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[NFC] EliminateDuplicatePHINodes(): small-size optimization: if there are <= 32 PHI's, O(n^2) algo is faster (geomean -0.08%) 

This is functionally equivalent to the old implementation.

As per https://llvm-compile-time-tracker.com/compare.php?from=5f4e9bf6416e45eba483a4e5e263749989fdb3b3&to=4739e6e4eb54d3736e6457249c0919b30f6c855a&stat=instructions
this is a clear geomean compile-time regression-free win with overall geomean of `-0.08%`

32 PHI's appears to be the sweet spot; both the 16 and 64 performed worse:
https://llvm-compile-time-tracker.com/compare.php?from=5f4e9bf6416e45eba483a4e5e263749989fdb3b3&to=c4efe1fbbfdf0305ac26cd19eacb0c7774cdf60e&stat=instructions
https://llvm-compile-time-tracker.com/compare.php?from=5f4e9bf6416e45eba483a4e5e263749989fdb3b3&to=e4989d1c67010d3339d1a40ff5286a31f10cfe82&stat=instructions

If we have more PHI's than that, we fall-back to the original DenseSet-based implementation,
so the not-so-fast cases will still be handled.

However compile-time isn't the main motivation here.
I can name at least 3 limitations of this CSE:
1. Assumes that all PHI nodes have incoming basic blocks in the same order (can be fixed while keeping the DenseMap)
2. Does not special-handle `undef` incoming values (i don't see how we can do this with hashing)
3. Does not special-handle backedge incoming values (maybe can be fixed by hashing backedge as some magical value)

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D87408
This commit is contained in:
Roman Lebedev 2020-09-17 11:08:26 +03:00
parent 6f6d389da5
commit aadf55d1ce
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GPG Key ID: 083C3EBB4A1689E0
1 changed files with 52 additions and 3 deletions
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55
llvm/lib/Transforms/Utils/Local.cpp
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@ -104,6 +104,12 @@ static cl::opt<bool> PHICSEDebugHash(
cl::desc("Perform extra assertion checking to verify that PHINodes's hash "
"function is well-behaved w.r.t. its isEqual predicate"));
static cl::opt<unsigned> PHICSENumPHISmallSize(
"phicse-num-phi-smallsize", cl::init(32), cl::Hidden,
cl::desc(
"When the basic block contains not more than this number of PHI nodes, "
"perform a (faster!) exhaustive search instead of set-driven one."));
// Max recursion depth for collectBitParts used when detecting bswap and
// bitreverse idioms
static const unsigned BitPartRecursionMaxDepth = 64;
@ -1132,9 +1138,39 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
return true;
}
// WARNING: this logic must be kept in sync with
// Instruction::isIdenticalToWhenDefined()!
bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
static bool EliminateDuplicatePHINodesNaiveImpl(BasicBlock *BB) {
// This implementation doesn't currently consider undef operands
// specially. Theoretically, two phis which are identical except for
// one having an undef where the other doesn't could be collapsed.
bool Changed = false;
// Examine each PHI.
// Note that increment of I must *NOT* be in the iteration_expression, since
// we don't want to immediately advance when we restart from the beginning.
for (auto I = BB->begin(); PHINode *PN = dyn_cast<PHINode>(I);) {
++I;
// Is there an identical PHI node in this basic block?
// Note that we only look in the upper square's triangle,
// we already checked that the lower triangle PHI's aren't identical.
for (auto J = I; PHINode *DuplicatePN = dyn_cast<PHINode>(J); ++J) {
if (!DuplicatePN->isIdenticalToWhenDefined(PN))
continue;
// A duplicate. Replace this PHI with the base PHI.
++NumPHICSEs;
DuplicatePN->replaceAllUsesWith(PN);
DuplicatePN->eraseFromParent();
Changed = true;
// The RAUW can change PHIs that we already visited.
I = BB->begin();
break; // Start over from the beginning.
}
}
return Changed;
}
static bool EliminateDuplicatePHINodesSetBasedImpl(BasicBlock *BB) {
// This implementation doesn't currently consider undef operands
// specially. Theoretically, two phis which are identical except for
// one having an undef where the other doesn't could be collapsed.
@ -1152,6 +1188,8 @@ bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
return PN == getEmptyKey() || PN == getTombstoneKey();
}
// WARNING: this logic must be kept in sync with
// Instruction::isIdenticalToWhenDefined()!
static unsigned getHashValueImpl(PHINode *PN) {
// Compute a hash value on the operands. Instcombine will likely have
// sorted them, which helps expose duplicates, but we have to check all
@ -1191,6 +1229,7 @@ bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
// Set of unique PHINodes.
DenseSet<PHINode *, PHIDenseMapInfo> PHISet;
PHISet.reserve(4 * PHICSENumPHISmallSize);
// Examine each PHI.
bool Changed = false;
@ -1213,6 +1252,16 @@ bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
return Changed;
}
bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
if (
#ifndef NDEBUG
!PHICSEDebugHash &&
#endif
hasNItemsOrLess(BB->phis(), PHICSENumPHISmallSize))
return EliminateDuplicatePHINodesNaiveImpl(BB);
return EliminateDuplicatePHINodesSetBasedImpl(BB);
}
/// enforceKnownAlignment - If the specified pointer points to an object that
/// we control, modify the object's alignment to PrefAlign. This isn't
/// often possible though. If alignment is important, a more reliable approach