primops: Mark actions evaluated by `catch*` as lazy

There is something very peculiar about the `catch` family of operations
with respect to strictness analysis: they turn divergence into
non-divergence. For this reason, it isn't safe to mark them as strict in
the expression whose exceptions they are catching. The reason is this:

Consider,

    let r = \st -> raiseIO# blah st
    in catch (\st -> ...(r st)..) handler st

If we give the first argument of catch a strict signature, we'll get a
demand 'C(S)' for 'r'; that is, 'r' is definitely called with one
argument, which indeed it is. The trouble comes when we feed 'C(S)' into
'r's RHS as the demand of the body as this will lead us to conclude that
the whole 'let' will diverge; clearly this isn't right.

This is essentially the problem in #10712, which arose when
7c0fff41789669450b02dc1db7f5d7babba5dee6 marked the `catch*` primops as
being strict in the thing to be evaluated. Here I've partially reverted
this commit, again marking the first argument of these primops as lazy.

Fixes #10712.

Test Plan: Validate checking `exceptionsrun001`

Reviewers: simonpj, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1616

GHC Trac Issues: #10712, #11222

1 files changed, 20 insertions, 10 deletions

diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp
index c16646e0f6..de14e30f76 100644
--- a/compiler/prelude/primops.txt.pp
+++ b/compiler/prelude/primops.txt.pp
@@ -1946,17 +1946,27 @@ necessarily the second.  Hence strictApply1Dmd and lazyApply1Dmd
 Howver, consider
     catch# (\st -> case x of ...) (..handler..) st
 We'll see that the entire thing is strict in 'x', so 'x' may be evaluated
-before the catch#.  So fi evaluting 'x' causes a divide-by-zero exception,
+before the catch#.  So if evaluting 'x' causes a divide-by-zero exception,
 it won't be caught.  This seems acceptable:
+
   - x might be evaluated somewhere else outside the catch# anyway
   - It's an imprecise eception anyway.  Synchronous exceptions (in the
     IO monad) will never move in this way.
-There was originally a comment
-  "Catch is actually strict in its first argument
-   but we don't want to tell the strictness
-   analyser about that, so that exceptions stay inside it."
-but tracing it back through the commit logs did not give any
-rationale.  And making catch# lazy has performance costs for everyone.
+
+Unfortunately, there is a tricky wrinkle here, as pointed out in #10712.
+Consider,
+
+    let r = \st -> raiseIO# blah st
+    in catch (\st -> ...(r st)..) handler st
+
+If we give the first argument of catch a strict signature, we'll get
+a demand 'C(S)' for 'r'; that is, 'r' is definitely called with one
+argument, which indeed it is. The trouble comes when we feed 'C(S)'
+into 'r's RHS as the demand of the body as this will lead us to conclude that
+the whole 'let' will diverge; clearly this isn't right.
+
+There's something very special about catch: it turns divergence into
+non-divergence.
 -}
 
 primop  CatchOp "catch#" GenPrimOp
@@ -1965,7 +1975,7 @@ primop  CatchOp "catch#" GenPrimOp
        -> State# RealWorld
        -> (# State# RealWorld, a #)
    with
-   strictness  = { \ _arity -> mkClosedStrictSig [strictApply1Dmd,lazyApply2Dmd,topDmd] topRes }
+   strictness  = { \ _arity -> mkClosedStrictSig [lazyApply1Dmd,lazyApply2Dmd,topDmd] topRes }
                  -- See Note [Strictness for mask/unmask/catch]
    out_of_line = True
    has_side_effects = True
@@ -2069,7 +2079,7 @@ primop  CatchRetryOp "catchRetry#" GenPrimOp
    -> (State# RealWorld -> (# State# RealWorld, a #) )
    -> (State# RealWorld -> (# State# RealWorld, a #) )
    with
-   strictness  = { \ _arity -> mkClosedStrictSig [strictApply1Dmd,lazyApply1Dmd,topDmd] topRes }
+   strictness  = { \ _arity -> mkClosedStrictSig [lazyApply1Dmd,lazyApply1Dmd,topDmd] topRes }
                  -- See Note [Strictness for mask/unmask/catch]
    out_of_line = True
    has_side_effects = True
@@ -2079,7 +2089,7 @@ primop  CatchSTMOp "catchSTM#" GenPrimOp
    -> (b -> State# RealWorld -> (# State# RealWorld, a #) )
    -> (State# RealWorld -> (# State# RealWorld, a #) )
    with
-   strictness  = { \ _arity -> mkClosedStrictSig [strictApply1Dmd,lazyApply2Dmd,topDmd] topRes }
+   strictness  = { \ _arity -> mkClosedStrictSig [lazyApply1Dmd,lazyApply2Dmd,topDmd] topRes }
                  -- See Note [Strictness for mask/unmask/catch]
    out_of_line = True
    has_side_effects = True