[PATCH] D59467: [clang] Adding the Likely Attribute from C++2a to AST

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D59467#1439585 <https://reviews.llvm.org/D59467#1439585>, @Tyker wrote:

> about the semantic issue.
>  with the standard's latitude on where we can put the attribute, the attribute can appear any where on the path of execution and must be matched with the condition it corresponds to. we need to do it for diagnostics purposes to detect conflicts and repetitions. and we need it again for the codegen. but where in the ast can we store this information ?


I feel like this could be tracked in the StmtBits as well as via an attributed stmt node. We need to be able to handle cases like:

  if (foo) [[likely]] {
    stmts...
  }
  
  if (bar) {
    stmts...
    [[likely]];
    stmts...
  }
  
  if (baz)
    [[likely]] stmt;

The attributed statement is required because we want ast pretty printing to also print out the `[[likely]]` attribute usage and AST matchers to appropriately find the attribute, etc, and we need to know which statement the attribute was attached to in order to get the positioning correct. However, for diagnostic purposes, we likely want to know information like "has this path already been attributed" and that can either be through some extra data on an AST node, or it could be an Analysis pass like we do for `[[fallthrough]]` support with a separate pass for CodeGen.

> is adding information to conditional statements Node acceptable ?

You can generally add information to AST nodes, but pay close attention to things like adding bits to a bit-field that suddenly cause the object size to increase and see if there are ways to avoid that if possible (and if not, it's still fine).

It might make sense to work your way backwards from the backend to see what possible ways we can support this feature. `__builtin_expect` may make sense for if statements, but for case and default labels in a switch, we may want to play with the branch weights. For catch handlers, we may need to thread information through yet another way. Once we know what kinds of paths we can actually do something interesting with, we may spot a pattern of how to expose the information through the AST.


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