I was trying to find out if it works but I ran into the problem I described. Yes, there were a few years where the latest version didn't work but in many years it did. ghc used to give a warning, this is an unsupported version but we'll try it anyways. I would like to return to that. 

In any case I believe ghc dev should test to see if it works and if so use it in HEAD. If it doesn't work than we should file a bug and fix it. In the past we got a few versions behind llvm. I think we want to be on the latest version available for each new release if possible. The earlier we look into this the more chance we have to succeed at that.

On Tue, Apr 1, 2025 at 1:27 PM Brandon Allbery <allbery.b@gmail.com> wrote:
Have you demonstrated that it works? IIRC the current behavior is because some LLVM version (16, IIRC) didn't work with GHC (threw errors from opt, I think).

On Tue, Apr 1, 2025 at 10:49 AM George Colpitts <george.colpitts@gmail.com> wrote:
llvm 20 is out but unlike in earlier versions of ghc moving to it means you can no longer use llvm:

compiling:
ghc -fllvm hello.hs
Loaded package environment from /Users/avie/.ghc/aarch64-darwin-9.12.2/environments/default
[1 of 2] Compiling Main             ( hello.hs, hello.o )
<no location info>: error: [GHC-66599]
    GHC was not configured with a supported LLVM toolchain
    Make sure you have installed LLVM between [13 and 20) and reinstall GHC to make -fllvm work

from configure:
configure: We only support llvm 13 upto 20 (non-inclusive) (found 20.1.1).

Can we move to llvm 20 on HEAD and can we revert to the old behavior on ghc 9.12.3?

Should I file an ER?

Thanks

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brandon s allbery kf8nh