> On May 19, 2016, at 9:07 AM, John McCall via swift-dev <swift-dev@swift.org>
> wrote:
>
>> On May 18, 2016, at 1:51 PM, Saleem Abdulrasool <compn...@compnerd.org>
>> wrote:
>> Hi,
>>
>> It seems that there are assumptions about the ability to create relative
>> address across sections which doesn't seem possible on Windows ARM.
>>
>> Consider the following swift code:
>>
>> final class _ContiguousArrayStorage<Element> { }
>>
>> When compiled for Windows x86 (via swiftc -c -target i686-windows
>> -parse-as-library -parse-stdlib -module-name Swift -o Swift.obj
>> reduced.swift) it will generate the metadata pattern as:
>>
>> __TMPCs23_ContiguousArrayStorage:
>> ...
>> .long
>> __TMnCs23_ContiguousArrayStorage-(__MPCs23_ContiguousArrayStorage+128)
>> ...
>>
>> This generates a IMAGE_REL_I386_REL32 relocation which is the 32-bit
>> relative displacement of the target.
>>
>> On Windows ARM (swiftc -c -target i686-windows -parse-pas-library
>> -parse-stdlib -module-name Swift -o Swift.obj reduced.swift) it will
>> generate similar assembly:
>>
>> _TMPCs23_ContiguousArrayStorage:
>> ...
>> .long
>> _TMnCs23_ContiguousArrayStorage-(_MPCs23_ContiguousArrayStorage+128)
>> ...
>>
>> However, this generates an IMAGE_REL_ARM_ADDR32 relocation which is the
>> 32-bit VA of the target. If the symbol are in the same section, it is
>> possible to get a relative value. However, I don't really see a way to
>> generate a relative offset across sections. There is no relocation in the
>> COFF ARM specification which provides the 32-bit relative displacement of
>> the target. There are 20, 23, and 24 bit relative displacements designed
>> specifically for branch instructions, but none that would operate on generic
>> data.
>>
>> Is there a good way to address this ABI issue? Or perhaps do we need
>> something more invasive to support such targets? Now, I might be completely
>> overlooking something simple that I didn't consider, so pointing that out
>> would be greatly appreciated as well.
>
> You can build PIC on Windows ARM, right? How does Microsoft compile this:
>
> static int x;
> int *get_x_addr() { return &x; }
I'm not familiar with Windows ARM, but if image loading works like Windows on
Intel, then PIC isn't a thing—everything is compiled for a fixed address, and
the kernel brute-force slides all the addresses at load time if it can't map an
exe or dll at its preferred address.
-Joe
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