Modules: Llvm (#13009)

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diff --git a/compiler/GHC/Llvm/MetaData.hs b/compiler/GHC/Llvm/MetaData.hs
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+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+
+module GHC.Llvm.MetaData where
+
+import GhcPrelude
+
+import GHC.Llvm.Types
+import Outputable
+
+-- The LLVM Metadata System.
+--
+-- The LLVM metadata feature is poorly documented but roughly follows the
+-- following design:
+-- * Metadata can be constructed in a few different ways (See below).
+-- * After which it can either be attached to LLVM statements to pass along
+-- extra information to the optimizer and code generator OR specifically named
+-- metadata has an affect on the whole module (i.e., linking behaviour).
+--
+--
+-- # Constructing metadata
+-- Metadata comes largely in three forms:
+--
+-- * Metadata expressions -- these are the raw metadata values that encode
+--   information. They consist of metadata strings, metadata nodes, regular
+--   LLVM values (both literals and references to global variables) and
+--   metadata expressions (i.e., recursive data type). Some examples:
+--     !{ !"hello", !0, i32 0 }
+--     !{ !1, !{ i32 0 } }
+--
+-- * Metadata nodes -- global metadata variables that attach a metadata
+--   expression to a number. For example:
+--     !0 = !{ [<metadata expressions>] !}
+--
+-- * Named metadata -- global metadata variables that attach a metadata nodes
+--   to a name. Used ONLY to communicated module level information to LLVM
+--   through a meaningful name. For example:
+--     !llvm.module.linkage = !{ !0, !1 }
+--
+--
+-- # Using Metadata
+-- Using metadata depends on the form it is in:
+--
+-- * Attach to instructions -- metadata can be attached to LLVM instructions
+--   using a specific reference as follows:
+--     %l = load i32* @glob, !nontemporal !10
+--     %m = load i32* @glob, !nontemporal !{ i32 0, !{ i32 0 } }
+--   Only metadata nodes or expressions can be attached, named metadata cannot.
+--   Refer to LLVM documentation for which instructions take metadata and its
+--   meaning.
+--
+-- * As arguments -- llvm functions can take metadata as arguments, for
+--   example:
+--     call void @llvm.dbg.value(metadata !{ i32 0 }, i64 0, metadata !1)
+--   As with instructions, only metadata nodes or expressions can be attached.
+--
+-- * As a named metadata -- Here the metadata is simply declared in global
+--   scope using a specific name to communicate module level information to LLVM.
+--   For example:
+--     !llvm.module.linkage = !{ !0, !1 }
+--
+
+-- | A reference to an un-named metadata node.
+newtype MetaId = MetaId Int
+               deriving (Eq, Ord, Enum)
+
+instance Outputable MetaId where
+    ppr (MetaId n) = char '!' <> int n
+
+-- | LLVM metadata expressions
+data MetaExpr = MetaStr !LMString
+              | MetaNode !MetaId
+              | MetaVar !LlvmVar
+              | MetaStruct [MetaExpr]
+              deriving (Eq)
+
+instance Outputable MetaExpr where
+  ppr (MetaVar (LMLitVar (LMNullLit _))) = text "null"
+  ppr (MetaStr    s ) = char '!' <> doubleQuotes (ftext s)
+  ppr (MetaNode   n ) = ppr n
+  ppr (MetaVar    v ) = ppr v
+  ppr (MetaStruct es) = char '!' <> braces (ppCommaJoin es)
+
+-- | Associates some metadata with a specific label for attaching to an
+-- instruction.
+data MetaAnnot = MetaAnnot LMString MetaExpr
+               deriving (Eq)
+
+-- | Metadata declarations. Metadata can only be declared in global scope.
+data MetaDecl
+    -- | Named metadata. Only used for communicating module information to
+    -- LLVM. ('!name = !{ [!<n>] }' form).
+    = MetaNamed !LMString [MetaId]
+    -- | Metadata node declaration.
+    -- ('!0 = metadata !{ <metadata expression> }' form).
+    | MetaUnnamed !MetaId !MetaExpr