[decompiler] rewrite set lets as just sets

[ManDude]

Rewrites specific kinds of lets where the return value of the set! itself is meant to be used to just be set!s. Implemented at the let rewrite level. Seems to work for Jak 2 so far.

Fixes #1854 .

[water111]

I think this change is unsafe to make - for example:

(let ((temp (foo)))
  (set! (-> (bar) field) temp)
  )

will call foo before bar, but

(set! (-> (bar) field) (foo))

will call bar before foo.

Which is why I think this detection has to happen before expression building.
If we do it here, we also lose the ability to detect things like *!

[ManDude]

I was wondering that too, but it looks like set! is an exception to the left-to-right eval order. The source is eval'd before the destination.

[ManDude]

Math macros like *! also seemed unaffected, looking at the refs.

[water111]

Yeah it looks like you're right - both the compiler and decompiler agree about this, and I made a comment about it being flipped here

I think that means this is safe, but I do want to think about it a little more.

The *! thing would only apply in cases where we don't detect *! currently - if you had something that was originally (*! foo bar), but it got split up due to this set/let issue, then this fix will give us (set! foo (* foo bar)). Which I wouldn't worry about too much.

A few other things I want to think about

• uint128/vf stuff : seems like it doesn't get applied for vf stuff because that doesn't become a set
• dropping casts for float/int convers: I think it won't but I want to review some more
• dropping casts for (the-as - not sure, I kinda think this will be wrong
• bitfield stuff: the compile makes set! on a bitfield return none, but we could change this

[ManDude]

Only problem might be that it won't cast things like enums or time-frame properly (see seagull). Hasn't caused actual issues so far though so I won't spend time fixing it right now.