[LLVMdev] Let's get rid of neverHasSideEffects

[Jakob Stoklund Olesen]

On Aug 21, 2012, at 4:41 PM, Chris Lattner <clattner at apple.com> wrote:

> Personally, I don't like the direction of making everything be redundantly specified with "let" clauses and in the patterns.  I agree that it is a problem that we're not inferring from Pat<> patterns, and that not all instructions are expressible with patterns, but I'd rather we solve *those* problems than throw out inference.

After discussing this, we have reached a workable compromise. Here's what we'll do:

The inferred instruction properties will become tristate and default to Unset:

  bit hasSideEffects = ?;
  bit mayLoad = ?;
  bit mayStore = ?;

TableGen will attempt to infer these properties from any instruction patterns.

If inference fails and a property is unset, TableGen will issue an error instead of guessing.

If inference succeeds and a property is set to an inconsistent value, TableGen will issue an error.

If inference succeeds and a property is set to a consistent value, TableGen will happily and silently go about its business.

This will also handle migration for out-of-tree targets. They can replace 'neverHasSideEffects = 1' with 'hasSideEffects = 0', and any instructions without patterns will be pointed out for audit.

/jakob