Eliminate more dead reg->reg moves in linear allocator

diff --git a/compiler/nativeGen/RegAllocLinear.hs b/compiler/nativeGen/RegAllocLinear.hs
index d9ff121..d761bae 100644 (file)
--- a/compiler/nativeGen/RegAllocLinear.hs
+++ b/compiler/nativeGen/RegAllocLinear.hs
@@ -498,7 +498,15 @@ genRaInsn block_live new_instrs instr r_dying w_dying =
     -- (j) free up stack slots for dead spilled regs
     -- TODO (can't be bothered right now)
 
-    return (patched_instr : w_spills ++ reverse r_spills
+    -- erase reg->reg moves where the source and destination are the same.
+    -- If the src temp didn't die in this instr but happened to be allocated
+    -- to the same real reg as the destination, then we can erase the move anyway.
+       squashed_instr  = case isRegRegMove patched_instr of
+                               Just (src, dst)
+                                | src == dst   -> []
+                               _               -> [patched_instr]
+
+    return (squashed_instr ++ w_spills ++ reverse r_spills
                 ++ clobber_saves ++ new_instrs,
            fixup_blocks)
   }}