[llvm] 274a9b0 - [DSE] Support redundant stores eliminated by memset.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2021-10-29T22:19:53+01:00
New Revision: 274a9b0f0b4e9f6a8520e92d9eff629815436e28

URL: https://github.com/llvm/llvm-project/commit/274a9b0f0b4e9f6a8520e92d9eff629815436e28
DIFF: https://github.com/llvm/llvm-project/commit/274a9b0f0b4e9f6a8520e92d9eff629815436e28.diff

LOG: [DSE] Support redundant stores eliminated by memset.

This patch adds support to remove stores that write the same value
as earlier memesets.

It uses isOverwrite to check that a memset completely overwrites a later
store. The candidate store must store the same bytewise value as the
byte stored by the memset.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D112321

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
    llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
index 26ae5877a7597..05ba76d57d96e 100644
--- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -1955,10 +1955,32 @@ struct DSEState {
       auto *UpperDef = dyn_cast<MemoryDef>(Def->getDefiningAccess());
       if (!UpperDef || MSSA.isLiveOnEntryDef(UpperDef))
         continue;
-      auto *DefInst = Def->getMemoryInst();
-      auto *UpperInst = UpperDef->getMemoryInst();
+
+      Instruction *DefInst = Def->getMemoryInst();
+      Instruction *UpperInst = UpperDef->getMemoryInst();
+      auto IsRedundantStore = [this, DefInst,
+                               UpperInst](MemoryLocation UpperLoc) {
+        if (DefInst->isIdenticalTo(UpperInst))
+          return true;
+        if (auto *MemSetI = dyn_cast<MemSetInst>(UpperInst)) {
+          if (auto *SI = dyn_cast<StoreInst>(DefInst)) {
+            auto MaybeDefLoc = getLocForWriteEx(DefInst);
+            if (!MaybeDefLoc)
+              return false;
+            int64_t InstWriteOffset = 0;
+            int64_t DepWriteOffset = 0;
+            auto OR = isOverwrite(UpperInst, DefInst, UpperLoc, *MaybeDefLoc,
+                                  InstWriteOffset, DepWriteOffset);
+            Value *StoredByte = isBytewiseValue(SI->getValueOperand(), DL);
+            return StoredByte && StoredByte == MemSetI->getOperand(1) &&
+                   OR == OW_Complete;
+          }
+        }
+        return false;
+      };
+
       auto MaybeUpperLoc = getLocForWriteEx(UpperInst);
-      if (!MaybeUpperLoc || !DefInst->isIdenticalTo(UpperInst) ||
+      if (!MaybeUpperLoc || !IsRedundantStore(*MaybeUpperLoc) ||
           isReadClobber(*MaybeUpperLoc, DefInst))
         continue;
       LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n  DEAD: " << *DefInst

diff  --git a/llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll b/llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll
index 7888a393f5218..8419dcadf3e19 100644
--- a/llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll
+++ b/llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll
@@ -403,8 +403,6 @@ declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i1 immarg)
 define void @test12_memset_simple(i8* %ptr) {
 ; CHECK-LABEL: @test12_memset_simple(
 ; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[PTR:%.*]], i8 0, i64 10, i1 false)
-; CHECK-NEXT:    [[PTR_5:%.*]] = getelementptr i8, i8* [[PTR]], i64 4
-; CHECK-NEXT:    store i8 0, i8* [[PTR_5]], align 1
 ; CHECK-NEXT:    ret void
 ;
   call void @llvm.memset.p0i8.i64(i8* %ptr, i8 0, i64 10, i1 false)