[llvm-dev] ORC JIT Weekly #2 -- COFF COMDAT Constants and Emulated TLS

[Machiel van Hooren via llvm-dev]

I'm very pleased that PR40074/44337 has been squashed, great work! This 
means I can now use unmodified LLVM for my application and perhaps soon 
target a stable release. Will it be part of the LLVM 10 release?

Machiel

On 24-Jan-20 04:27, Lang Hames via llvm-dev wrote:
> Hi All,
>
> This week I've been focused on removing some of the blockers for 
> people transitioning from ORCv1 to ORCv2.
>
> Issue #1 (http://llvm.org/PR40074, http://llvm.org/PR44337):
>
> When LLVM codegens floating point constants for COFF we produce named 
> constant pool entries of the form __real@<bitval>. These are stored in 
> COFF COMDAT sections [1] which allow duplicate symbol definitions to 
> be discarded (roughly equivalent to the concept of a weak symbol). 
> This posed two problems: (1) ORC had no idea about the (rough) mapping 
> between COMDAT and Weak, and (2) did not support late introduction of 
> weak symbols via MaterializationResponsibility::defineMaterializing. 
> This caused use of the same floating point constant in multiple files 
> to result in duplicate definition errors for the constant pool entries.
>
> Both issues have been addressed in 84217ad6611, and floating point 
> constants should now work in COFF. If you run in to any trouble with 
> them please file a bug and CC me.
>
> Issue #2: Emulated TLS and name mangling (raised by Geoff Levner on 
> llvm-dev):
>
> Use of emulated-tls affects the name mangling of LLVM IR symbols. For 
> a thread local symbol with name <Name>, when emulated-tls is enabled 
> we will produce at least one emulated-tls symbol: __emutls_v.<Name> 
> (which must then be linker-mangled to get the final symbol name). If 
> that symbol has a non-zero initializer a second symbol will also be 
> produced: __emutls_t.<Name>. ORCv2 requires up-front specification of 
> the exact set of /linker-level/ symbol names that a module produces, 
> so that it can dispatch and track compiles, however none of the 
> existing mangling code took emulated-tls into account. This has been 
> fixed in ce2207abaf9 (with follow up fixes for the various examples I 
> broke), and you should now be able to use thread-locals in ORCv2.
>
> Besides these two bugs, I have continued work on the big static 
> initializer patch. Unfortunately issue #2 interacts with it, so I have 
> to do some integration work before it is ready to land, but I'm 
> hopeful that it will be ready some time next week.
>
> Finally, I have added a new example (bb7a5707ac0)to 
> llvm/examples/LLJITExamples: LLJITWithObjectLinkingLayerPlugin 
> demonstrating how to write a custom ObjectLinkingLayer plugin. 
> ObjectLinkingLayer plugins can be used to receive and respond to 
> events in the linking layer, and to inspect and modify JITLink 
> LinkGraphs during the linking process. ORC uses this internally to 
> register EH frames, but the system is likely to be of interest to 
> anyone who wants low level control/visibility into the JIT linker.
>
> — Lang.
>
> [1] 
> https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#comdat-sections-object-only 
>
>
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