Allow packing constructor fields

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

1 files changed, 4 insertions, 0 deletions

diff --git a/includes/stg/Types.h b/includes/stg/Types.h
index af6a51791c..91ad446993 100644
--- a/includes/stg/Types.h
+++ b/includes/stg/Types.h
@@ -68,6 +68,8 @@ typedef uint8_t                  StgWord8;
 #define STG_INT8_MAX             INT8_MAX
 #define STG_WORD8_MAX            UINT8_MAX
 
+#define FMT_Word8                PRIu8
+
 typedef int16_t                  StgInt16;
 typedef uint16_t                 StgWord16;
 
@@ -75,6 +77,8 @@ typedef uint16_t                 StgWord16;
 #define STG_INT16_MAX            INT16_MAX
 #define STG_WORD16_MAX           UINT16_MAX
 
+#define FMT_Word16               PRIu16
+
 typedef int32_t                  StgInt32;
 typedef uint32_t                 StgWord32;