Work around LLVM's reltable lookup optimization (#1214)

Clang 14.0.0+ introduce a relative lookup table optimization that produces code
that subtracts two pointers from different allocation regions, which is
something that `crucible-llvm` is ill equipped to reason about. For lack of a
viable alternative, we work around this issue by "undoing" this optimization
before translating the code to Crucible. For the full details, see the new
`Note [Undoing LLVM's relative table lookup conversion pass]` in
`Lang.Crucible.LLVM.Globals`.

Fixes #1174.

crucible-llvm/src/Lang/Crucible/LLVM/Globals.hs

@@ -46,7 +46,7 @@ import           Control.Monad (foldM)
 import           Control.Monad.IO.Class (MonadIO(..))
 import           Control.Monad.Except (MonadError(..))
 import           Control.Lens hiding (op, (:>) )
-import           Data.List (foldl')
+import           Data.List (foldl', genericLength, isPrefixOf)
 import           Data.Map.Strict (Map)
 import qualified Data.Map.Strict as Map
 import qualified Data.Set as Set
@@ -107,16 +107,17 @@ makeGlobalMap :: forall arch wptr. (?lc :: TypeContext, HasPtrWidth wptr)
               => LLVMContext arch
               -> L.Module
               -> GlobalInitializerMap
-makeGlobalMap ctx m = foldl' addAliases globalMap (Map.toList (llvmGlobalAliases ctx))
+makeGlobalMap ctx m = foldl' addAliases globalMap1 (Map.toList (llvmGlobalAliases ctx))
 
   where
    addAliases mp (glob, aliases) =
         case Map.lookup glob mp of
           Just initzr -> insertAll (map L.aliasName (Set.toList aliases)) initzr mp
           Nothing     -> mp -- should this be an error/exception?
 
-   globalMap = Map.fromList $ map (L.globalSym &&& (id &&& globalToConst))
-                                  (L.modGlobals m)
+   globalMap0 = Map.fromList $ map (\g -> (L.globalSym g, g)) (L.modGlobals m)
+   globalMap1 = Map.map (id &&& globalToConst) globalMap0
+   loadRelConstInitMap = buildLoadRelConstInitMap globalMap0 m
 
    insertAll ks v mp = foldr (flip Map.insert v) mp ks
 
@@ -135,11 +136,20 @@ makeGlobalMap ctx m = foldl' addAliases globalMap (Map.toList (llvmGlobalAliases
                   => L.Global -> m (MemType, Maybe LLVMConst)
    globalToConst' g =
      do let ?lc  = ctx^.llvmTypeCtx -- implicitly passed to transConstant
-        let gty  = L.globalType g
-        let gval = L.globalValue g
-        mt  <- liftMemType gty
-        val <- traverse (transConstant' mt) gval
-        return (mt, val)
+        let (gty, mbGval) =
+              -- Check if a global variable was passed as an argument to
+              -- llvm.load.relative.i* (i.e., if it is reltable-like), and if
+              -- so, use an altered value for the constant initializer that uses
+              -- `bitcast`. See
+              -- Note [Undoing LLVM's relative table lookup conversion pass].
+              case Map.lookup (L.globalSym g) loadRelConstInitMap of
+                Just (L.Typed constInitTy constInitVal) ->
+                  (constInitTy, Just constInitVal)
+                Nothing ->
+                  (L.globalType g, L.globalValue g)
+        mt <- liftMemType gty
+        mbVal <- traverse (transConstant' mt) mbGval
+        return (mt, mbVal)
 
