Brendan Hansknecht (May 01 2024 at 20:27): Brendan Hansknecht (May 01 2024 at 20:27):Just was listening to some LLVM terrible compilation time issues:
In this example, someone made a tuple with 2000 elements (all u8). Took 3 seconds to compile.
They then made a tuple with 5000 elements.... it took 2 minutes to compile.
This is for like 7 lines of code that just creates a zeroed tuple and then sets a few values in it.
Brendan Hansknecht (May 01 2024 at 20:30):The reason was that the generated llvm ir did something like:
%val = load [2000 x u8] %mem
And in the 5000 case, just swap out the number with 5000.
Why does this explode llvm: ssa values in llvm are meant to be passed in registers
Brendan Hansknecht (May 01 2024 at 20:30):So this was asking llvm to simply load the value into the 2000 registers it has lying around.
Brendan Hansknecht (May 01 2024 at 20:31):Or 5000 in the worse case.
Brendan Hansknecht (May 01 2024 at 20:31):Fundamental comment/lesson: llvm ssa values are meant to fit in roughly 2 or 3 registers. Anything else should never be an ssa value.
Brendan Hansknecht (May 01 2024 at 20:32):llvm is not built for that and will explode.
Brendan Hansknecht (May 01 2024 at 20:35):Just a general reminder of things we should make sure to avoid in our llvm ir.
Last updated: Jul 26 2025 at 12:14 UTC