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This is another step for fixing #13825 and is based on D38 by Simon
Marlow.
The change allows storing multiple constructor fields within the same
word. This currently applies only to `Float`s, e.g.,
```
data Foo = Foo {-# UNPACK #-} !Float {-# UNPACK #-} !Float
```
on 64-bit arch, will now store both fields within the same constructor
word. For `WordX/IntX` we'll need to introduce new primop types.
Main changes:
- We now use sizes in bytes when we compute the offsets for
constructor fields in `StgCmmLayout` and introduce padding if
necessary (word-sized fields are still word-aligned)
- `ByteCodeGen` had to be updated to correctly construct the data
types. This required some new bytecode instructions to allow pushing
things that are not full words onto the stack (and updating
`Interpreter.c`). Note that we only use the packed stuff when
constructing data types (i.e., for `PACK`), in all other cases the
behavior should not change.
- `RtClosureInspect` was changed to handle the new layout when
extracting subterms. This seems to be used by things like `:print`.
I've also added a test for this.
- I deviated slightly from Simon's approach and use `PrimRep` instead
of `ArgRep` for computing the size of fields. This seemed more
natural and in the future we'll probably want to introduce new
primitive types (e.g., `Int8#`) and `PrimRep` seems like a better
place to do that (where we already have `Int64Rep` for example).
`ArgRep` on the other hand seems to be more focused on calling
functions.
Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>
Test Plan: ./validate
Reviewers: bgamari, simonmar, austin, hvr, goldfire, erikd
Reviewed By: bgamari
Subscribers: maoe, rwbarton, thomie
GHC Trac Issues: #13825
Differential Revision: https://phabricator.haskell.org/D3809
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