The existing way of creating the predicate in the guard blocks uses
a boolean value per outgoing block. This increases the number of live
booleans as the number of outgoing blocks increases. The new way added
in this change is to store one integer to represent the outgoing block
we want to branch to, then at each guard block, an integer equality
check is performed to decide which a specific outgoing block is taken.
Using an integer reduces the number of live values and decreases
register pressure especially in cases where there are a large number
of outgoing blocks. The integer based approach is used when the
number of outgoing blocks crosses a threshold, which is currently set
to 32.
Patch by Ruiling Song.
Differential review: https://reviews.llvm.org/D127831
This is the first patch in a series intended for removing flag
-enable-new-pm=0 from lit tests. This is part of a bigger
effort of completely removing legacy code related to legacy
pass manager in favor of currently default new pass manager.
In this patch flag has been removed only from tests where no significant
change has been required because checks has been duplicated for
both PMs.
Reviewed By: fhahn
Differential Revision: https://reviews.llvm.org/D134150
This reverts commit e13248ab0e79b59d5e5ac73e2fe57d82ce485ce1.
Need to revert because the transformation cannot occur for basic
blocks that contain convergent instructions.
UnifyLoopExits creates a single exit, a control flow hub, for
loops with multiple exits. There is an input to the block for
each loop exiting block and an output from the block for each
loop exit block. Multiple checks, or guard blocks, are needed
to branch to the correct exit block.
For large loops with lots of exit blocks, all the extra guard
blocks cause problems for StructurizeCFG and subsequent passes.
This patch reduces the number of guard blocks needed when the
exit blocks branch to a common block (e.g., an unreachable
block). The guard blocks are reduced by changing the inputs
and outputs of the control flow hub. The inputs are the exit
blocks and the outputs are the common block.
Reducing the guard blocks enables StructurizeCFG to reorder the
basic blocks in the CFG to reduce the values that exit a loop
with multiple exits. This reduces the compile-time of
StructurizeCFG and also reduces register pressure.
Differential Revision: https://reviews.llvm.org/D123230
The NewDefault was used to simplify the updating of PHI nodes, but it
causes some inefficiency for target that will run structurizer later. For
example, for a simple two-case switch, the extra NewDefault is causing
unstructured CFG like:
O
/ \
O O
/ \ / \
C1 ND C2
\ | /
\ | /
D
The change is to avoid the ND(NewDefault) block, that is we will get a
structured CFG for above example like:
O
/ \
/ \
O O
/ \ / \
C1 \ / C2
\-> D <-/
The IR change introduced by this patch should be trivial to other targets,
so I am doing this unconditionally.
Fall-through among the cases will also cause unstructured CFG, but it need
more work and will be addressed in a separate change.
Reviewed by: arsenm
Differential Revision: https://reviews.llvm.org/D123607
For each natural loop with multiple exit blocks, this pass creates a
new block N such that all exiting blocks now branch to N, and then
control flow is redistributed to all the original exit blocks.
The bulk of the tranformation is a new function introduced in
BasicBlockUtils that an redirect control flow from a set of incoming
blocks to a set of outgoing blocks via a common "hub".
This is a useful workaround for a limitation in the structurizer which
incorrectly orders blocks when processing a nest of loops. This pass
bypasses that issue by ensuring that each natural loop is recognized
as a separate region. Since the structurizer is a region pass, it no
longer sees a nest of loops in a single region, and instead processes
each "level" in the nesting as a separate region.
The AMDGPU backend provides a new option to enable this pass before
the structurizer, which may eventually be enabled by default.
Reviewers: madhur13490, arsenm, nhaehnle
Reviewed By: nhaehnle
Differential Revision: https://reviews.llvm.org/D75865
