FIX #1052 (NCG doesn't realise shift instructions trash shifted input)

See comments with shift_code

diff --git a/compiler/nativeGen/MachCodeGen.hs b/compiler/nativeGen/MachCodeGen.hs
index 3ff958d..0bab29c 100644 (file)
--- a/compiler/nativeGen/MachCodeGen.hs
+++ b/compiler/nativeGen/MachCodeGen.hs
@@ -1148,16 +1148,30 @@ getRegister e@(CmmMachOp mop [x, y]) -- dyadic MachOps
          -- in
          return (Any rep code)
         
-    {- Case2: shift length is complex (non-immediate) -}
+    {- Case2: shift length is complex (non-immediate)
+      * y must go in %ecx.
+      * we cannot do y first *and* put its result in %ecx, because
+        %ecx might be clobbered by x.
+      * if we do y second, then x cannot be 
+        in a clobbered reg.  Also, we cannot clobber x's reg
+        with the instruction itself.
+      * so we can either:
+        - do y first, put its result in a fresh tmp, then copy it to %ecx later
+        - do y second and put its result into %ecx.  x gets placed in a fresh
+          tmp.  This is likely to be better, becuase the reg alloc can
+          eliminate this reg->reg move here (it won't eliminate the other one,
+          because the move is into the fixed %ecx).
+    -}
     shift_code rep instr x y{-amount-} = do
-        (x_reg, x_code) <- getNonClobberedReg x
+        x_code <- getAnyReg x
+       tmp <- getNewRegNat rep
         y_code <- getAnyReg y
        let 
-          code = x_code `appOL`
+          code = x_code tmp `appOL`
                  y_code ecx `snocOL`
-                 instr (OpReg ecx) (OpReg x_reg)
+                 instr (OpReg ecx) (OpReg tmp)
         -- in
-        return (Fixed rep x_reg code)
+        return (Fixed rep tmp code)
 
     --------------------
     add_code :: MachRep -> CmmExpr -> CmmExpr -> NatM Register