cmd/compile/internal/inline: hairyVisitor doesn't properly account for conditionally-inlined PGO calls

[prattmic]

In hairVisitor.doNode we give calls a cost of 57, unless the call is inlineable, in which case we use the cost of the callee, under the assumption that this call will get inlined.

"Inlineable" here more-or-less just means that fn.Inl != nil. In a non-PGO build, we only set fn.Inl if the function cost is < 80, making it inlineable ~anywhere.

In a PGO build, fn.Inl is set for any function involved in a hot callsite (e.g., a -> b), but we will only inline at the hot callsite. Any other callsite (e.g., c -> b) will not inline. This makes the assumption incorrect and means that the cold caller (c) will have an artificially inflated cost, potentially making it too expensive to inline.

I will send a CL to fix this.

cc @cherrymui @thanm @mdempsky

[gopherbot]

Change https://go.dev/cl/483196 mentions this issue: cmd/compile: synchronize inlinability logic between hairyVisitor and mkinlcall