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diff --git a/compiler/GHC/CmmToAsm/Reg/Utils.hs b/compiler/GHC/CmmToAsm/Reg/Utils.hs
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+++ b/compiler/GHC/CmmToAsm/Reg/Utils.hs
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+module GHC.CmmToAsm.Reg.Utils
+    ( toRegMap, toVRegMap )
+where
+
+{- Note [UniqFM and the register allocator]
+   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+   Before UniqFM had a key type the register allocator
+   wasn't picky about key types, using VirtualReg, Reg
+   and Unique at various use sites for the same map.
+
+   This is safe.
+   * The Unique values come from registers at various
+     points where we lose a reference to the original
+     register value, but the unique is still valid.
+
+   * VirtualReg is a subset of the registers in Reg's type.
+     Making a value of VirtualReg into a Reg in fact doesn't
+     change its unique. This is because Reg consists of virtual
+     regs and real regs, whose unique values do not overlap.
+
+   * Since the code was written in the assumption that keys are
+     not typed it's hard to reverse this assumption now. So we get
+     some gnarly but correct code where we often pass around Uniques
+     and switch between using Uniques, VirtualReg and RealReg as keys
+     of the same map. These issues were always there. But with the
+     now-typed keys they become visible. It's a classic case of not all
+     correct programs type checking.
+
+   We reduce some of the burden by providing a way to cast
+
+        UniqFM VirtualReg a
+
+   to
+
+        UniqFM Reg a
+
+    in this module. This is safe as Reg is the sum of VirtualReg and
+    RealReg. With each kind of register keeping the same unique when
+    treated as Reg.
+
+   TODO: If you take offense to this I encourage you to refactor this
+   code. I'm sure we can do with less casting of keys and direct use
+   of uniques. It might also be reasonable to just use a IntMap directly
+   instead of dealing with UniqFM at all.
+
+
+-}
+import GHC.Types.Unique.FM
+import GHC.Platform.Reg
+
+-- These should hopefully be zero cost.
+
+toRegMap :: UniqFM VirtualReg elt -> UniqFM Reg elt
+toRegMap = unsafeCastUFMKey
+
+toVRegMap :: UniqFM Reg elt -> UniqFM VirtualReg elt
+toVRegMap = unsafeCastUFMKey
+