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>
Celegans12
Dean Galvin
STL1te