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, 6 insertions, 3 deletions

diff --git a/compiler/cmm/SMRep.hs b/compiler/cmm/SMRep.hs
index 34048fe116..1469ae1bd3 100644
--- a/compiler/cmm/SMRep.hs
+++ b/compiler/cmm/SMRep.hs
@@ -9,7 +9,7 @@ module SMRep (
         -- * Words and bytes
         WordOff, ByteOff,
         wordsToBytes, bytesToWordsRoundUp,
-        roundUpToWords,
+        roundUpToWords, roundUpTo,
 
         StgWord, fromStgWord, toStgWord,
         StgHalfWord, fromStgHalfWord, toStgHalfWord,
@@ -79,8 +79,11 @@ type ByteOff = Int
 -- | Round up the given byte count to the next byte count that's a
 -- multiple of the machine's word size.
 roundUpToWords :: DynFlags -> ByteOff -> ByteOff
-roundUpToWords dflags n =
-  (n + (wORD_SIZE dflags - 1)) .&. (complement (wORD_SIZE dflags - 1))
+roundUpToWords dflags n = roundUpTo n (wORD_SIZE dflags)
+
+-- | Round up @base@ to a multiple of @size@.
+roundUpTo :: ByteOff -> ByteOff -> ByteOff
+roundUpTo base size = (base + (size - 1)) .&. (complement (size - 1))
 
 -- | Convert the given number of words to a number of bytes.
 --