Revert ad15c2, which causes Windows seg-faults (Trac #8834)

We don't yet understand WHY commit ad15c2, which is to do with
CmmSink, causes seg-faults on Windows, but it certainly seems to.  So
reverting it is a stop-gap, but we need to un-block the 7.8 release.

Many thanks to awson for identifying the offending commit.

1 files changed, 21 insertions, 64 deletions

diff --git a/compiler/cmm/CmmSink.hs b/compiler/cmm/CmmSink.hs
index c404a2e932..635b0021f1 100644
--- a/compiler/cmm/CmmSink.hs
+++ b/compiler/cmm/CmmSink.hs
@@ -3,6 +3,8 @@ module CmmSink (
      cmmSink
   ) where
 
+import CodeGen.Platform (callerSaves)
+
 import Cmm
 import CmmOpt
 import BlockId
@@ -236,9 +238,11 @@ some tool like perf or VTune and make decisions what to inline based on that.
 -- global) and literals.
 --
 isTrivial :: CmmExpr -> Bool
-isTrivial (CmmReg _) = True
-isTrivial (CmmLit _) = True
-isTrivial _          = False
+isTrivial (CmmReg (CmmLocal _)) = True
+-- isTrivial (CmmLit _) = True  -- Disabled because it used to make thing worse.
+                                -- Needs further investigation
+isTrivial _ = False
+
 
 --
 -- annotate each node with the set of registers live *after* the node
@@ -501,8 +505,7 @@ regsUsedIn ls e = wrapRecExpf f e False
 -- nor the NCG can do it.  See Note [Register parameter passing]
 -- See also StgCmmForeign:load_args_into_temps.
 okToInline :: DynFlags -> CmmExpr -> CmmNode e x -> Bool
-okToInline dflags expr node@(CmmUnsafeForeignCall{}) =
-    not (globalRegistersConflict dflags expr node)
+okToInline dflags expr CmmUnsafeForeignCall{} = not (anyCallerSavesRegs dflags expr)
 okToInline _ _ _ = True
 
 -- -----------------------------------------------------------------------------
@@ -515,23 +518,23 @@ okToInline _ _ _ = True
 conflicts :: DynFlags -> Assignment -> CmmNode O x -> Bool
 conflicts dflags (r, rhs, addr) node
 
-  -- (1) node defines registers used by rhs of assignment. This catches
-  -- assignmnets and all three kinds of calls. See Note [Sinking and calls]
-  | globalRegistersConflict dflags rhs node                       = True
-  | localRegistersConflict  dflags rhs node                       = True
-
-  -- (2) node uses register defined by assignment
+  -- (1) an assignment to a register conflicts with a use of the register
+  | CmmAssign reg  _ <- node, reg `regUsedIn` rhs                 = True
   | foldRegsUsed dflags (\b r' -> r == r' || b) False node        = True
 
-  -- (3) a store to an address conflicts with a read of the same memory
+  -- (2) a store to an address conflicts with a read of the same memory
   | CmmStore addr' e <- node
   , memConflicts addr (loadAddr dflags addr' (cmmExprWidth dflags e)) = True
 
-  -- (4) an assignment to Hp/Sp conflicts with a heap/stack read respectively
+  -- (3) an assignment to Hp/Sp conflicts with a heap/stack read respectively
   | HeapMem    <- addr, CmmAssign (CmmGlobal Hp) _ <- node        = True
   | StackMem   <- addr, CmmAssign (CmmGlobal Sp) _ <- node        = True
   | SpMem{}    <- addr, CmmAssign (CmmGlobal Sp) _ <- node        = True
 
+  -- (4) assignments that read caller-saves GlobalRegs conflict with a
+  -- foreign call.  See Note [Unsafe foreign calls clobber caller-save registers]
+  | CmmUnsafeForeignCall{} <- node, anyCallerSavesRegs dflags rhs = True
+
   -- (5) foreign calls clobber heap: see Note [Foreign calls clobber heap]
   | CmmUnsafeForeignCall{} <- node, memConflicts addr AnyMem      = True
 
@@ -541,57 +544,11 @@ conflicts dflags (r, rhs, addr) node
   -- (7) otherwise, no conflict
   | otherwise = False
 
--- Returns True if node defines any global registers that are used in the
--- Cmm expression
-globalRegistersConflict :: DynFlags -> CmmExpr -> CmmNode e x -> Bool
-globalRegistersConflict dflags expr node =
-    foldRegsDefd dflags (\b r -> b || (CmmGlobal r) `regUsedIn` expr) False node
-
--- Returns True if node defines any local registers that are used in the
--- Cmm expression
-localRegistersConflict :: DynFlags -> CmmExpr -> CmmNode e x -> Bool
-localRegistersConflict dflags expr node =
-    foldRegsDefd dflags (\b r -> b || (CmmLocal  r) `regUsedIn` expr) False node
-
--- Note [Sinking and calls]
--- ~~~~~~~~~~~~~~~~~~~~~~~~
---
--- We have three kinds of calls: normal (CmmCall), safe foreign (CmmForeignCall)
--- and unsafe foreign (CmmUnsafeForeignCall). We perform sinking pass after
--- stack layout (see Note [Sinking after stack layout]) which leads to two
--- invariants related to calls:
---
---   a) during stack layout phase all safe foreign calls are turned into
---      unsafe foreign calls (see Note [Lower safe foreign calls]). This
---      means that we will never encounter CmmForeignCall node when running
---      sinking after stack layout
---
---   b) stack layout saves all variables live across a call on the stack
---      just before making a call (remember we are not sinking assignments to
---      stack):
---
---       L1:
---          x = R1
---          P64[Sp - 16] = L2
---          P64[Sp - 8]  = x
---          Sp = Sp - 16
---          call f() returns L2
---       L2:
---
---      We will attempt to sink { x = R1 } but we will detect conflict with
---      { P64[Sp - 8]  = x } and hence we will drop { x = R1 } without even
---      checking whether it conflicts with { call f() }. In this way we will
---      never need to check any assignment conflicts with CmmCall. Remember
---      that we still need to check for potential memory conflicts.
---
--- So the result is that we only need to worry about CmmUnsafeForeignCall nodes
--- when checking conflicts (see Note [Unsafe foreign calls clobber caller-save registers]).
--- This assumption holds only when we do sinking after stack layout. If we run
--- it before stack layout we need to check for possible conflicts with all three
--- kinds of calls. Our `conflicts` function does that by using a generic
--- foldRegsDefd and foldRegsUsed functions defined in DefinerOfRegs and
--- UserOfRegs typeclasses.
---
+anyCallerSavesRegs :: DynFlags -> CmmExpr -> Bool
+anyCallerSavesRegs dflags e = wrapRecExpf f e False
+  where f (CmmReg (CmmGlobal r)) _
+         | callerSaves (targetPlatform dflags) r = True
+        f _ z = z
 
 -- An abstraction of memory read or written.
 data AbsMem