diff options
Diffstat (limited to 'compiler/cmm/ZipCfgCmmRep.hs')
| -rw-r--r-- | compiler/cmm/ZipCfgCmmRep.hs | 29 |
1 files changed, 28 insertions, 1 deletions
diff --git a/compiler/cmm/ZipCfgCmmRep.hs b/compiler/cmm/ZipCfgCmmRep.hs index b289fdccc4..43e310c80c 100644 --- a/compiler/cmm/ZipCfgCmmRep.hs +++ b/compiler/cmm/ZipCfgCmmRep.hs @@ -64,7 +64,7 @@ data Middle | MidStore CmmExpr CmmExpr -- Assign to memory location. Size is -- given by cmmExprType of the rhs. - | MidForeignCall -- A foreign call; + | MidForeignCall -- A foreign call; see Note [Foreign calls] ForeignSafety -- Is it a safe or unsafe call? MidCallTarget -- call target and convention CmmFormals -- zero or more results @@ -142,6 +142,33 @@ data ValueDirection = Arguments | Results -- Arguments go with procedure definitions, jumps, and arguments to calls -- Results go with returns and with results of calls. deriving Eq + +{- Note [Foreign calls] +~~~~~~~~~~~~~~~~~~~~~~~ +A MidForeign call is used *all* foreign calls, both *unsafe* and *safe*. +Unsafe ones are easy: think of them as a "fat machine instruction". + +Safe ones are trickier. A safe foreign call + r = f(x) +ultimately expands to + push "return address" -- Never used to return to; + -- just points an info table + save registers into TSO + call suspendThread + r = f(x) -- Make the call + call resumeThread + restore registers + pop "return address" +We cannot "lower" a safe foreign call to this sequence of Cmms, because +after we've saved Sp all the Cmm optimiser's assumptions are broken. +Furthermore, currently the smart Cmm constructors know the calling +conventions for Haskell, the garbage collector, etc, and "lower" them +so that a LastCall passes no parameters or results. But the smart +constructors do *not* (currently) know the foreign call conventions. + +For these reasons use MidForeignCall for all calls. The only annoying thing +is that a safe foreign call needs an info table. +-} ---------------------------------------------------------------------- ----- Splicing between blocks |