Segfault in 'Simplify the CFG' pass when discarding a capture in a match inside a loop

[burjui]

actor Main
  new create(env: Env) =>
    let x: (None | U8) = 1
    while true do
      let _: None = match x
        | let _: U8 => None
      end
    end

This causes a segfault on Arch Linux x86_64 with Pony 0.22.6 and on issue-2760 branch:

$ ponyc
...
0.      Running pass 'Simplify the CFG' on function '@Main_tag_create_oo'
[1]    18802 segmentation fault (core dumped)  ponyc

GDB stacktrace:

#0  0x00007ffff4f747c4 in llvm::removeUnreachableBlocks(llvm::Function&, llvm::LazyValueInfo*) () from /usr/lib/libLLVM-6.0.so
#1  0x00007ffff5376dab in ?? () from /usr/lib/libLLVM-6.0.so
#2  0x00007ffff47efdc1 in llvm::FPPassManager::runOnFunction(llvm::Function&) ()
   from /usr/lib/libLLVM-6.0.so
#3  0x00007ffff47efebd in llvm::legacy::FunctionPassManagerImpl::run(llvm::Function&) ()
   from /usr/lib/libLLVM-6.0.so
#4  0x00007ffff47f0405 in llvm::legacy::FunctionPassManager::run(llvm::Function&) ()
   from /usr/lib/libLLVM-6.0.so
#5  0x0000555555611a43 in optimise(compile_t*, bool) ()
#6  0x0000555555611bef in genopt ()
#7  0x000055555560b45f in genexe ()
#8  0x000055555559806d in codegen ()
#9  0x000055555557ba9e in main ()

[SeanTAllen]

This goes back to the introduction of _.

This

actor Main
  new create(env: Env) =>
    let x: (None | U8) = 1
    while true do
      let y: None = match x
        | let z: U8 => None
      end
    end

compiles. So this is related to _.

I haven't determined exactly the commit when this happened. It might have always happened. I gave up eventually.

[dipinhora]

I tried to figure out when this was introduced. It was in #1499 when this syntax was first allowed. Prior to that commit, the syntax wasn't allowed.

Also, the following compiles successfully:

actor Main
  new create(env: Env) =>
    let x: (None | U8) = 1
    while true do
      let y: None = match x
        | let _: U8 => None
      end
    end

However, when compiled without --debug argument to ponyc, it results in an Illegal instruction error when the program is run.

The following also results in the segfault (Running pass 'Simplify the CFG' on function '@Main_tag_create_oo'):

actor Main
  new create(env: Env) =>
    let x: (None | U8) = 1
    while true do
      let _: None = match x
        | let y: U8 => None
      end
    end

Same segfault for this:

actor Main
  new create(env: Env) =>
    while true do
      let _: None = None
    end

However, the following compiles but has the same Illegal Instruction issue:

actor Main
  new create(env: Env) =>
    while true do
      let _: None = None
      let y: None = None
    end

The following also has the same segfault as the original report:

actor Main
  new create(env: Env) =>
    var y: Bool = true
    while y do
      let _: None = None
    end

However, the following compiles and runs successfully:

actor Main
  new create(env: Env) =>
    var y: Bool = true
    while y do
      let _: None = None
      y = false
    end

My guess is that this is related to optimizations that try and remove the while loop altogether because it is an infinite loop with no exit condition.

[burjui]

Just a note: such an "optimization" would violate the semantics of a program. It is safe to remove code after the infinite loop (with a warning, of course), because it's unreachable, but not the loop itself.

[SeanTAllen]

@burjui can you clarify what you mean by "remove the code after the infinite loop"?

[Praetonus]

This is a bug somewhere in genmatch.c where a terminator instruction isn't being generated for a basic block. I'll investigate.

[Praetonus]

Looks like there's also a type checking bug involved in this.

[burjui]

@SeanTAllen I mean that if there is some code after an infinite loop, it can be considered unreachable and safe to remove.

[SeanTAllen]

I'm no longer able to reproduce this bug.