Clang is using the wrong memory model

[Agner Fog]

Thanks for your replies.

A Cygwin application with -mcmodel=small appears to work fine.

As I explained, -mcmodel=small does something else when the target is 
Windows. It does not require addresses to be below 2GB, it only requires 
the distance between a code section and it's data section to be below 2GB.

Your concern would be right if -mcmodel=small was doing the same thing 
as it does in Linux, but this is not the case.

It does not matter where the application and the Cygwin DLL are placed 
because Windows allows EXEs and DLLs to be placed at any address. There 
is no static linking between an application and the Cygwin DLL, so the 
distance between application and DLL does not matter.

I don't see any problem with fork() either, because there is no static 
link between parent and child process. The OS puts everything in place 
using virtual addresses.

Agner


On 16/08/2019 10.26, Corinna Vinschen wrote:
> On Aug 16 08:06, Agner Fog wrote:
>> Cygwin Clang is using -mcmodel=medium as default for Win64, according to my
>> tests, while the right model is -mcmodel=small
> -mcmodel=small is *only* the right model if the target is native
> Windows.  If the target is a Cygwin application it *must* at least
> be compiled with -mcmodel=medium.  The reason is the standarized
> memory layout of Cygwin application and DLLs.
>
>> Linux Clang with --target=x86_64-pc-cygwin gives the small memory model.
> Which is wrong.
>
>> I took this to the LLVM Bugzilla as you asked me to:
>> https://bugs.llvm.org/show_bug.cgi?id=42983
>>
>> This gave the following conclusion:
>>
>> -mcmodel=small does something different when the target is Windows. This
>> difference appears to be undocumented. The small memory model with a Linux
>> target puts everything below the 2GB limit so that 32-bit absolute addresses
>> can be used. The small memory model with a Windows target is using 32-bit
>> relative addresses instead, which is the correct thing to do in Windows.
> Yes, but not for Cygwin applications and DLLs.  The reason is that Cygwin
> apps and DLLs reside in the memory beyond the first 2 Gigs in a standarized
> way so as not to collide with Windows code and datastructures.  And that
> in turn was necessary to make fork() more reliable on 64 bit.
>
> Here's the memory layout:
>
> 0000:00000000	Windows
> 0000:80000000	Thread stacks
> 0001:00400000	Executable
> 0001:80000000	*The* Cygwin DLL
> 0002:00000000	Rebased Cygwin DLLs
> 0004:00000000	Unrebased Cygwin DLLs(*)
>
> 0006:00000000	Heap (up to Cygwin 3.0)
> 0008:00000000	Heap (starting with Cygwin 3.1)
>
> 0700:00000000	Top-down start address for mmaps up to Windows 8
> 		or up to Cygwin 3.0
> 7000:00000000	Top-down start address for mmaps starting with Windows 8.1
> 		and Cygwin 3.1
>
> So there's a difference between non-Cygwin (-mcmodel=small as default)
> and Cygwin (at least -mcmodel=medium).
>
>
> Corinna
>

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