[PATCH] D90328: Eliminates dead store of an exisiting value

[Daniel McCrevan via Phabricator via llvm-commits]

dmccrevan updated this revision to Diff 302880.
dmccrevan edited the summary of this revision.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D90328/new/

https://reviews.llvm.org/D90328

Files:
  llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
  llvm/test/Transforms/DeadStoreElimination/MSSA/pr16520.ll


Index: llvm/test/Transforms/DeadStoreElimination/MSSA/pr16520.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/DeadStoreElimination/MSSA/pr16520.ll
@@ -0,0 +1,37 @@
+; RUN: opt -basic-aa -dse -S < %s | FileCheck %s
+; PR16520
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @_Z1fbRb(i1 zeroext %b, i8* nocapture %r) {
+; CHECK-LABEL: @_Z1fbRb(
+; CHECK-NEXT: entry:
+; CHECK-NEXT:   store i8 1, i8* %r, align 1
+; CHECK-NEXT:   br i1 %b, label %if.then, label %if.else
+; CHECK:      if.then:                                          ; preds = %entry
+; CHECK-NEXT:   tail call void @_Z1gv()
+; CHECK-NEXT:   br label %if.end
+; CHECK:      if.else:                                          ; preds = %entry
+; CHECK-NEXT:   tail call void @_Z1hv()
+; CHECK-NEXT:   br label %if.end
+; CHECK:      if.end:                                           ; preds = %if.else, %if.then
+; CHECK-NEXT:   ret void
+; CHECK:      declare void @_Z1gv()
+; CHECK:      declare void @_Z1hv()
+;
+entry:
+  store i8 1, i8* %r, align 1
+  br i1 %b, label %if.then, label %if.else
+if.then:                                          ; preds = %entry
+  store i8 1, i8* %r, align 1
+  tail call void @_Z1gv()
+  br label %if.end
+if.else:                                          ; preds = %entry
+  tail call void @_Z1hv()
+  br label %if.end
+if.end:                                           ; preds = %if.else, %if.then
+  ret void
+}
+declare void @_Z1gv()
+declare void @_Z1hv()
Index: llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -2383,6 +2383,44 @@
     return false;
   }
 
+  /// Eliminates writes to variables where the value that is being written
+  /// is already stored in the variable
+  bool eliminateDeadStoresOfExisitingValues() {
+    bool MadeChange = false;
+    LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs that write the "
+                         "already exisiting value\n");
+    for (int I = MemDefs.size() - 1; I >= 0; I--) {
+      MemoryDef *Def = MemDefs[I];
+      MemoryAccess *DefAccess = dyn_cast<MemoryAccess>(Def);
+      if (DefAccess == NULL)
+        continue;
+      if (isa<MemoryPhi>(DefAccess))
+        continue;
+      Instruction *DefInst = cast<MemoryUseOrDef>(DefAccess)->getMemoryInst();
+      if (DefInst == NULL) {
+        continue;
+      }
+      for (Use &U : DefAccess->uses()) {
+        MemoryAccess *UseAccess = cast<MemoryAccess>(U.getUser());
+        if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess)) {
+          MemoryAccess *UseDefAccess = UseDef->getDefiningAccess();
+          if (UseDefAccess == NULL) {
+            continue;
+          }
+          Instruction *UseInst = cast<MemoryUseOrDef>(UseDefAccess)->getMemoryInst();
+          if (UseInst == NULL) {
+            continue;
+          }
+          if (UseInst->isIdenticalTo(DefInst)) {
+            deleteDeadInstruction(DefInst);
+            MadeChange = true;
+          }
+        }
+      }
+    }
+    return MadeChange;
+  }
+
   /// Eliminate writes to objects that are not visible in the caller and are not
   /// accessed before returning from the function.
   bool eliminateDeadWritesAtEndOfFunction() {
@@ -2653,6 +2691,8 @@
     }
   }
 
+  MadeChange |= State.eliminateDeadStoresOfExisitingValues();
+
   if (EnablePartialOverwriteTracking)
     for (auto &KV : State.IOLs)
       MadeChange |= removePartiallyOverlappedStores(State.DL, KV.second, TLI);


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