Tweak EarlyCSE to recognize series of dead stores

[reames]

Summary

EarlyCSE is giving up on the current instruction immediately when it recognizes that the current instruction makes a previous store trivially dead. There's no reason to do this. Once the previous store has been deleted, it's perfectly legal to remember the value of the current store (for value forwarding) and the fact the store occurred (it could be dead too!).

[hfinkel]

Seems to make sense. LGTM.

[reames]

Closed by commit rL222241 (authored by @reames).

llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp

if (Inst->mayWriteToMemory()) {
         if (LastStore &&
             LastStore->getPointerOperand() == SI->getPointerOperand()) {
           DEBUG(dbgs() << "EarlyCSE DEAD STORE: " << *LastStore << "  due to: "
                        << *Inst << '\n');
           LastStore->eraseFromParent();
           Changed = true;
           ++NumDSE;
           LastStore = nullptr;
-          continue;
+          // fallthrough - we can exploit information about this store
         }
 
         // Okay, we just invalidated anything we knew about loaded values.  Try
         // to salvage *something* by remembering that the stored value is a live
         // version of the pointer.  It is safe to forward from volatile stores
         // to non-volatile loads, so we don't have to check for volatility of
         // the store.
         AvailableLoads->insert(SI->getPointerOperand(),

llvm/trunk/test/Transforms/EarlyCSE/basic.ll

define i32 @test8(i32 *%P) {
   %V1 = call i32 @func(i32* %P) readnone
   store i32 4, i32* %P
   %V2 = call i32 @func(i32* %P) readnone
   %Diff = sub i32 %V1, %V2
   ret i32 %Diff
   ; CHECK: ret i32 0
 }
 
+;; Trivial DSE can't be performed across a readonly call.  The call
+;; can observe the earlier write.
+; CHECK-LABEL: @test9(
+define i32 @test9(i32 *%P) {
+  store i32 4, i32* %P
+  %V1 = call i32 @func(i32* %P) readonly
+  store i32 5, i32* %P        
+  ret i32 %V1
+  ; CHECK: store i32 4, i32* %P        
+  ; CHECK-NEXT: %V1 = call i32 @func(i32* %P)
+  ; CHECK-NEXT: store i32 5, i32* %P        
+  ; CHECK-NEXT: ret i32 %V1
+}
+
+;; Trivial DSE can be performed across a readnone call.
+; CHECK-LABEL: @test10
+define i32 @test10(i32 *%P) {
+  store i32 4, i32* %P
+  %V1 = call i32 @func(i32* %P) readnone
+  store i32 5, i32* %P        
+  ret i32 %V1
+  ; CHECK-NEXT: %V1 = call i32 @func(i32* %P)
+  ; CHECK-NEXT: store i32 5, i32* %P        
+  ; CHECK-NEXT: ret i32 %V1
+}
+
+;; Trivial dead store elimination - should work for an entire series of dead stores too.
+; CHECK-LABEL: @test11(
+define void @test11(i32 *%P) {
+  store i32 42, i32* %P
+  store i32 43, i32* %P
+  store i32 44, i32* %P
+  store i32 45, i32* %P
+  ret void
+  ; CHECK-NEXT: store i32 45
+  ; CHECK-NEXT: ret void
+}
+