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.

1 files changed, 9 insertions, 6 deletions

diff --git a/compiler/codeGen/StgCmmExpr.hs b/compiler/codeGen/StgCmmExpr.hs
index c2f2efed3d..22fcfaf412 100644
--- a/compiler/codeGen/StgCmmExpr.hs
+++ b/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