A selection outside loops whose one arm only returns and whose other arm
branches to a join it does not dominate gets an empty block on that edge
as its merge (splitOneReturnJoin), but only when every other predecessor
of the join dominates the selection or is dominated by it. When the
parent selection's other arm is a region that ends in the same join, the
selection has no structured merge and the shader fails to recompile.
When every structurization attempt fails, Structurize now runs them again
and accepts that split for any join the selection does not dominate. The
split only inserts an empty block on the edge from the selection to the
join. Graphs that structurize on main keep their result.
A synthetic CFG and a synthetic program cover the shape; both fail on main
with "conditional block N has no structured merge".
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fnrk5Dazjb2tLooVSpdRQq
A loop whose exit paths leave the loop construct before they meet got
the nearest common post-dominator of its exits as merge, and
splitSharedMergeBlocks never separated it. An exit continues the
enclosing loop, so its path to that merge runs through the outer
header again, or an exit enters a block that a branch before the loop
also reaches. splitOneLoopMerge gave the loop a forwarding merge block
each round, the other path still bypassed it, and the post-dominator
stayed the same block until "CFG shared merge splitting exceeded
budget". These shapes from #2642 fail that way on main, on d59a9167,
with #2471 and with both:
{{1}, {2}, {3, 4}, {2, 5}, {}, {1}}
{{1}, {2, 3}, {4, 5}, {}, {3, 6}, {1}, {2}}
{{1, 3}, {2, 4}, {3, 1}, {4}, {}}
{{1, 2}, {3, 2}, {4}, {4, 1}, {}}
{{1, 2}, {4}, {3, 1}, {4, 2}, {}}
When the plain and the privatized pass both fail, Structurize now runs
a third pass. It routes such a loop's exits through one junction with
routeLoopExits, as loops whose exits have no common post-dominator are
routed already, and gives an outer loop whose only latch lies in an
inner loop its own continue block. In that pass a loop exit conditional
that is neither a direct break or continue nor has an exit tail of its
own is structured as a selection, and an exit tail that the other arm
reaches is not used as one. Without that, graphs the routing unblocks
keep a conditional without merge (the rule from #2762) or a selection
entered past its header. Graphs that the first two passes structure
are unchanged, and when the third pass fails too, the second pass's
error is reported.
Differential fuzz against main, reducible graphs with 5 to 9 blocks,
each with 0 estimated words per block and again with one block at 1000
words (the privatizeSharedReturns path). Oracle: exact post-dominators,
requireStructuredBranches, merges dominated by their headers, the same
executions as the original graph, and spirv-val (SPIRV-Tools v2026.4,
Vulkan 1.3) on a module emitted from the structured graph.
- 200,000 random graphs. main structures 178,743 (0 words) and 178,750
(1000 words); the output is identical on all of them. Of main's
14,256 and 14,311 budget failures 13,527 and 13,596 now structure,
and 2,576 and 2,565 of its refusals. No new output is invalid. The
invalid outputs are main's 530, plus at 1000 words one graph that
d59a9167 already turns from a budget failure into a back edge to a
block that is not a loop header.
- 24,000 structured programs from orishamir/anyps5-cfgfuzz's
generator (if/else, loops, break, continue, return). main structures
21,093 and 21,022; the output is identical. 619 of 2,448 and 634 of
2,429 budget failures now structure. No new output is invalid; the
invalid outputs are main's 92.
agc_structurizer has the five shapes; requireOriginalBranches now
follows route variables. agc_shader_control_flow has two RDNA programs
of the first two shapes. The non-GPU tests and the 253 agc_driver GPU
tests pass on Windows (RTX 5070 Laptop).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A selection in a loop whose arms both reach their nearest common
post-dominator within the iteration got that post-dominator as its
merge. When an arm can also continue the loop, the post-dominator
follows the back edge: the continuing path re-enters the loop header
and reaches the outer selection's merge without passing the block where
the arms meet. In the loop {1}, {2, 4}, {3, 4}, {4, 5}, {1, 6}, {1}, {}
from #2642 (latches 4 and 5) the selections at 1 and 2 both merged at 4.
splitSharedMergeBlocks gave 2 a forwarding merge block, recomputed 4 for
it because 3 continues through 5, and repeated until "CFG shared merge
splitting exceeded budget: originalBlocks=9 currentBlocks=45
splitBudget=36".
When that post-dominator is inside the loop construct, findSelectionMerge
now takes the arms' nearest common post-dominator over the paths of the
iteration that reach it, so paths that leave through the continue block
or the loop's merge no longer move the merge past the arms' join. When
no arm leaves early the result is the same block as before. The loop
structures with two forwarding blocks and no clones or route variables;
so do three latches, the selections on the false side, and an if-else
whose one arm continues.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Intersect copied fixed-point sets without allocating each intermediate.
