[PATCH] D156175: [clang][ConstExprEmitter] handle NullToPointer ImplicitCastExpr

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers added inline comments.


================
Comment at: clang/lib/CodeGen/CGExprConstant.cpp:1131-1132
+    case CK_NullToPointer: {
+      if (llvm::Constant *C = Visit(subExpr, destType))
+        if (C->isNullValue())
+          return CGM.EmitNullConstant(destType);
----------------
efriedma wrote:
> nickdesaulniers wrote:
> > FWIW, I would have thought these might be unnecessary, but we do trip 2 tests without these guards IIRC.
> The fact that the Visit can fail doesn't surprise me; you can write arbitrary expressions of type nullptr_t (for example, `nullptr_t f(); void* g = f();`.  I can't think of any reason you'd need to check C->isNullValue(), though; do you have a testcase for that?
Failed Tests (2):
  Clang :: CodeGenCXX/const-init-cxx11.cpp
  Clang :: CodeGenCXX/cxx11-thread-local-instantiated.cpp

Looking at the first:

```
decltype(nullptr) null();
int *p = null();
```
and the corresponding AST:
```
|-FunctionDecl 0x5650ebbb44a8 <x.cpp:2:3, col:26> col:21 used null 'decltype(nullptr) ()'
`-VarDecl 0x5650ebbb45e0 <line:3:3, col:17> col:8 p 'int *' cinit
  `-ImplicitCastExpr 0x5650ebbb4740 <col:12, col:17> 'int *' <NullToPointer>
    `-CallExpr 0x5650ebbb4720 <col:12, col:17> 'decltype(nullptr)':'std::nullptr_t'
      `-ImplicitCastExpr 0x5650ebbb4708 <col:12> 'decltype(nullptr) (*)()' <FunctionToPointerDecay>
        `-DeclRefExpr 0x5650ebbb4690 <col:12> 'decltype(nullptr) ()' lvalue Function 0x5650ebbb44a8 'null' 'decltype(nullptr) ()'
```


we change the existing test case from having a `__cxx_global_var_init` routine, i.e.:
```
@p = dso_local global ptr null, align 8
@llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @_GLOBAL__sub_I_x.cpp, ptr null }]

; Function Attrs: noinline uwtable
define internal void @__cxx_global_var_init() #0 section ".text.startup" {
entry:
  %call = call ptr @_Z4nullv()
  store ptr null, ptr @p, align 8
  ret void
}

declare ptr @_Z4nullv() #1

; Function Attrs: noinline uwtable
define internal void @_GLOBAL__sub_I_x.cpp() #0 section ".text.startup" {
entry:
  call void @__cxx_global_var_init()
  ret void
}
```
to simply:
```
@p = dso_local global ptr null, align 8
```
So it seems nice to skip having the static constructors, but then `@_Z4nullv` is never invoked.  That seems problematic? (I expect the fast path to fail once it recurses down to the DeclRefExpr, but that requires the subexpr of the ImplicitCastExpr be visited).

---

So I think the code as written is good to go. WDYT?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D156175/new/

https://reviews.llvm.org/D156175