Use lengthIs and friends in more places

While investigating #12545, I discovered several places in the code
that performed length-checks like so:

```
length ts == 4
```

This is not ideal, since the length of `ts` could be much longer than 4,
and we'd be doing way more work than necessary! There are already a slew
of helper functions in `Util` such as `lengthIs` that are designed to do
this efficiently, so I found every place where they ought to be used and
did just that. I also defined a couple more utility functions for list
length that were common patterns (e.g., `ltLength`).

Test Plan: ./validate

Reviewers: austin, hvr, goldfire, bgamari, simonmar

Reviewed By: bgamari, simonmar

Subscribers: goldfire, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3622

5 files changed, 8 insertions, 7 deletions

diff --git a/compiler/codeGen/StgCmmClosure.hs b/compiler/codeGen/StgCmmClosure.hs
index 37572b7d4e..8eaee795a5 100644
--- a/compiler/codeGen/StgCmmClosure.hs
+++ b/compiler/codeGen/StgCmmClosure.hs
@@ -553,7 +553,7 @@ getCallMethod dflags _ id _ n_args v_args _cg_loc
               (Just (self_loop_id, block_id, args))
   | gopt Opt_Loopification dflags
   , id == self_loop_id
-  , n_args - v_args == length args
+  , args `lengthIs` (n_args - v_args)
   -- If these patterns match then we know that:
   --   * loopification optimisation is turned on
   --   * function is performing a self-recursive call in a tail position
diff --git a/compiler/codeGen/StgCmmExpr.hs b/compiler/codeGen/StgCmmExpr.hs
index edf97eeb0a..6e6ad7e9d7 100644
--- a/compiler/codeGen/StgCmmExpr.hs
+++ b/compiler/codeGen/StgCmmExpr.hs
@@ -562,7 +562,7 @@ chooseReturnBndrs bndr (PrimAlt _) _alts
   = assertNonVoidIds [bndr]
 
 chooseReturnBndrs _bndr (MultiValAlt n) [(_, ids, _)]
-  = ASSERT2(n == length ids, ppr n $$ ppr ids $$ ppr _bndr)
+  = ASSERT2(ids `lengthIs` n, ppr n $$ ppr ids $$ ppr _bndr)
     assertNonVoidIds ids     -- 'bndr' is not assigned!
 
 chooseReturnBndrs bndr (AlgAlt _) _alts
diff --git a/compiler/codeGen/StgCmmLayout.hs b/compiler/codeGen/StgCmmLayout.hs
index dc80036b55..b123420d58 100644
--- a/compiler/codeGen/StgCmmLayout.hs
+++ b/compiler/codeGen/StgCmmLayout.hs
@@ -274,7 +274,7 @@ direct_call :: String
             -> CLabel -> RepArity
             -> [(ArgRep,Maybe CmmExpr)] -> FCode ReturnKind
 direct_call caller call_conv lbl arity args
-  | debugIsOn && real_arity > length args  -- Too few args
+  | debugIsOn && args `lengthLessThan` real_arity  -- Too few args
   = do -- Caller should ensure that there enough args!
        pprPanic "direct_call" $
             text caller <+> ppr arity <+>
diff --git a/compiler/codeGen/StgCmmPrim.hs b/compiler/codeGen/StgCmmPrim.hs
index e0a68f68d8..1ecd72f9db 100644
--- a/compiler/codeGen/StgCmmPrim.hs
+++ b/compiler/codeGen/StgCmmPrim.hs
@@ -619,7 +619,7 @@ emitPrimOp dflags [res] (VecBroadcastOp vcat n w) [e] = do
 
 emitPrimOp dflags [res] (VecPackOp vcat n w) es = do
     checkVecCompatibility dflags vcat n w
-    when (length es /= n) $
+    when (es `lengthIsNot` n) $
         panic "emitPrimOp: VecPackOp has wrong number of arguments"
     doVecPackOp (vecElemInjectCast dflags vcat w) ty zeros es res
   where
@@ -637,7 +637,7 @@ emitPrimOp dflags [res] (VecPackOp vcat n w) es = do
 
 emitPrimOp dflags res (VecUnpackOp vcat n w) [arg] = do
     checkVecCompatibility dflags vcat n w
-    when (length res /= n) $
+    when (res `lengthIsNot` n) $
         panic "emitPrimOp: VecUnpackOp has wrong number of results"
     doVecUnpackOp (vecElemProjectCast dflags vcat w) ty arg res
   where
diff --git a/compiler/codeGen/StgCmmTicky.hs b/compiler/codeGen/StgCmmTicky.hs
index 8e4e5ece5a..8d86e37ddf 100644
--- a/compiler/codeGen/StgCmmTicky.hs
+++ b/compiler/codeGen/StgCmmTicky.hs
@@ -124,6 +124,7 @@ import Id
 import BasicTypes
 import FastString
 import Outputable
+import Util
 
 import DynFlags
 
@@ -381,7 +382,7 @@ tickyUnboxedTupleReturn arity
 -- Ticks at a *call site*:
 tickyDirectCall :: RepArity -> [StgArg] -> FCode ()
 tickyDirectCall arity args
-  | arity == length args = tickyKnownCallExact
+  | args `lengthIs` arity = tickyKnownCallExact
   | otherwise = do tickyKnownCallExtraArgs
                    tickySlowCallPat (map argPrimRep (drop arity args))
 
@@ -412,7 +413,7 @@ tickySlowCallPat :: [PrimRep] -> FCode ()
 tickySlowCallPat args = ifTicky $
   let argReps = map toArgRep args
       (_, n_matched) = slowCallPattern argReps
-  in if n_matched > 0 && n_matched == length args
+  in if n_matched > 0 && args `lengthIs` n_matched
      then bumpTickyLbl $ mkRtsSlowFastTickyCtrLabel $ concatMap (map Data.Char.toLower . argRepString) argReps
      else bumpTickyCounter $ fsLit "VERY_SLOW_CALL_ctr"