[GlobalOpt] Delete write-only constants with cast users

[jdoerfert]

Summary

We handled constant bit/addrspace-casts and GEPs but no
bit/addrspace-cast instructions before.

[lebedev.ri]

LGTM

llvm/lib/Transforms/IPO/GlobalOpt.cpp

330

Recursion within worklist, really?!

llvm/test/Transforms/GlobalOpt/writeonly-internal-bitcast.ll

2

Precommit?

[jdoerfert]

Check for pointer type, ptr2int test

[lebedev.ri]

This review seems to be stuck/dead, consider abandoning if no longer relevant.

[arsenm]

llvm/lib/Transforms/IPO/GlobalOpt.cpp

326–328

This is less useful with pointer bitcasts effectively gone. Is this still interesting if restricted to addrspacecast?

llvm/test/Transforms/GlobalOpt/writeonly-internal-bitcast.ll

35

May need a test with addrspacecast

llvm/lib/Transforms/IPO/GlobalOpt.cpp

if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
         Constant *SubInit = nullptr;
         if (Init)
           SubInit = ConstantFoldLoadThroughGEPConstantExpr(Init, CE);
         Changed |= CleanupConstantGlobalUsers(CE, SubInit, DL, GetTLI);
       } else if ((CE->getOpcode() == Instruction::BitCast &&
                   CE->getType()->isPointerTy()) ||
                  CE->getOpcode() == Instruction::AddrSpaceCast) {
         // Pointer cast, delete any stores and memsets to the global.
+        // TODO: We should try to cast the Init value to the new type.
         Changed |= CleanupConstantGlobalUsers(CE, nullptr, DL, GetTLI);
       }
 
       if (CE->use_empty()) {
         CE->destroyConstant();
         Changed = true;
       }
+    } else if (CastInst *CI = dyn_cast<CastInst>(U)) {
+      if (CI->getType()->isPointerTy()) {
+        // Pointer cast, delete any stores and memsets to the global.
+        // TODO: We should try to cast the Init value to the new type.
+        Changed |= CleanupConstantGlobalUsers(CI, nullptr, DL, GetTLI);
+        if (CI->use_empty()) {
+          CI->eraseFromParent();
+          Changed = true;
+        }
+      }
     } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
       // Do not transform "gepinst (gep constexpr (GV))" here, because forming
       // "gepconstexpr (gep constexpr (GV))" will cause the two gep's to fold
       // and will invalidate our notion of what Init is.
       Constant *SubInit = nullptr;
       if (!isa<ConstantExpr>(GEP->getOperand(0))) {
         ConstantExpr *CE = dyn_cast_or_null<ConstantExpr>(
             ConstantFoldInstruction(GEP, DL, &GetTLI(*GEP->getFunction())));

llvm/test/Transforms/GlobalOpt/writeonly-internal-bitcast.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -globalopt %s -S -o - | FileCheck %s
 
 %struct.S = type { i32, [2048 x i8] }
 
 @DirectAccess = internal global %struct.S undef, align 4
 @CastAccess = internal global %struct.S undef, align 4
 @CastAccessConst = internal global %struct.S undef, align 4
 @ASCastAccess = internal addrspace(3) global %struct.S undef, align 4
 @P2ICastAccess = internal global %struct.S undef, align 4
 
 define void @f(i32 %I32)  {
 ; CHECK-LABEL: @f(
-; CHECK-NEXT:    [[G1:%.*]] = getelementptr inbounds [[STRUCT_S:%.*]], %struct.S* @CastAccess, i32 0, i32 1, i32 [[I32:%.*]]
-; CHECK-NEXT:    [[BC1:%.*]] = bitcast i8* [[G1]] to i32*
-; CHECK-NEXT:    store i32 0, i32* [[BC1]], align 4
-; CHECK-NEXT:    [[G2:%.*]] = getelementptr inbounds [[STRUCT_S]], %struct.S* @CastAccessConst, i32 0, i32 1, i32 42
-; CHECK-NEXT:    [[BC2:%.*]] = bitcast i8* [[G2]] to i32*
-; CHECK-NEXT:    store i32 0, i32* [[BC2]], align 4
-; CHECK-NEXT:    [[AS3:%.*]] = addrspacecast [[STRUCT_S]] addrspace(3)* @ASCastAccess to %struct.S*
-; CHECK-NEXT:    [[G3:%.*]] = getelementptr inbounds [[STRUCT_S]], %struct.S* [[AS3]], i32 0, i32 1, i32 [[I32]]
-; CHECK-NEXT:    [[BC3:%.*]] = bitcast i8* [[G3]] to i32*
-; CHECK-NEXT:    store i32 0, i32* [[BC3]], align 4
-; CHECK-NEXT:    [[G4:%.*]] = getelementptr inbounds [[STRUCT_S]], %struct.S* @P2ICastAccess, i32 0, i32 1, i32 42
+; CHECK-NEXT:    [[G4:%.*]] = getelementptr inbounds [[STRUCT_S:%.*]], %struct.S* @P2ICastAccess, i32 0, i32 1, i32 42
 ; CHECK-NEXT:    [[PTR2INT:%.*]] = ptrtoint i8* [[G4]] to i64
 ; CHECK-NEXT:    [[INT2PTR:%.*]] = inttoptr i64 [[PTR2INT]] to i32*
 ; CHECK-NEXT:    store i32 0, i32* [[INT2PTR]], align 4
 ; CHECK-NEXT:    ret void
 ;
   %g0 = getelementptr inbounds %struct.S, %struct.S* @DirectAccess, i32 0, i32 1, i32 %I32
   store i8 0, i8* %g0, align 4
   %g1 = getelementptr inbounds %struct.S, %struct.S* @CastAccess, i32 0, i32 1, i32 %I32
   %bc1 = bitcast i8* %g1 to i32*
   store i32 0, i32* %bc1, align 4
   %g2 = getelementptr inbounds %struct.S, %struct.S* @CastAccessConst, i32 0, i32 1, i32 42
   %bc2 = bitcast i8* %g2 to i32*
   store i32 0, i32* %bc2, align 4
   %as3 = addrspacecast %struct.S addrspace(3)* @ASCastAccess to %struct.S*
   %g3 = getelementptr inbounds %struct.S, %struct.S* %as3, i32 0, i32 1, i32 %I32
   %bc3 = bitcast i8* %g3 to i32*
   store i32 0, i32* %bc3, align 4
   %g4 = getelementptr inbounds %struct.S, %struct.S* @P2ICastAccess, i32 0, i32 1, i32 42
   %ptr2int = ptrtoint i8* %g4 to i64
   %int2ptr = inttoptr i64 %ptr2int to i32*
   store i32 0, i32* %int2ptr, align 4
   ret void
 }