[Git][ghc/ghc][master] bytecode: Do not generate `SLIDE x 0` instructions

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
bf8c7d6e by Matthew Pickering at 2025-01-29T02:31:44-05:00
bytecode: Do not generate `SLIDE x 0` instructions

SLIDE x 0 is a no-op as it means to shift x elements of the stack by no
spaces. In the interpreter, this results in a loop which copies an array
element into the same place.

I have instrumented GHCi to count how many of these instructions are interpreted.
The workload was `ghc` compiling two simple modules.

Total no-op slides: 7793476
Total slides: 11413289
Percentage useless (slides): 68%
Percentage uselss of total instructions: 9%

- - - - -


1 changed file:

- compiler/GHC/StgToByteCode.hs


Changes:

=====================================
compiler/GHC/StgToByteCode.hs
=====================================
@@ -548,7 +548,7 @@ returnUnliftedReps d s szb reps = do
                         PUSH_BCO tuple_bco `consOL`
                         unitOL RETURN_TUPLE
     return ( mkSlideB platform szb (d - s) -- clear to sequel
-             `consOL` ret)                 -- go
+             `appOL` ret)                 -- go
 
 -- construct and return an unboxed tuple
 returnUnboxedTuple
@@ -812,7 +812,7 @@ doTailCall init_d s p fn args = do
         platform <- profilePlatform <$> getProfile
         assert (sz == wordSize platform) return ()
         let slide = mkSlideB platform (d - init_d + wordSize platform) (init_d - s)
-        return (push_fn `appOL` (slide `consOL` unitOL ENTER))
+        return (push_fn `appOL` (slide `appOL` unitOL ENTER))
   do_pushes !d args reps = do
       let (push_apply, n, rest_of_reps) = findPushSeq reps
           (these_args, rest_of_args) = splitAt n args
@@ -1531,7 +1531,7 @@ generatePrimCall d s p target _mb_unit _result_ty args
               (push_target `consOL`
                push_info `consOL`
                PUSH_BCO args_bco `consOL`
-               (mkSlideB platform szb (d - s) `consOL` unitOL PRIMCALL))
+               (mkSlideB platform szb (d - s) `appOL` unitOL PRIMCALL))
 
 -- -----------------------------------------------------------------------------
 -- Deal with a CCall.
@@ -2266,8 +2266,8 @@ unsupportedCConvException = throwGhcException (ProgramError
   ("Error: bytecode compiler can't handle some foreign calling conventions\n"++
    "  Workaround: use -fobject-code, or compile this module to .o separately."))
 
-mkSlideB :: Platform -> ByteOff -> ByteOff -> BCInstr
-mkSlideB platform nb db = SLIDE n d
+mkSlideB :: Platform -> ByteOff -> ByteOff -> OrdList BCInstr
+mkSlideB platform nb db = mkSlideW n d
   where
     !n = bytesToWords platform nb
     !d = bytesToWords platform db



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/bf8c7d6edf224c083c90ab3ddac49979ac18ad39

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/bf8c7d6edf224c083c90ab3ddac49979ac18ad39
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