[PATCH] D111727: [DSE] Eliminates dead store of an exisiting value (PR16520)

[Dawid Jurczak via Phabricator via llvm-commits]

yurai007 created this revision.
yurai007 added reviewers: fhahn, nikic, xbolva00, asbirlea, george.burgess.iv, lebedev.ri, dmccrevan.
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That's https://reviews.llvm.org/D90328 follow-up.

This change eliminates writes to variables where the value that is being written is already stored in the variable. This achieves the goal by looping through all memory definitions in the current state & each definitions uses. When there is a use where the write instruction is identical to the original instruction that created the memory definition, it will remove this redundant write.

For example:

void f() {

  x = 1;
  if foo() {
     x = 1;
     g();
  } else {
    h();
  }

}
void g();
void h();

The second x=1 will be eliminated since it is rewriting 1 to x. This pass will produce this:

void f() {

  x = 1;
  if foo() {
     g();
  } else {
    h();
  }

}
void g();
void h();


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D111727

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


Index: llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll
===================================================================
--- llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll
+++ llvm/test/Transforms/DeadStoreElimination/stores-of-existing-values.ll
@@ -11,7 +11,6 @@
 ; CHECK-NEXT:    store i8 1, i8* [[R:%.*]], align 1
 ; CHECK-NEXT:    br i1 [[B:%.*]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    store i8 1, i8* [[R]], align 1
 ; CHECK-NEXT:    tail call void @fn_mayread_or_clobber()
 ; CHECK-NEXT:    br label [[IF_END:%.*]]
 ; CHECK:       if.else:
@@ -38,8 +37,6 @@
 }
 
 declare void @fn_mayread_or_clobber()
-
-
 declare void @fn_readonly() readonly
 
 define void @test2(i1 %b, i8* nocapture %r) {
@@ -54,7 +51,6 @@
 ; CHECK-NEXT:    tail call void @fn_readonly()
 ; CHECK-NEXT:    br label [[IF_END]]
 ; CHECK:       if.end:
-; CHECK-NEXT:    store i8 1, i8* [[R]], align 1
 ; CHECK-NEXT:    ret void
 ;
 entry:
@@ -181,7 +177,6 @@
 ; CHECK-NEXT:    [[C1:%.*]] = call i1 @cond()
 ; CHECK-NEXT:    br i1 [[C1]], label [[FOR_BODY:%.*]], label [[END:%.*]]
 ; CHECK:       for.body:
-; CHECK-NEXT:    store i32 1, i32* [[P]], align 4
 ; CHECK-NEXT:    [[LV:%.*]] = load i32, i32* [[P]], align 4
 ; CHECK-NEXT:    br label [[FOR_HEADER]]
 ; CHECK:       end:
@@ -216,7 +211,6 @@
 ; CHECK:       bb2:
 ; CHECK-NEXT:    ret void
 ; CHECK:       bb3:
-; CHECK-NEXT:    store i32 0, i32* [[P]], align 4
 ; CHECK-NEXT:    ret void
 ;
   store i32 0, i32* %P
@@ -268,7 +262,6 @@
 ; CHECK-NEXT:    call void @fn_mayread_or_clobber()
 ; CHECK-NEXT:    ret void
 ; CHECK:       bb3:
-; CHECK-NEXT:    store i32 0, i32* [[P]], align 4
 ; CHECK-NEXT:    ret void
 ;
   store i32 0, i32* %P
Index: llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -1941,6 +1941,32 @@
 
     return false;
   }
+
+  /// Eliminates writes to locations where the value that is being written
+  /// is already stored at the same location.
+  bool eliminateRedundantStoresOfExisitingValues() {
+    auto MadeChange = false;
+    LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs that write the "
+                         "already exisiting value\n");
+    for (auto *Def : MemDefs) {
+      if (SkipStores.contains(Def) || MSSA.isLiveOnEntryDef(Def) ||
+          !isRemovable(Def->getMemoryInst()))
+        continue;
+      auto UpperDef = dyn_cast_or_null<MemoryDef>(Def->getDefiningAccess());
+      if (!UpperDef || MSSA.isLiveOnEntryDef(UpperDef))
+        continue;
+      auto *DefInst = Def->getMemoryInst();
+      auto *UpperInst = UpperDef->getMemoryInst();
+      auto MaybeUpperLoc = getLocForWriteEx(UpperInst);
+      if (!MaybeUpperLoc || !DefInst->isIdenticalTo(UpperInst) ||
+          isReadClobber(*MaybeUpperLoc, DefInst))
+        continue;
+      deleteDeadInstruction(DefInst);
+      NumRedundantStores++;
+      MadeChange = true;
+    }
+    return MadeChange;
+  }
 };
 
 static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
@@ -2104,6 +2130,7 @@
     }
   }
 
+  MadeChange |= State.eliminateRedundantStoresOfExisitingValues();
   if (EnablePartialOverwriteTracking)
     for (auto &KV : State.IOLs)
       MadeChange |= removePartiallyOverlappedStores(State.DL, KV.second, TLI);


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