Preserve sorted results and the existing analysis iteration order.
Compact oversized storage only when publishing a changed set.
Keep an allocating control switch outside the cache key.
Compare complete fixed points over empty intersections, loops and DAGs.
Register the same equivalence fixtures for the allocating control.
Both dominance analyses start from sorted unique sets and intersect them.
Insert the current block ID with lower_bound instead of sorting again.
Preserve every intermediate set and the existing ID-order iteration.
Keep the generic unsorted helper and all graph rewrite logic unchanged.
Compare both analyses against the original algorithms on six raw CFGs.
Cover finite loops, closed SCCs, unreachable islands and permuted IDs.
A pixel shader whose early discards branch to one small ending block
gave that block several conditional predecessors outside any loop, and
structurizing failed. When the regular pass throws, the structurizer
restarts from the original graph and first gives each conditional
predecessor its own copy of such a return block (within the clone word
floor and a budget of four clones per block).
Uncharted: Legacy of Thieves Collection (PPSA05684) stopped at such a pixel shader with "conditional block 15 has no structured merge".
computePostDominators iterates its sets to a fixed point, visiting the
blocks in id order. Post-dominance flows from a block's successors back to
it, and successors mostly have higher ids, so each pass moved the sets back
by about one block: a long program needed about as many passes as it has
blocks, each costing O(blocks^2). recomputeAnalyses runs it after every
split of splitSharedMergeBlocks, up to four times per block.
Visit the blocks in post-order of the CFG (successors first), then any the
entry does not reach. The sets start from every block and only shrink, so
the iteration still converges to the same greatest fixed point, now in a
few passes.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A loop whose exits have no common post-dominator, such as a normal exit
to one s_endpgm tail beside an early-out to a kill tail, got no merge
block, and Structurize threw "loop at block N has no structured
merge/continue". With three or more exits, detectNaturalLoops threw
first, on a lookup of InvalidControlFlowId.
Such a loop now leaves through one new merge block that dispatches on
route variables. They are cleared before the header and set on each
exit edge to a target other than the latch's exit. Inner loops are
routed first; a routing that would change the natural loops throws.
A loop-exit tail branch whose in-body target is the continue block no
longer merges there, which spirv-val rejects ("Header block ... is
contained in the loop construct ..., but its merge block ... is not").
A selection whose one arm only returns (kill or s_endpgm) and whose other
arm branches straight to the merge of an enclosing selection had no
structured merge, so the draw was skipped. PPSA21564's BotEye G-buffer
pixel shaders (eight variants) have this shape.
When no other split applies, the edge from such a header to the shared
join is now split into an empty block, which becomes the header's merge.
A conditional inside a loop with one target in the loop body and the
other outside it was taken for a break and emitted without a merge. When
the outside target is not the loop's merge but a tail of blocks that
reaches it, SPIR-V has no construct for that branch: spirv-val rejects
the module ("Selection must be structured") and the driver is free to
compile it into anything.
Such a branch now merges at its in-body target when the tail is private
to it (every tail block is dominated by the tail's first block, stays in
the loop construct and leaves only for the loop's merge), so the tail is
an "if (c) { tail; break; }" the loop already allows. Merges are given
to these after every other construct's, and one already taken leaves the
branch as before.
All 13 PPSA21564 programs whose V#s are read from GPU memory (the
deferred lighting and ray-traced passes) had two such branches in their
traversal loops. With this change none of the 74 dumped title programs
has one, and the 209 modules of a title-to-cinematic run validate apart
from three unrelated image-offset and entry-point errors.
Tests: agc_structurizer_tests structures a loop that exits through one
tail block and a loop with two exit tails, and requires every merge-less
conditional to target its loop's merge or continue.
Only the lines this branch added lose their comments; the commits before
keep them, with the reasoning behind each change, for whoever digs into
that code later.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
findSelectionMerge took the nearest common post-dominator of a selection's
targets, computed across the loop's back edge. When one arm only continues
(reaching that block in a later iteration) and the other target is shared
with an enclosing selection, the shared merge was split off the header, the
post-dominator stayed where it was, and splitting repeated until the budget
threw.
Inside a loop, an arm that reaches the post-dominator only past the continue
or merge block now leaves the selection by continuing, and the merge is the
other arm's target; after one split that is the header's own synthetic merge.
agc_structurizer_tests builds the smallest such graph; it hit the budget
before.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Jan Šlégl
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Dean Galvin
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