Kena: Bridge of Spirits (PPSA01802) refuses ten pixel shaders with
packed pixel ancillary input has an unsupported live use (used by SelectU32 ? ?)
at any visit limit. All ten have 30 to 33 phis on loop back edges (reported
on Linux with an RX 9070 XT).
Two rules stopped the proof at a loop, and the first one fires before the
second is ever asked:
- The walk from the select gave up at the first use inside a loop. A value
derived from the select reaches a loop phi as soon as its register is
written inside the loop, because the masked write becomes
Select(loopExec, new, phi). A loop that leaves exec alone needs nothing
more than this.
- An exec that a loop narrows and carries around its back edge is a phi
inside the loop, and such phis counted as not implying the write's exec.
The walk now follows uses into loops. A phi inside a loop implies the
write's exec when all of its incoming values do, where the phi itself counts
as implying it while its back-edge values are checked. This is sound because
each rule of the proof takes a lane bit of one value from a value computed
earlier: a phi's value on each iteration is the incoming value computed
before the edge was taken, a set lane of an And is set in both operands, a
set lane of a ballot is a lane whose predicate held. If any value in the
proof had a lane set where the write's exec is clear, the earliest such
value would take that lane from an earlier value of the proof that is set
there too, which contradicts it being the earliest. Two conditions keep this
true:
- The write's exec must be computed once, outside every loop. Otherwise a
phi can carry a ballot of it from an earlier iteration, which is not the
value the select used. Masks computed inside a loop keep the old rules.
- A result may only be kept if the assumptions it used held. When a value
that was assumed turns out not to be implied, every result recorded since
it was assumed is dropped. Results that say "not implied" are kept; they
only make the proof refuse more.
A lane that is turned back on inside the loop makes the back-edge exec wider
than the write's exec, so the phi is not implied and a read under it keeps
the select. A lane that is turned back on but never reads the derived value
does not.
New tests cover a loop that carries the write with exec saved, narrowed and
restored (read inside and after the loop), a loop that leaves exec alone,
and the helper-only ancillary select feeding such loops. The refusing tests
each fail when the matching check is removed: a revived lane that reads the
write inside the loop (ignoring back-edge values), a result proven under an
assumption that later fails (keeping it), and a write mask computed inside
an earlier loop (allowing such masks). Two existing tests expected a loop to
keep the select although every read is under the write's own exec; they now
expect it to go, next to new counterparts with a wider exec.
On the Taxi Chaos 2 requests, compute shaders with a loop inside a branch now
lose the branch's masked writes: 0x194000b400 removes 207 selects instead of
36, 0x19002bd200 111 instead of 28, and seven more gain 5 to 43. In
0x16400cf700 the 26 new removals are writes under the branch condition
tid < count; the loop's exec starts as a ballot of that condition, only loses
lanes at the latch, and every store and atomic inside the loop runs under it.
No proof on these requests had to drop a result.
Walks now go on into loops, so the pass takes longer on loop shaders, but it
stays below main. Time in MaskedSelectEliminator per prepare (median of 3,
the pass runs twice per prepare), 54 Taxi Chaos 2 requests, one run:
shader previous commit this commit main 5edae353
0x1ac0413000 334 ms 349 ms 424 ms
0x19002bd200 38 ms 103 ms 112 ms
0x19c016aa00 9 ms 74 ms 16 ms
0x194000b400 13 ms 31 ms 32 ms
all 54 547 ms 805 ms 977 ms
0x19c016aa00 removes nothing more; its walks only end later inside its
loops. The 144 requests of the NVIDIA dump go from 1283 to 1589 ms and gain
removals in the copies of the same shaders. 197 of the 198 requests prepare,
as before.
Related: #2548
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
LEGO Horizon Adventures (PPSA14632) still refuses a pixel shader with
packed pixel ancillary input has an unsupported live use (used by SelectU32 ? ?)
although it has the form the previous commit handles: the only whole-word
reader is SelectU32(IEqual32(HelperInvocation, 0), BitFieldUExtract(x, 0, 11),
ancillary), the exec words are Ballot(not helper) & Ballot(true), and there
are no image ops, stores or loop phis. About 10,250 IR values derive from the
word, so the walk from that select stops at its fixed limit of 4096 visited
values and the select is kept. With the limit raised to 8192 the shader
prepares (reported on Linux with an RX 9070 XT).
