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[DSE,MSSA] Reorder DSE blocking checks. 

Currently we stop exploring candidates too early in some cases.

In particular, we can continue checking the defining accesses of
non-removable MemoryDefs and defs without analyzable write location
(read clobbers are already ruled out using MemorySSA at this point).
This commit is contained in:
Florian Hahn 2020-06-22 16:24:27 +01:00
parent c52bee61e9
commit 328c8642e2
2 changed files with 28 additions and 26 deletions
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48
llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
View File

@ -1900,22 +1900,21 @@ struct DSEState {
// * A memory instruction that may throw and \p SI accesses a non-stack
// object.
// * Atomic stores stronger that monotonic.
bool isDSEBarrier(Instruction *SI, const Value *SILocUnd,
Instruction *NI) const {
bool isDSEBarrier(const Value *SILocUnd, Instruction *NI) const {
// If NI may throw it acts as a barrier, unless we are to an alloca/alloca
// like object that does not escape.
if (NI->mayThrow() && !InvisibleToCallerBeforeRet.count(SILocUnd))
return true;
// If NI is an atomic load/store stronger than monotonic, do not try to
// eliminate/reorder it.
if (NI->isAtomic()) {
if (auto *NSI = dyn_cast<StoreInst>(NI)) {
if (isStrongerThanMonotonic(NSI->getOrdering()))
return true;
} else
llvm_unreachable(
"Other instructions should be modeled/skipped in MemorySSA");
if (auto *LI = dyn_cast<LoadInst>(NI))
return isStrongerThanMonotonic(LI->getOrdering());
if (auto *SI = dyn_cast<StoreInst>(NI))
return isStrongerThanMonotonic(SI->getOrdering());
llvm_unreachable("other instructions should be skipped in MemorySSA");
}
return false;
}
};
@ -2029,6 +2028,22 @@ bool eliminateDeadStoresMemorySSA(Function &F, AliasAnalysis &AA,
Instruction *NI = NextDef->getMemoryInst();
LLVM_DEBUG(dbgs() << " (" << *NI << ")\n");
// Before we try to remove anything, check for any extra throwing
// instructions that block us from DSEing
if (State.mayThrowBetween(SI, NI, SILocUnd)) {
LLVM_DEBUG(dbgs() << " ... skip, may throw!\n");
break;
}
// Check for anything that looks like it will be a barrier to further
// removal
if (State.isDSEBarrier(SILocUnd, NI)) {
LLVM_DEBUG(dbgs() << " ... skip, barrier\n");
continue;
}
ToCheck.insert(NextDef->getDefiningAccess());
if (!hasAnalyzableMemoryWrite(NI, TLI)) {
LLVM_DEBUG(dbgs() << " ... skip, cannot analyze def\n");
continue;
@ -2039,20 +2054,6 @@ bool eliminateDeadStoresMemorySSA(Function &F, AliasAnalysis &AA,
continue;
}
// Check for anything that looks like it will be a barrier to further
// removal
if (State.isDSEBarrier(SI, SILocUnd, NI)) {
LLVM_DEBUG(dbgs() << " ... skip, barrier\n");
continue;
}
// Before we try to remove anything, check for any extra throwing
// instructions that block us from DSEing
if (State.mayThrowBetween(SI, NI, SILocUnd)) {
LLVM_DEBUG(dbgs() << " ... skip, may throw!\n");
break;
}
if (!DebugCounter::shouldExecute(MemorySSACounter))
continue;
@ -2088,7 +2089,6 @@ bool eliminateDeadStoresMemorySSA(Function &F, AliasAnalysis &AA,
}
}
ToCheck.insert(NextDef->getDefiningAccess());
if (OR == OW_Complete) {
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *NI
<< "\n KILLER: " << *SI << '\n');

6
llvm/test/Transforms/DeadStoreElimination/MSSA/simple.ll
View File

@ -612,10 +612,12 @@ entry:
}
; Some tests where volatile may block removing a store.
; Here we can remove the first non-volatile store. We cannot remove the
; volatile store.
define void @test44_volatile(i32* %P) {
; CHECK-LABEL: @test44_volatile(
; CHECK-NEXT: store i32 1, i32* [[P:%.*]], align 4
; CHECK-NEXT: store volatile i32 2, i32* [[P]], align 4
; CHECK-NEXT: store volatile i32 2, i32* [[P:%.*]], align 4
; CHECK-NEXT: store i32 3, i32* [[P]], align 4
; CHECK-NEXT: ret void
;