carbon-lang/toolchain/docs/lower.md

Lower

Table of contents

• Overview
• Mangling
  • Examples

Overview

Lowering takes the SemIR and produces LLVM IR. At present, this is done in a single pass, although it's possible we may need to do a second pass so that we can first generate type information for function arguments.

Lowering is done per SemIR::InstBlock. This minimizes changes to the IRBuilder insertion point, something that is both expensive and potentially fragile.

Mangling

Part of lowering is choosing deterministically unique identifiers for each lowered entity to use in platform object files. Any feature of an entity (such as parent namespaces or overloaded function parameters) that would create a distinct entity must be included in some way in the generated identifier.

The current rudimentary name mangling scheme is as follows:

• As a special case, Main.Run is emitted as main.

Otherwise the resulting name consists of:

1. _C
2. The unqualified function name (function name mangling is the only thing implemented at the moment).
3. .
4. If the function being mangled is a member of:
   • an impl, then add:
     1. The implementing type, per the scope mangling.
     2. :
     3. The interface type, per the scope mangling.
   • a type or namespace, then add:
     1. The scope, per the scope mangling.

The scope mangling scheme is as follows:

1. The unqualified name of the type or namespace.
2. If the type or namespace is within another type or namespace:
   1. .
   2. The enclosing scope, per the scope mangling.
3. .
4. The package name.

Examples

package P1;
interface Interface {
  fn Op[self: Self]();
}

namespace NameSpace;
class NameSpace.Implementation {
  // Mangled as:
  // `_COp.Implementation.NameSpace.Main:Interface.P1`
  impl as P1.Interface {
    fn Op[self: Self]() {
    }
  }
}
// Mangled as `main`.
fn Run() {
  var v: NameSpace.Implementation;
  v.(P1.Interface.Op)();
}