From ac049528f38c7b85ac570b37a3724cf5d01487d4 Mon Sep 17 00:00:00 2001
From: simonpj <unknown>
Date: Tue, 24 Aug 1999 08:52:25 +0000
Subject: [PATCH] [project @ 1999-08-24 08:52:25 by simonpj] Make
primOpHasSideEffects return True for the various Write primops. Amazingly,
this wasn't done, and that meant that Write operations could be silently
discarded! This broke the GHC typechecker (actually the implementation of
checkNoErrsTc, showed up in deriving code)
---
ghc/compiler/prelude/PrimOp.lhs | 50 ++++++++++++++++++++++++++++-----
1 file changed, 43 insertions(+), 7 deletions(-)
diff --git a/ghc/compiler/prelude/PrimOp.lhs b/ghc/compiler/prelude/PrimOp.lhs
index 24bd867afd91..5535f9759f31 100644
--- a/ghc/compiler/prelude/PrimOp.lhs
+++ b/ghc/compiler/prelude/PrimOp.lhs
@@ -1967,36 +1967,56 @@ primOpOutOfLine op
_ -> False
\end{code}
+
+primOpOkForSpeculation
+~~~~~~~~~~~~~~~~~~~~~~
Sometimes we may choose to execute a PrimOp even though it isn't
certain that its result will be required; ie execute them
``speculatively''. The same thing as ``cheap eagerness.'' Usually
this is OK, because PrimOps are usually cheap, but it isn't OK for
(a)~expensive PrimOps and (b)~PrimOps which can fail.
+PrimOps that have side effects also should not be executed speculatively.
+
+Ok-for-speculation also means that it's ok *not* to execute the
+primop. For example
+ case op a b of
+ r -> 3
+Here the result is not used, so we can discard the primop. Anything
+that has side effects mustn't be dicarded in this way, of course!
+
See also @primOpIsCheap@ (below).
-PrimOps that have side effects also should not be executed speculatively
-or by data dependencies.
\begin{code}
primOpOkForSpeculation :: PrimOp -> Bool
+ -- See comments with CoreUtils.exprOkForSpeculation
primOpOkForSpeculation op
= not (primOpCanFail op || primOpHasSideEffects op || primOpOutOfLine op)
\end{code}
+
+primOpIsCheap
+~~~~~~~~~~~~~
@primOpIsCheap@, as used in \tr{SimplUtils.lhs}. For now (HACK
WARNING), we just borrow some other predicates for a
what-should-be-good-enough test. "Cheap" means willing to call it more
than once. Evaluation order is unaffected.
\begin{code}
+primOpIsCheap :: PrimOp -> Bool
+ -- See comments with CoreUtils.exprOkForSpeculation
primOpIsCheap op = not (primOpHasSideEffects op || primOpOutOfLine op)
\end{code}
+primOpIsDupable
+~~~~~~~~~~~~~~~
primOpIsDupable means that the use of the primop is small enough to
duplicate into different case branches. See CoreUtils.exprIsDupable.
\begin{code}
+primOpIsDupable :: PrimOp -> Bool
+ -- See comments with CoreUtils.exprIsDupable
primOpIsDupable (CCallOp _ _ might_gc _) = not might_gc
-- If the ccall can't GC then the call is pretty cheap, and
-- we're happy to duplicate
@@ -2035,11 +2055,6 @@ duplicated.
\begin{code}
primOpHasSideEffects :: PrimOp -> Bool
-primOpHasSideEffects TakeMVarOp = True
-primOpHasSideEffects DelayOp = True
-primOpHasSideEffects WaitReadOp = True
-primOpHasSideEffects WaitWriteOp = True
-
primOpHasSideEffects ParOp = True
primOpHasSideEffects ForkOp = True
primOpHasSideEffects KillThreadOp = True
@@ -2056,6 +2071,27 @@ primOpHasSideEffects MakeStableNameOp = True
primOpHasSideEffects EqStablePtrOp = True -- SOF
primOpHasSideEffects DeRefStablePtrOp = True -- ??? JSM & ADR
+-- In general, writes are considered a side effect, but
+-- reads and variable allocations are not
+-- Why? Because writes must not be omitted, but reads can be if their result is not used.
+-- (Sequencing of reads is maintained by data dependencies on the resulting
+-- world state.)
+primOpHasSideEffects WriteArrayOp = True
+primOpHasSideEffects (WriteByteArrayOp _) = True
+primOpHasSideEffects (WriteOffAddrOp _) = True
+primOpHasSideEffects WriteMutVarOp = True
+
+primOpHasSideEffects UnsafeFreezeArrayOp = True
+primOpHasSideEffects UnsafeFreezeByteArrayOp = True
+primOpHasSideEffects UnsafeThawArrayOp = True
+primOpHasSideEffects UnsafeThawByteArrayOp = True
+
+primOpHasSideEffects TakeMVarOp = True
+primOpHasSideEffects PutMVarOp = True
+primOpHasSideEffects DelayOp = True
+primOpHasSideEffects WaitReadOp = True
+primOpHasSideEffects WaitWriteOp = True
+
primOpHasSideEffects ParGlobalOp = True
primOpHasSideEffects ParLocalOp = True
primOpHasSideEffects ParAtOp = True
--
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