yaxunl marked 3 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:
> > > 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.
> Are you sure? All of my attempts to produce these constants in LLVM seem to
> get instantly folded, even without a target set in the IR:
> i32 ptrtoint (i8* inttoptr (i32 -1 to i8*) to i32)
> i64 ptrtoint (i8* inttoptr (i64 -1 to i8*) to i64)
> This folding literally occurs within ConstantExpr::getPtrToInt, without any
> passes running, and it seems to happen regardless of the pointer having an
> addrspace.
>
> I suspect that the problem is that you additionally have an addrspacecast
> constant in place, which almost certainly does block the constant folder.
> The right fix for that is to ensure that your target's implementation of
> performAddrSpaceCast makes an effort to peephole casts of Constants.
In amdgpu target, null pointer in addr space 0 is represented as `addrspacecast
int addrspace(4)* null to int*` instead of `inttoptr i32 -1 to i8*`. As
addrspacecast is opaque to target-independent LLVM passes, they don't know how
to fold it. Only the backend knows how to fold it.
This could be a general situation, i.e., the non-zero null pointer is
represented in some target specific form which LLVM does not know how to fold.
https://reviews.llvm.org/D26196
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