Fix seq# case of exprOkForSpeculation

This subtle patch fixes Trac #5129 (again; comment:20
and following).

I took the opportunity to document seq# properly; see
Note [seq# magic] in PrelRules, and Note [seq# and expr_ok]
in CoreUtils.

compiler/codeGen/StgCmmExpr.hs

@@ -60,7 +60,8 @@ cgExpr  :: StgExpr -> FCode ReturnKind
 
 cgExpr (StgApp fun args)     = cgIdApp fun args
 
-{- seq# a s ==> a -}
+-- seq# a s ==> a
+-- See Note [seq# magic] in PrelRules
 cgExpr (StgOpApp (StgPrimOp SeqOp) [StgVarArg a, _] _res_ty) =
   cgIdApp a []
 
@@ -447,13 +448,14 @@ cgCase scrut@(StgApp v []) _ (PrimAlt _) _
 
 {- Note [Handle seq#]
 ~~~~~~~~~~~~~~~~~~~~~
-case seq# a s of v
-  (# s', a' #) -> e
+See Note [seq# magic] in PrelRules.
+The special case for seq# in cgCase does this:
 
+  case seq# a s of v
+    (# s', a' #) -> e
 ==>
-
-case a of v
-  (# s', a' #) -> e
+  case a of v
+    (# s', a' #) -> e
 
 (taking advantage of the fact that the return convention for (# State#, a #)
 is the same as the return convention for just 'a')
@@ -461,6 +463,7 @@ is the same as the return convention for just 'a')
 
 cgCase (StgOpApp (StgPrimOp SeqOp) [StgVarArg a, _] _) bndr alt_type alts
   = -- Note [Handle seq#]
+    -- And see Note [seq# magic] in PrelRules
     -- Use the same return convention as vanilla 'a'.
     cgCase (StgApp a []) bndr alt_type alts
 

compiler/coreSyn/CoreUtils.hs

@@ -1449,8 +1449,11 @@ app_ok primop_ok fun args
               -- Often there is a literal divisor, and this
               -- can get rid of a thunk in an inner loop
 
+        | SeqOp <- op    -- See Note [seq# and expr_ok]
+        -> all (expr_ok primop_ok) args
+
         | otherwise
-        -> primop_ok op     -- Check the primop itself
+        -> primop_ok op  -- Check the primop itself
         && and (zipWith arg_ok arg_tys args)  -- Check the arguments
 
       _other -> isUnliftedType (idType fun)          -- c.f. the Var case of exprIsHNF
@@ -1607,6 +1610,25 @@ See also Note [dataToTag#] in primops.txt.pp.
 
 Bottom line:
   * in exprOkForSpeculation we simply ignore all lifted arguments.
+  * except see Note [seq# and expr_ok] for an exception
+
+
+Note [seq# and expr_ok]
+~~~~~~~~~~~~~~~~~~~~~~~
+Recall that
+   seq# :: forall a s . a -> State# s -> (# State# s, a #)
+must always evaluate its first argument.  So it's really a
+counter-example to Note [Primops with lifted arguments]. In
+the case of seq# we must check the argument to seq#.  Remember
+item (d) of the specification of exprOkForSpeculation:
+
+  -- Precisely, it returns @True@ iff:
+  --  a) The expression guarantees to terminate,
+         ...
+  --  d) without throwing a Haskell exception
+
+The lack of this special case caused Trac #5129 to go bad again.
+See comment:24 and following
 
 
 ************************************************************************

compiler/prelude/PrelRules.hs

@@ -942,7 +942,56 @@ dataToTagRule = a `mplus` b
 ************************************************************************
 -}
 
--- seq# :: forall a s . a -> State# s -> (# State# s, a #)
+{- Note [seq# magic]
+~~~~~~~~~~~~~~~~~~~~
+The primop
+   seq# :: forall a s . a -> State# s -> (# State# s, a #)
+
+is /not/ the same as the Prelude function seq :: a -> b -> b
+as you can see from its type.  In fact, seq# is the implementation
+mechanism for 'evaluate'
+
+   evaluate :: a -> IO a
+   evaluate a = IO $ \s -> seq# a s
+
+The semantics of seq# is
+  * evaluate its first argument
+  * and return it
+
+Things to note
+
+* Why do we need a primop at all?  That is, instead of
+      case seq# x s of (# x, s #) -> blah
+  why not instead say this?
+      case x of { DEFAULT -> blah)
+
+  Reason (see Trac #5129): if we saw
+    catch# (\s -> case x of { DEFAULT -> raiseIO# exn s }) handler
+
+  then we'd drop the 'case x' because the body of the case is bottom
+  anyway. But we don't want to do that; the whole /point/ of
+  seq#/evaluate is to evaluate 'x' first in the IO monad.
+
+  In short, we /always/ evaluate the first argument and never
+  just discard it.
+
+* Why return the value?  So that we can control sharing of seq'd
+  values: in
+     let x = e in x `seq` ... x ...
+  We don't want to inline x, so better to represent it as
+       let x = e in case seq# x RW of (# _, x' #) -> ... x' ...
+  also it matches the type of rseq in the Eval monad.
+
+Implementing seq#.  The compiler has magic for SeqOp in
+
+- PrelRules.seqRule: eliminate (seq# <whnf> s)
+
+- StgCmmExpr.cgExpr, and cgCase: special case for seq#
+
+- CoreUtils.exprOkForSpeculation;
+  see Note [seq# and expr_ok] in CoreUtils
+-}
+
 seqRule :: RuleM CoreExpr
 seqRule = do
   [Type ty_a, Type _ty_s, a, s] <- getArgs

compiler/prelude/primops.txt.pp

@@ -2697,13 +2697,7 @@ primop SparkOp "spark#" GenPrimOp
 
 primop SeqOp "seq#" GenPrimOp
    a -> State# s -> (# State# s, a #)
-
-   -- why return the value?  So that we can control sharing of seq'd
-   -- values: in
-   --    let x = e in x `seq` ... x ...
-   -- we don't want to inline x, so better to represent it as
-   --    let x = e in case seq# x RW of (# _, x' #) -> ... x' ...
-   -- also it matches the type of rseq in the Eval monad.
+   -- See Note [seq# magic] in PrelRules
 
 primop GetSparkOp "getSpark#" GenPrimOp
    State# s -> (# State# s, Int#, a #)

testsuite/tests/codeGen/should_run/all.T

@@ -94,7 +94,7 @@ test('T5149', omit_ways(['ghci']), multi_compile_and_run,
 test('T5129',
      # The bug is in simplifier when run with -O1 and above, so only run it
      # optimised, using any backend.
-     [ only_ways(['optasm']), expect_broken(5129) ],
+     only_ways(['optasm']),
      compile_and_run, [''])
 
 test('T5626', exit_code(1), compile_and_run, [''])