applied = lora_sdnq.apply_select(module, network_layer_name, per_net, component_wanted)
if applied is not None:
if applied and component_wanted:
applied_layers.append(network_layer_name)
applied_weight += 1
module.network_current_names = component_wanted
module.network_current_stack = stack_sig
if task is not None:
pbar.update(task, advance=1)
continue
lora_stack.warn_once('select-unridable', f'Network stack: mode={lora_stack.mode()} layer="{network_layer_name}" fallback=sum') # a pair the channel cannot carry (bias delta or malformed member) sums like any unsupported set
elif getattr(module, 'sdnq_dequantizer', None) is not None: # hosting disabled: quantized layers have no side-channel to carry segments and packed backups cannot flip, so the sum paths below take the layer
if any(net.modules.get(network_layer_name, None) is not None for net in l.loaded_networks):
lora_stack.warn_once('select-host-disabled', f'Network stack: mode={lora_stack.mode()} quant=sdnq host=disabled fallback=sum')
else: # other layers select by recomputing the winner from the pristine backup at schedule time
sel_backup = network_backup_weights(module, network_layer_name, component_wanted, fuse)
if tensor_backup(module) is not None: # a flip recomputes the winner from the pristine tensor, which fuse mode does not keep
if lora_stack.register_weight_pair_cached(network_layer_name, module, component_wanted): # a stored score record registers without assembling the pair
backup_size += sel_backup
network_apply_weights(module, None, None, device=device) # pristine until the schedule applies the winner
applied_layers.append(network_layer_name)
applied_weight += 1
module.network_current_names = component_wanted
module.network_current_stack = stack_sig
if task is not None:
pbar.update(task, advance=1)
continue
per_net, sel_bias = network_calc_weights(module, network_layer_name, elimit=elimit, per_net=True)
if sel_bias is None and lora_stack.register_weight_pair(network_layer_name, module, per_net, component_wanted):
backup_size += sel_backup # counted only when this branch keeps the layer; the fallthrough re-enters the shared backup call below, which counts it then
network_apply_weights(module, None, None, device=device) # pristine until the schedule applies the winner
applied_layers.append(network_layer_name)
applied_weight += 1
module.network_current_names = component_wanted
module.network_current_stack = stack_sig
if task is not None:
pbar.update(task, advance=1)
continue
if lora_sdnq.factor_candidate(module, network_layer_name, component_wanted):
restore_pristine(module, device) # an earlier non-factorable set may have requantized this layer
applied = lora_sdnq.apply_factors(module, network_layer_name, component_wanted)
if applied is not None: # exact path took the layer; None falls through to hosting or requantize
if applied and component_wanted:
applied_layers.append(network_layer_name)
applied_weight += 1
module.network_current_names = component_wanted
module.network_current_stack = stack_sig
if task is not None:
pbar.update(task, advance=1)
continue
if lora_sdnq.host_candidate(module, network_layer_name, component_wanted):
restore_pristine(module, device) # the hosted delta is measured against the pristine base
hosted = lora_sdnq.apply_cached(module, network_layer_name, component_wanted) # a stored entry serves the layer before the delta is assembled
if hosted is None:
batch_updown, batch_ex_bias = network_calc_weights(module, network_layer_name, elimit=elimit)
calced = True
if batch_ex_bias is None: # bias deltas need the plain path; weight-only sets ride the side-channel without a weight backup
hosted = lora_sdnq.apply_hosted(module, network_layer_name, batch_updown, component_wanted)
if hosted is not None:
batch_updown, batch_ex_bias = None, None
del batch_updown, batch_ex_bias
if hosted is not None:
if hosted and component_wanted:
applied_layers.append(network_layer_name)
applied_weight += 1
module.network_current_names = component_wanted
module.network_current_stack = stack_sig
if task is not None:
pbar.update(task, advance=1)
continue
stripped = lora_sdnq.remove_factors(module) # the mechanism gate can decline a layer still carrying attached factors; the weight path must start from the pristine channel
if stripped and not component_wanted: # factor-mode layers have no tensor backup, dropping the factors is the whole restore
module.network_current_names = ()
module.network_current_stack = stack_sig
if task is not None:
pbar.update(task, advance=1)
if ctx.group_offload and component not in ctx.group_stripped and group_will_mutate(module, network_layer_name, l.loaded_networks):
ctx.device = group_offload_strip(ctx.sd_model, component, ctx.group_stripped)
if try_select(ctx, module, network_layer_name):
continue
backup_size += network_backup_weights(module, network_layer_name, component_wanted, fuse)
if not component_wanted:
lora_stack.drop(network_layer_name) # a restored layer must leave the selection schedule
if tensor_backup(module) is None: # fuse mode has no tensor backup, restore stays with network_deactivate
if task is not None:
pbar.update(task, advance=1)
continue
batch_updown, batch_ex_bias = None, None # restore-only pass, apply with no weights reverts to backup
else:
if not calced: # the host branch may have assembled the delta already; a declined layer reuses it
batch_updown, batch_ex_bias = network_calc_weights(module, network_layer_name, elimit=elimit)
if batch_updown is not None:
lora_sdnq.note_fallback(module, network_layer_name) # only layers whose quantized weight actually takes a delta
if fuse:
