This is experimental, and has not tests.
It's been used to debug validation of structured control flow.
- Has a legend describing special arcs to merge blocks and continue
targets.
- Labels the function entry block, with the Id of the function.
Previously we use vectors of objects and move semantics to handle
ownership. That approach has the flaw that inserting an object into
the middle of a vector, which may trigger a vector reallocation,
can invalidate some addresses taken from instructions.
Now the in-memory representation internally uses vector of unique
pointers to handle ownership. Since objects are explicitly heap-
allocated now, pointers to them won't be invalidated by vector
resizing anymore.
The NEW behavior is to not dereference variables or interpret keywords
that have been quoted or bracketed.
For more information, see
https://cmake.org/cmake/help/v3.1/policy/CMP0054.html.
This is to suppress a warning when using CMake 3.1.3+.
- Find unreachable continue targets. Look for back edges
with a DFS traversal separate from the dominance traversals,
where we count the OpLoopMerge from the header to the continue
target as an edge in the graph.
- It's ok for a loop to have multiple back edges, provided
they are all from the same block, and we call that the latch block.
This may require a clarification/fix in the SPIR-V spec.
- Compute postdominance correctly for infinite loop:
Bias *predecessor* traversal root finding so that you use
a later block in the original list. This ensures that
for certain simple infinite loops in the CFG where neither
block branches to a node without successors, that we'll
compute the loop header as dominating the latch block, and the
latch block as postdominating the loop header.
Fixes dominance calculation when there is a forward arc from an
unreachable block A to a reachable block B. Before this fix, we would
say that B is not dominated by the graph entry node, and instead say
that the immediate dominator of B is the psuedo-entry node of the
augmented CFG.
The fix:
- Dominance is defined in terms of a traversal from the entry block
of the CFG. So the forward DFS should start from the function
entry block, not the pseudo-entry-block.
- When following edges backward during dominance calculations, only go to
nodes that are actually reachable in the forward traversal.
Important: the sense of reachability flips around when computing
post-dominance.
Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/297
