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JIT: Factor SSA's DFS and profile synthesis's loop finding (#95251)

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Factor out SSA's general DFS (that takes EH into account) and
encapsulate it in a `FlowGraphDfsTree` class.

Factor out profile synthesis's loop finding and encapsulate it in a
`FlowGraphNaturalLoops` class. Switch construction of it to use the
general DFS instead of the restricted one (that does not account for
exceptional flow).

Optimize a few things in the process:
* Avoid storing loop blocks in a larger than necessary bit vector; store
  them starting from the loop header's postorder index instead.
* Provide post-order and reverse post-order visitors for the loop
  blocks; switch profile synthesis to use this in a place

No diffs are expected. A small amount of diffs are expected when profile
synthesis is enabled due to the modelling of exceptional flow and also
from handling unreachable predecessors (which would reject some loops as
unnatural loops before).

My future plans are to proceed to replace the loop representation of
loops with this factored version, removing the lexicality requirement in
the process, and hopefully fixing some of our deficiencies.
This commit is contained in:
Jakob Botsch Nielsen 2023-11-28 12:02:21 +01:00 • committed by GitHub
parent 8ba8da3e9e
commit f106d7ecd1
Signed by: github
GPG key ID: 4AEE18F83AFDEB23
15 changed files with 1054 additions and 635 deletions
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9
src/coreclr/jit/valuenum.cpp
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@ -9668,8 +9668,7 @@ public:
//
bool IsReachable(BasicBlock* bb)
{
return (bb->bbPostorderNum < m_comp->fgSSAPostOrderCount) &&
(m_comp->fgSSAPostOrder[bb->bbPostorderNum] == bb) &&
return m_comp->m_dfs->Contains(bb) &&
!BitVecOps::IsMember(&m_blockTraits, m_provenUnreachableBlocks, bb->bbNum);
}
@ -9850,9 +9849,11 @@ PhaseStatus Compiler::fgValueNumber()
// SSA has already computed a post-order taking EH successors into account.
// Visiting that in reverse will ensure we visit a block's predecessors
// before itself whenever possible.
for (unsigned i = fgSSAPostOrderCount; i != 0; i--)
BasicBlock** postOrder = m_dfs->GetPostOrder();
unsigned postOrderCount = m_dfs->GetPostOrderCount();
for (unsigned i = postOrderCount; i != 0; i--)
{
BasicBlock* block = fgSSAPostOrder[i - 1];
BasicBlock* block = postOrder[i - 1];
JITDUMP("Visiting " FMT_BB "\n", block->bbNum);
if (block != fgFirstBB)