A walk may now take max(4096, 2 * instructions) steps. A step is a value
visited for the first time or an implication evaluated for the first time.
The walk only visits values derived from the select, at most one per
instruction, so a walk is only cut short when the implications it evaluates
cost as much as the program itself. On the 198 Taxi Chaos 2 requests no walk
needs more than 0.79 * instructions steps. One walk stays linear in the
program, O(instructions + operands): each value is expanded once, and with
the per-walk memo each implication is evaluated once.
The walk is also cheaper, so that the larger limit costs nothing:
- visited values are marked in a table indexed by value id instead of a hash
set, and the table, the work list and the memo are reused across walks;
- blocks outside loops are looked up in a table indexed by block id instead
of a hash set. This lookup ran once per use and was most of the pass's time.
Time in MaskedSelectEliminator per prepare (median of 3, the pass runs twice
per prepare), 54 Taxi Chaos 2 requests:
shader before 40a8a5a7 this commit main 5edae353
0x1ac0413000 689 ms 285 ms 401 ms
0x19002bd200 102 ms 34 ms 100 ms
0x18c07bf000 191 ms 52 ms (refused)
all 54 1294 ms 484 ms 919 ms
0x1ac0413000 and its three copies in the NVIDIA dump now remove 659 selects
instead of 625: their walks no longer stop at 4096. No other request changes
what it removes. 197 of the 198 requests prepare before and after; the other
one fails the same way in both (v_readlane_b32 of lane 63 on a 32-lane host
subgroup).
Related: #2548
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Taxi Chaos 2 (PPSA30279) pixel shader 0x18c07bf000 is refused when it is
registered, and the title stops:
packed pixel ancillary input has an unsupported live use (used by SelectU32 ? ?)
The shader reads the ancillary VGPR (v2) once, with v_bfe_u32 v3, v2, 16, 11,
which #1904 lowers to Layer. The next write to v2, v_rcp_f32 v2, s33, runs
before any kill under the initial exec and becomes
SelectU32(IEqual32(HelperInvocation, 0), rcp, ancillary). The raw word
therefore survives only in helper lanes. The shader has no s_wqm, no DPP,
readlane or swizzle, no stores, and only *_lz samples and gathers. Every
helper-lane value that comes from v2 is dropped by a later exec select, a
compare that is ANDed with exec before its ballot, or an export whose exec
excludes helper lanes.
MaskedSelectEliminator exists to remove selects like this one, but it could
not prove the case. The kill paths save and restore exec, so the later execs
are phis at block merges, and the AND chains the kills build go deeper than
its eight-level limit.
When it checks whether a later exec implies the write's exec, it now:
- accepts a phi when the phi and each incoming value lie outside loops and
every incoming value implies that exec;
- pairs low and high exec words merged by phis in one block, predecessor by
predecessor;
- memoises results for each select and follows up to 64 levels. A cycle
counts as not implied.
Once the select is gone, nothing reads the ancillary word, so no bit of it
is produced. A select whose false lanes can still be observed is kept and
the shader is still refused. That covers a data test that leaves the word in
non-helper lanes that later export, an export under the full wave, and an
unmasked ballot.
Related: #2548
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A use of the packed ancillary VGPR other than a constant field extract (the word passed through a
select before its sample number is extracted) reads the word rebuilt from the sample number (bit 8)
and the render target index (bit 16). Extracts of other bits straight from the word stay refused, and
the refusal now names the uses.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Any other live use of the packed register keeps throwing, as on main, instead of
reading SampleId << 8 | Layer << 16 with the other bits zero. The field range
check no longer underflows for an extract that starts past a field's end.
Adds the pixel_ancillary recompiler test.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
AlxcBTW
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Joey Fisher
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