Prior to this commit rename_variables_block() is recursively called,
performing a depth-first traversal of the control flow graph. The
function uses a non-trivial amount of stack space for local variables,
which puts us in danger of smashing the stack, given a sufficiently deep
dominance tree.
XCOM: Enemy Within contains a shader with such a dominance tree (1574
nir_blocks in total, depth of at least 143).
Jason tells me that he believes that any walk over the nir_blocks that
respects dominance is sufficient (a DFS might have been necessary prior
to the introduction of nir_phi_builder).
In fact, the introduction of nir_phi_builder made the problem worse:
rename_variables_block(), walks to the bottom of the dominance tree
before calling nir_phi_builder_value_get_block_def() which walks back to
the top of the dominance tree...
In any case, this patch ensures we avoid that problem as well.
Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97225
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Right now, we have phi placement code in two places and there are other
places where it would be nice to be able to do this analysis. Instead of
repeating it all over the place, this commit adds a helper for placing all
of the needed phi nodes for a value.
v2: Add better documentation
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Emil Velikov
Matt Turner
Jason Ekstrand