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Generating testcases

Get the offending Rust code as LLVM IR

cargo build --release --target=./arduino.json --verbose
# grab the `rustc ...` invocation
# add `--emit=llvm-ir`
# Look for `target/arduino/release/deps/foo.0.ll`

Build LLVM

Basically, recreate what the build system does

cmake \
  -D CMAKE_BUILD_TYPE=Debug \
  -D LLVM_ENABLE_ASSERTIONS=ON \
  -D LLVM_TARGETS_TO_BUILD="X86" \
  -D LLVM_EXPERIMENTAL_TARGETS_TO_BUILD="AVR" \
  -D LLVM_INCLUDE_EXAMPLES=OFF \
  -D LLVM_INCLUDE_TESTS=OFF \
  -D LLVM_INCLUDE_DOCS=OFF \
  -D LLVM_ENABLE_ZLIB=OFF \
  -D WITH_POLLY=OFF \
  -D LLVM_ENABLE_TERMINFO=OFF \
  -D LLVM_ENABLE_LIBEDIT=OFF \
  ..

Reduce the testcase

Create the two supporting files, then run it against your LLVM IR (./bugpoint.sh myfile.ll). This will produce a bugpoint-reduced-simplified.bc, which you can convert back to IR with llvm-dis.

bugpoint.sh

#!/bin/bash

export PATH=../debug/bin/:$PATH

bugpoint $1 -compile-custom -compile-command ./repro.sh -safe-tool-args='-march=avr -mcpu=atmega328p -filetype=obj'

repro.sh

Edit this file to look for your error

#!/bin/bash

set -eux

! llc -march=avr -mcpu=atmega328p -filetype=obj $1 2>&1 | grep 'No instruction defining live value'

Apply human touch

Look at the LLVM IR. Are there a bunch of defined types or functions that aren't needed? Try removing them. If you remove a bunch, try throwing it back into bugpoint. Repeat until it's "small enough" or you give up.

Metadata generally lives at the bottom of the IR file and is prefixed with !. You should be able to remove metadata from the testcase in almost every single case and it will have no effect on the test itself. If metadata is deleted, make sure to delete references to it.

Metadata references usually live after function declarations like so

define void @foo() !3 {
   ...
}

In this case, to remove the metadata reference, simply remove !3