llvm dev - Jul 2018 - Zero-sized globals in LLVM IR

Hello the list,

What is the correct type for a global of size zero?  I need the compiler to be
able to generate one, so that the linker will insert it at a specific position
without perturbing the location of anything else in the section.  I have tried a
zero-length array (generates something at least one byte).  At Nuno’s
suggestion, I tried a structure with no fields.  In release builds, this works
correctly, but in debug builds it triggers an assertion that the type must be
sized.

What is the recommended way of doing this?

David

I believe C says you can't have one. Every variable must have a distinct
address. I don't know what LLVM IR says, but you could maybe insert an asm
label.


On Tue, Jul 17, 2018 at 3:58 AM, David Chisnall via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello the list,
>
> What is the correct type for a global of size zero?  I need the compiler
> to be able to generate one, so that the linker will insert it at a specific
> position without perturbing the location of anything else in the section.
> I have tried a zero-length array (generates something at least one byte).
> At Nuno’s suggestion, I tried a structure with no fields.  In release
> builds, this works correctly, but in debug builds it triggers an assertion
> that the type must be sized.
>
> What is the recommended way of doing this?
>
> David
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On 17 Jul 2018, at 12:16, Bruce Hoult <brucehoult at sifive.com>
wrote:
> 
> I believe C says you can't have one. Every variable must have a
distinct address. I don't know what LLVM IR says, but you could maybe insert
an asm label.
I’m aware that C doesn’t permit this, but LLVM is intended to be a portable
abstraction between source languages and target binary formats, and all
supported object file formats do support it.  In PE/COFF, this is the standard
way of implementing the equivalent of the GNU linker’s __start_ magic symbols,
for example.

David

How did you come to the conclusion that a zero-length array generates 
something of at least of size 1?  I don't see a significant difference 
between the assembly output of "@a1 = global [0 x i8] []" and
"@a2 =
global {} {}".

	.type	a1, at object              # @a1
	.data
	.globl	a1
a1:
	.size	a1, 0

	.type	a2, at object              # @a2
	.bss
	.globl	a2
	.p2align	3
a2:
	.size	a2, 0

Subjectively, I'd consider "@a2 = global {} {}" to be the
"idiomatic"
way to define a zero-sized global.  IMHO it shouldn't trigger an 
assertion.

On 2018-07-17 12:58, David Chisnall via llvm-dev wrote:
> Hello the list,
> 
> What is the correct type for a global of size zero?  I need the
> compiler to be able to generate one, so that the linker will insert it
> at a specific position without perturbing the location of anything
> else in the section.  I have tried a zero-length array (generates
> something at least one byte).  At Nuno’s suggestion, I tried a
> structure with no fields.  In release builds, this works correctly,
> but in debug builds it triggers an assertion that the type must be
> sized.
> 
> What is the recommended way of doing this?
> 
> David
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Hi David,

You wrote
>>>but in debug builds it triggers an assertion that the type must be
sized
I am curious as to which specific assert you're encountering.

Context: zero-sized structures and arrays are legal in the Go language; so
far in our (in-progress) gollvm work we haven't seen issues or asserts with
these sorts of constructs.

Thanks, Than

On Tue, Jul 17, 2018 at 6:59 AM David Chisnall via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello the list,
>
> What is the correct type for a global of size zero?  I need the compiler
> to be able to generate one, so that the linker will insert it at a specific
> position without perturbing the location of anything else in the section.
> I have tried a zero-length array (generates something at least one byte).
> At Nuno’s suggestion, I tried a structure with no fields.  In release
> builds, this works correctly, but in debug builds it triggers an assertion
> that the type must be sized.
>
> What is the recommended way of doing this?
>
> David
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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