Clang is using the wrong memory model

[Agner Fog]

Cygwin Clang is using -mcmodel=medium as default for Win64, according to 
my tests, while the right model is -mcmodel=small

Linux Clang with --target=x86_64-pc-cygwin gives the small memory model.

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.

I told the LLVM guy that this difference needs to be documented so that 
you can rely on it, but this request has so far been ignored.

He says that you must have modified the source code to change the 
default memory model.

The medium memory model gives inefficient code because it uses an extra 
instruction to load a 64-bit absolute address into a register before 
every access to static data.

I cannot blame you Cygwin people for not using the small memory model as 
long as it is undocumented.

I will ask you to please join the discussion at 
https://bugs.llvm.org/show_bug.cgi?id=42983 so that we can clarify how 
to solve this problem.


On 14/08/2019 07.51, Agner Fog wrote:
> It's a difference in memory model.
>
> clang 6.0.0 under ubuntu with --target=x86_64-pc-cygwin gives relative 
> addresses, unless you specify -mcmodel=large.
>
> Cygwin clang with -mcmodel=small does the right thing: use relative 
> addresses.
>
> The -mcmodel=small option appears to work differently for Linux and 
> for Windows targets. I cannot find any documentation of this 
> difference. See:
>
> https://bugs.llvm.org/show_bug.cgi?id=42983
>
>
> On 12/08/2019 11.45, falk.tannhauser@free.fr wrote:
>> References: <578eb489-9391-9009-82ad-676eeb4c1c92@agner.org>
>> In-Reply-To: <578eb489-9391-9009-82ad-676eeb4c1c92@agner.org>
>>
>> Could the different behaviour between Cygwin and Linux simply be due 
>> to different Clang versions?
>> The current version under Cygwin is 5.0.1, while the latest version 
>> available under Linux
>> appears to be 8.0.1 .
>>
>> Falk
>>
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