[PATCH] D26196: Add support for non-zero null pointer for C and OpenCL

[Yaxun Liu via Phabricator via cfe-commits]

yaxunl marked 4 inline comments as done.
yaxunl added inline comments.


================
Comment at: lib/CodeGen/CGExprScalar.cpp:1532
+      return llvm::ConstantInt::get(ConvertType(DestTy),
+          CGF.getContext().getTargetNullPtrValue(E->getType()));
     assert(!DestTy->isBooleanType() && "bool should use PointerToBool");
----------------
rjmccall wrote:
> yaxunl wrote:
> > rjmccall wrote:
> > > Why is this necessary?  ptrtoint on the recursively-emitted null pointer should do this automatically.
> > Since the target knows the value in the null pointers, it can fold a null pointer to integer literal directly.
> > 
> > The above code does that, e.g.
> > 
> > ```
> > void test_cast_null_pointer_to_sizet_calee(size_t arg_private,
> >                                            size_t arg_local,
> >                                            size_t arg_global,
> >                                            size_t arg_constant,
> >                                            size_t arg_generic);
> > 
> > // CHECK-LABEL: test_cast_null_pointer_to_sizet
> > // CHECK: call void @test_cast_null_pointer_to_sizet_calee(i64 -1, i64 -1, i64 0, i64 0, i64 0)
> > void test_cast_null_pointer_to_sizet(void) {
> >   test_cast_null_pointer_to_sizet_calee((size_t)((private char*)0),
> >                                         (size_t)((local char*)0),
> >                                         (size_t)((global char*)0),
> >                                         (size_t)((constant char*)0),
> >                                         (size_t)((generic char*)0));
> > }
> > 
> > ```
> > 
> > Without the above code, we only get ptrtoint instructions.
> Oh, does the constant folder not know how to fold ptrtoint(inttoptr(X)) -> X?  I guess that's probably LLVM's nominal target-independence rearing its head.
> 
> Is getting a slightly LLVM constant actually important here?  I would prefer to avoid this complexity (and unnecessary recursive walk) if possible.
Since it's target dependent, it won't be constant folded by the target agnostic passes until very late in the backend, which may lose optimization opportunities. I think it's better folded by FE like other constants.


https://reviews.llvm.org/D26196