[FixIrreducible] Use CycleInfo instead of a custom SCC traversal
1. CycleInfo efficiently locates all cycles in a single pass, while the
SCC is
repeated inside every natural loop.
2. CycleInfo provides a hierarchy of irreducible cycles, and the new
implementation transforms each cycle in this hierarchy separately
instead of
reducing an entire irreducible SCC in a single step. This reduces the
number
of control-flow paths that pass through the header of each newly created
loop. This is evidenced by the reduced number of predecessors on the
"guard"
blocks in the lit tests, and fewer operands on the corresponding PHI
nodes.
3. When an entry of an irreducible cycle is the header of a child
natural loop,
the original implementation destroyed that loop. This is now preserved,
since the incoming edges on non-header entries are not touched.
4. In the new implementation, if an irreducible cycle is a superset of a
natural
loop with the same header, then that natural loop is destroyed and
replaced
by the newly created loop.
CreateControlFlowHub is a method that redirects control flow edges from a set of
incoming blocks to a set of outgoing blocks through a new set of "guard" blocks.
This is now refactored into a separate file with one enhancement: The input to
the method is now a set of branches rather than two sets of blocks.
The original implementation reroutes every edge from incoming blocks to outgoing
blocks. But it is possible that for some incoming block InBB, some successor S
might be in the set of outgoing blocks, but that particular edge should not be
rerouted. The new implementation makes this possible by allowing the user to
specify the targets of each branch that need to be rerouted.
This is needed when improving the implementation of FixIrreducible #101386.
Current use in FixIrreducible does not demonstrate this finer control over the
edges being rerouted. But in UnifyLoopExits, when only one successor of an
exiting block is an exit block, this refinement now reroutes only the relevant
control-flow through the edge; the non-exit successor is not rerouted. This
results in fewer branches and PHI nodes in the hub.
Some passes has limitation that only support simple terminators:
branch/unreachable/return. Right now, they ask the pass manager to add
LowerSwitch pass to eliminate `switch`. Let's manage such kind of pass
dependency by ourselves. Also add the assertion in the related passes.
The modified function was incorrectly (not unnecessarily) ignoring grandchild
loops, and this change fixes the bug. In particular, this fixes the handling of
the loop { inner, body }. The TODO in the same function is talking about the b1
self loop, which may be "unnecessarily" lost, but that is a different issue.
In the NPM, a pass cannot depend on another non-analysis pass. So pin
the test that tests that -lowerswitch is run automatically to legacy PM.
Reviewed By: sameerds
Differential Revision: https://reviews.llvm.org/D89051
This fixes warnings raised by Clang's new -Wsuggest-override, in preparation for enabling that warning in the LLVM build. This patch also removes the virtual keyword where redundant, but only in places where doing so improves consistency within a given file. It also removes a couple unnecessary virtual destructor declarations in derived classes where the destructor inherited from the base class is already virtual.
Differential Revision: https://reviews.llvm.org/D83709
Summary:
- Skip unreachable predecessors during header detection in SCC. Those
unreachable blocks would be generated in the switch lowering pass in
the corner cases or other frontends. Even though they could be removed
through the CFG simplification, we should skip them during header
detection.
Reviewers: sameerds
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D83562
Summary:
When an irreducible SCC is converted into a new natural loop, existing
loops included in that SCC now become children of the new loop. The
logic that moves these loops from the parent loop to the new loop
invoked undefined behaviour when it modified the container that it was
iterating over. Fixed this by first extracting all the loops that are
to be removed from the parent.
Fixes bug 45623.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D78544
This is related to commit 8c11bc0cd06ffc18786a1a51ee5ce5b1cc4923f6
which introduces the FixIrreducible pass. The warning seems hard to
reproduce locally. The latest attempt ought to work.
An irreducible SCC is one which has multiple "header" blocks, i.e., blocks
with control-flow edges incident from outside the SCC. This pass converts an
irreducible SCC into a natural loop by introducing a single new header
block and redirecting all the edges on the original headers to this
new block.
This is a useful workaround for a limitation in the structurizer
which, which produces incorrect control flow in the presence of
irreducible regions. The AMDGPU backend provides an option to
enable this pass before the structurizer, which may eventually be
enabled by default.
Reviewed By: nhaehnle
Differential Revision: https://reviews.llvm.org/D77198
This restores commit 2ada8e2525dd2653f30c8696a27162a3b1647d66.
Originally reverted with commit 44e09b59b869a91bf47d76e8bc569d9ee91ad145.
This reverts commit 2ada8e2525dd2653f30c8696a27162a3b1647d66.
Buildbots produced compilation errors which I was not able to quickly
reproduce locally. Need more time to investigate.
An irreducible SCC is one which has multiple "header" blocks, i.e., blocks
with control-flow edges incident from outside the SCC. This pass converts an
irreducible SCC into a natural loop by introducing a single new header
block and redirecting all the edges on the original headers to this
new block.
This is a useful workaround for a limitation in the structurizer
which, which produces incorrect control flow in the presence of
irreducible regions. The AMDGPU backend provides an option to
enable this pass before the structurizer, which may eventually be
enabled by default.
Reviewed By: nhaehnle
Differential Revision: https://reviews.llvm.org/D77198