weight_written, bias_written = network_apply_direct(module, batch_updown, batch_ex_bias, device=device)
else:
weight_written, bias_written = network_apply_weights(module, batch_updown, batch_ex_bias, device=device)
if batch_updown is not None or batch_ex_bias is not None:
applied_layers.append(network_layer_name)
applied_weight += 1 if weight_written else 0
applied_bias += 1 if bias_written else 0
refused += (batch_updown is not None and not weight_written) + (batch_ex_bias is not None and not bias_written) # a delta the module would not take leaves that layer on its base value
batch_updown, batch_ex_bias = None, None
del batch_updown, batch_ex_bias
module.network_current_names = component_wanted
module.network_current_stack = stack_sig
if task is not None:
bs = round(backup_size/1024/1024/1024, 2) if backup_size > 0 else None
pbar.update(task, advance=1, description=f'networks={len(l.loaded_networks)} modules={active_components} layers={total} weights={applied_weight} bias={applied_bias} backup={bs} device={device}')
if task is not None and len(applied_layers) == 0:
pbar.remove_task(task) # hide progress bar for no action
if try_factors(ctx, module, network_layer_name):
continue
hosted, batch = try_hosted(ctx, module, network_layer_name)
if hosted:
continue
apply_generic(ctx, module, network_layer_name, batch)
if ctx.task is not None and len(applied_layers) == 0:
ctx.pbar.remove_task(ctx.task) # hide progress bar for no action
global native_active, refused_writes # pylint: disable=global-statement
lora_sdnq.report_fallbacks()
native_active = len(l.loaded_networks) > 0
refused_writes = refused
l.last_backup_size = backup_size
refused_writes = ctx.refused
l.last_backup_size = ctx.backup_size
l.timer.activate += time.time() - t0
if refused > 0:
log.error(f'Network load: type=LoRA networks={[n.name for n in l.loaded_networks]} weights={applied_weight} bias={applied_bias} refused={refused} network partially applied')
if ctx.refused > 0:
log.error(f'Network load: type=LoRA networks={[n.name for n in l.loaded_networks]} weights={ctx.applied_weight} bias={ctx.applied_bias} refused={ctx.refused} network partially applied')
if l.debug and len(l.loaded_networks) > 0:
log.debug(f'Network load: type=LoRA networks={[n.name for n in l.loaded_networks]} modules={active_components} layers={total} weights={applied_weight} bias={applied_bias} refused={refused} backup={round(backup_size/1024/1024/1024, 2)} fuse={fuse}:{shared.opts.lora_fuse_diffusers} device={device} time={l.timer.summary}')
log.debug(f'Network load: type=LoRA networks={[n.name for n in l.loaded_networks]} modules={ctx.active_components} layers={ctx.total} weights={ctx.applied_weight} bias={ctx.applied_bias} refused={ctx.refused} backup={round(ctx.backup_size/1024/1024/1024, 2)} fuse={ctx.fuse}:{shared.opts.lora_fuse_diffusers} device={ctx.device} time={l.timer.summary}')
modules.clear()
if len(applied_layers) > 0 or shared.opts.diffusers_offload_mode == "sequential" or len(group_stripped) > 0:
sd_models.set_diffuser_offload(sd_model, op="model")
if len(applied_layers) > 0 or shared.opts.diffusers_offload_mode == "sequential" or len(ctx.group_stripped) > 0:
sd_models.set_diffuser_offload(ctx.sd_model, op="model")
def effective_mode():
+37 -1
View File
@@ -900,6 +900,41 @@ def test_route_fat_dense_delta_requantizes():
return True
def test_declined_host_delta_is_not_recomputed():
layer = build_layer('uint4')
torch.manual_seed(21)
D = torch.randn(OUT_F, IN_F, device=DEVICE) * 1e-2 # fat and full-rank: hosting assembles the delta and then routes it to the grid
net = make_dense_net('recompute', layer, D)
with host_rank(256), mock_model(lin=layer), counting_calc() as calls:
activate(net)
assert not hasattr(layer, 'sdnq_lora_svd_stash'), 'the fixture must reach the weight path, not the side channel'
assert calls['n'] == 1, f'a declined host hands its delta on instead of assembling it twice, got {calls["n"]}'
return True
def test_pass_presents_one_wanted_names_tuple():
from modules.lora import lora_factor_cache as fc
layer = build_layer('uint4')
_A, _B, D = make_delta(sigma=3e-3)
net = make_dense_net('identity', layer, D)
seen = [] # holds the objects, so a freed tuple cannot lend its address to the next one
real = fc.begin_pass
def recording(wanted_names):
seen.append(wanted_names)
return real(wanted_names)
fc.begin_pass = recording
try:
with host_rank(64), mock_model(lin=layer):
activate(net)
finally:
fc.begin_pass = real
assert len(seen) >= 2, f'the hosted path must consult the cache more than once for this to prove anything, got {len(seen)}'
assert all(x is seen[0] for x in seen), 'one walk must present one tuple: the cache memoizes its entry on identity, and an equal rebuild rereads it from disk'
return True
def test_route_rule_terms_gate_both_ways():
layer = build_layer('uint4')
torch.manual_seed(23)
@@ -2914,7 +2949,8 @@ def run_tests():
log.warning('=== Hosting ===')
for fn in [test_hosted_low_rank_delta_is_kept, test_hosted_dense_delta_beats_requant, test_hosted_skips_int8,
test_hosted_disabled_by_option, test_hosted_transitions_and_rng_isolation,
test_route_fat_dense_delta_requantizes, test_route_rule_terms_gate_both_ways, test_route_low_rank_fat_delta_stays_hosted,
test_route_fat_dense_delta_requantizes, test_declined_host_delta_is_not_recomputed, test_pass_presents_one_wanted_names_tuple,
test_route_rule_terms_gate_both_ways, test_route_low_rank_fat_delta_stays_hosted,
test_route_mixed_set_keeps_hosting, test_route_svd_checkpoint_keeps_hosting, test_route_dense_stack_keeps_hosting,
test_route_replay_from_cache, test_hosted_null_tail_collapses_to_effective_rank, test_hosted_flat_spectrum_keeps_cap]:
run_test(CAT_HOST, fn)