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Re: Clang is using the wrong memory model

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:
> 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 Vinschen
Cygwin Maintainer

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