[DSE] Cache liveOnEntry as clobbering access

[reames]

Summary

This builds on @fhahn's D112313, and caches the liveOnEntry node as a optimized access. D112313 tied to only cache a known clobber. This change adds caching the fact that no clobber exists. It still does not cache may-clobber results.

[nikic]

LGTM, I believe this is correct.

[fhahn]

LGTM thanks!

IIUC it's not possible to write a test for this easily, because the only use of the optimized access in DSE is eliminateRedundantStoresOfExistingValues, which is not applicable if the optimized accesses is liveOnEntry, right?

[reames]

In D120842#3357155, @fhahn wrote:

IIUC it's not possible to write a test for this easily, because the only use of the optimized access in DSE is eliminateRedundantStoresOfExistingValues, which is not applicable if the optimized accesses is liveOnEntry, right?

I came across this as a compile time improvement. (Edit: For clarity, this was split off a compile time improvement idea which didn't work out. This patch seemed useful potentially, even though the broader idea didn't work.) I could maybe test for the memory ssa printer pass after DSE, but that feels too fragile to be worthwhile.

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

for (;; Current = cast<MemoryDef>(Current)->getDefiningAccess()) {
           dbgs() << " (" << *cast<MemoryUseOrDef>(Current)->getMemoryInst()
                  << ")";
         dbgs() << "\n";
       });
 
       // Reached TOP.
       if (MSSA.isLiveOnEntryDef(Current)) {
         LLVM_DEBUG(dbgs() << "   ...  found LiveOnEntryDef\n");
+        if (CanOptimize && Current != KillingDef->getDefiningAccess())
+          // The first clobbering def is... none.
+          KillingDef->setOptimized(Current);
         return None;
       }
 
       // Cost of a step. Accesses in the same block are more likely to be valid
       // candidates for elimination, hence consider them cheaper.
       unsigned StepCost = KillingDef->getBlock() == Current->getBlock()
                               ? MemorySSASameBBStepCost
                               : MemorySSAOtherBBStepCost;