[cfe-dev] Mixed 32b/64b pointers in same translation unit

[Patrik Eklöf via cfe-dev]

Useful information.

Thanks.

Regards,
Patrik Eklöf



From: Friedman, Eli [mailto:efriedma at codeaurora.org]
Sent: 02 February 2017 19:18
To: Patrik Eklöf <patrik.eklof at energymachines.com>; Reid Kleckner <rnk at google.com>
Cc: cfe-dev at lists.llvm.org
Subject: Re: [cfe-dev] Mixed 32b/64b pointers in same translation unit

On 2/2/2017 10:00 AM, Patrik Eklöf via cfe-dev wrote:
Thanks for the answer, Reid.

I tried your suggestion and it worked on simpler types where I initialize pointers to some constant value, e.g.

constexpr __attribute__((address_space(0))) int* HiIAmA64BitPtr{ nullptr };
constexpr __attribute__((address_space(4))) int* HiIAmA32BitPtr{ nullptr };

int main()
{
                static_assert(sizeof(HiIAmA32BitPtr) == 4);
                static_assert(sizeof(HiIAmA64BitPtr) == 8);
}

But when I tried something more complex, it stops working. For example:

void foo() {}
__attribute((address_space(4))) void bar() {}

constexpr __attribute__((address_space(0))) auto* HiIAmA64BitPtr{ &foo };
constexpr __attribute__((address_space(4))) auto* HiIAmA32BitPtr{ &bar };

int main()
{
                constexpr auto SizeOf32 = sizeof(HiIAmA32BitPtr);
                constexpr auto SizeOf64 = sizeof(HiIAmA64BitPtr);
                static_assert(sizeof(HiIAmA32BitPtr) == 4);
                static_assert(sizeof(HiIAmA64BitPtr) == 8);
}

From what I can gather, it tries to take the address space of the parent type (or the dereferenced type of a pointer). My guess is that in the former experiment, the attribute binds to the dereferenced type and not the pointer? In this case, it doesn’t seem to work, though. Presumably because the attribute binds to the pointer type? I’m not sure. Regardless, it doesn’t work, and I’m still not sure why.

The assert that the 32-bit pointer is 4 bytes fails.

Any ideas?


clang doesn't currently support address spaces on function pointers.  You're getting a weird result because the address space is getting attached to the return type; clang should reject that, but currently doesn't.

-Eli


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