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 -- Corinna Vinschen Cygwin Maintainer
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