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[MemCpyOpt] Optimize MemoryDef insertion 

When converting a store into a memset, we currently insert the new
MemoryDef after the store MemoryDef, which requires all uses to be
renamed to the new def using a whole block scan. Instead, we can
insert the new MemoryDef before the store and not rename uses,
because we know that the location is immediately overwritten, so
all uses should still refer to the old MemoryDef. Those uses will
get renamed when the old MemoryDef is actually dropped, which is
efficient.

I expect something similar can be done for some of the other MSSA
updates in MemCpyOpt. This is an alternative to D107513, at least
for this particular case.

Differential Revision: https://reviews.llvm.org/D107702
This commit is contained in:
Nikita Popov 2021-08-07 23:09:09 +02:00
parent 2f946eaa9d
commit 17db125b48
1 changed files with 6 additions and 5 deletions
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11
llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
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@ -799,11 +799,12 @@ bool MemCpyOptPass::processStore(StoreInst *SI, BasicBlock::iterator &BBI) {
LLVM_DEBUG(dbgs() << "Promoting " << *SI << " to " << *M << "\n");
assert(isa<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(SI)));
auto *LastDef =
cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(SI));
auto *NewAccess = MSSAU->createMemoryAccessAfter(M, LastDef, LastDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
// The newly inserted memset is immediately overwritten by the original
// store, so we do not need to rename uses.
auto *StoreDef = cast<MemoryDef>(MSSA->getMemoryAccess(SI));
auto *NewAccess = MSSAU->createMemoryAccessBefore(
M, StoreDef->getDefiningAccess(), StoreDef);
MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/false);
eraseInstruction(SI);
NumMemSetInfer++;