LLVM error when optimizations are turned off

[dylanmckay]

When using rustc to compile for the AVR target, an LLVM error is generated:

LLVM ERROR: regalloc=... not currently supported with -O0

Which can be tracked back to llvm/lib/CodeGen/LiveVariables.cpp:635. There is a note at this line mentioning that this bug is an artifact of 'bizarre pass dependencies' and that it should be fixed. This leads me to believe that this error can be solved by either writing the fix specified on the aforementioned line, or simply changing the order in which Rust adds LLVM passes.

The AVR LLVM backend can successfully generate executables on its own (using llc), and so this bug must lay in Rust itself.

[dylanmckay]

The issue can be fixed by changing the optimisation level. This error will occur when compiling an AVR target with no optimisations (the default), but passing the -O' or--opt-level=2` flags will lead to a successful compilation.

[dylanmckay]

The issue is described in avr-llvm/llvm issue #15, and has now been fixed.

The bug no longer persists in avr-rust because we updated LLVM.