 -------------------------------------------------------------------------
 -- initializeMemory
@@ -187,11 +197,32 @@ initializeMemory predicate bak llvm_ctx llvmModl = do
    -- Allocate global values
    let globAliases = llvmGlobalAliases llvm_ctx
    let globals     = L.modGlobals llvmModl
+   let globalMap   = Map.fromList $ map (\g -> (L.globalSym g, g)) globals
+   let loadRelConstInitMap = buildLoadRelConstInitMap globalMap llvmModl
    gs_alloc <- mapM (\g -> do
                         let err msg = malformedLLVMModule
                                     ("Invalid type for global" <> fromString (show (L.globalSym g)))
                                     [fromString msg]
-                        ty <- either err return $ liftMemType $ L.globalType g
+                        -- Check if a global variable was passed as an argument
+                        -- to llvm.load.relative.i* (i.e., if it is
+                        -- reltable-like), and if so, use an altered type that
+                        -- uses pointers instead of `i32`s. Also, do not use the
+                        -- original global's alignment. See
+                        -- Note [Undoing LLVM's relative table lookup conversion pass].
+                        (ty, mbGlobAlign) <-
+                          case Map.lookup (L.globalSym g) loadRelConstInitMap of
+                            Just constInit -> do
+                              ty <- either err return $
+                                    liftMemType $
+                                    L.typedType constInit
+                              -- Return Nothing for the alignment so that we
+                              -- will instead use crucible-llvm's alignment
+                              -- inference to compute the alignment of the
+                              -- new constant initializer.
+                              pure (ty, Nothing)
+                            Nothing -> do
+                              ty <- either err return $ liftMemType $ L.globalType g
+                              pure (ty, L.globalAlign g)
                         let sz      = memTypeSize dl ty
                         let tyAlign = memTypeAlign dl ty
                         let aliases = map L.aliasName . Set.toList $
@@ -207,7 +238,7 @@ initializeMemory predicate bak llvm_ctx llvmModl = do
                         -- specified, the global is forced to have
                         -- exactly that alignment.
                         alignment <-
-                          case L.globalAlign g of
+                          case mbGlobAlign of
                             Just a | a > 0 ->
                               case toAlignment (toBytes a) of
                                 Nothing -> fail $ "Invalid alignemnt: " ++ show a ++ "\n  " ++
@@ -374,3 +405,213 @@ populateGlobal bak gl memty cval giMap mem =
      ptr <- doResolveGlobal bak mem (L.globalSym gl)
      (val, mem') <- runStateT (constToLLVMValP sym populateRec cval) mem
      storeConstRaw bak mem' ptr ty alignment val
+
+------------------------------------------------------------------------
+-- ** llvm.load.relative constant initializers
+
+-- | A map of global variable names ('L.Symbol's) that appear as arguments to
+-- calls to the @llvm.load.relative.i*@ intrinsic. See
+-- @Note [Undoing LLVM's relative table lookup conversion pass]@ for why we need
+-- this.
+type LoadRelConstInitMap = Map L.Symbol (L.Typed L.Value)
+
+-- | @buildLoadRelConstInitMap globalMap m@ takes a 'L.Module' (@m@) and a map
+-- of global variable symbols to their definitions (@globalMap@) and computes
+-- a 'LoadRelConstInitMap'. See
+-- @Note [Undoing LLVM's relative table lookup conversion pass]@ for why we need
+-- to do this.
+buildLoadRelConstInitMap ::
+  Map L.Symbol L.Global ->
+  L.Module ->
+  LoadRelConstInitMap
+buildLoadRelConstInitMap globalMap m = foldMap defineConstInits (L.modDefines m)
+  where
+    defineConstInits :: L.Define -> LoadRelConstInitMap
+    defineConstInits def = foldMap basicBlockConstInits (L.defBody def)
+
+    basicBlockConstInits :: L.BasicBlock -> LoadRelConstInitMap
+    basicBlockConstInits bb = foldMap stmtConstInits (L.bbStmts bb)
+
+    stmtConstInits :: L.Stmt -> LoadRelConstInitMap
+    stmtConstInits (L.Result _ instr _) = instrConstInits instr
+    stmtConstInits (L.Effect instr _)   = instrConstInits instr
+
+    instrConstInits :: L.Instr -> LoadRelConstInitMap
+    instrConstInits (L.Call _ _ (L.ValSymbol fun) [ptr, _offset])
+      | L.Symbol funStr <- fun
+      , "llvm.load.relative.i" `isPrefixOf` funStr
+      , Just (gs, foldedConstTy, foldedConstInit) <-
+          foldLoadRelConstInit (L.typedValue ptr)
+      = Map.singleton gs (L.Typed foldedConstTy foldedConstInit)
+    instrConstInits _ =
+      Map.empty
+
+    -- Check if the first argument to a call to llvm.load.relative.i* is
+    -- "reltable-like", and if so, return @Just (symb, ty, val)@, where:
+    --
+    -- - @symb@ is the name of the global variable corresponding to the
+    --   argument.
+    --
+    -- - @ty@ is the type of the global variable's new constant initializer.
+    --
+    -- - @val@ is the new constant initializer value.
+    --
+    -- See Note [Undoing LLVM's relative table lookup conversion pass] for an
+    -- explanation of what "reltable-like" means.
+    foldLoadRelConstInit :: L.Value -> Maybe (L.Symbol, L.Type, L.Value)
+    foldLoadRelConstInit (L.ValSymbol s)
+      | Just global <- Map.lookup s globalMap
+      , Just constInit <- L.globalValue global
+      -- Check that the type of the global variable is
+      -- [<constInitElems> x i32].
+      , L.ValArray (L.PrimType (L.Integer 32)) constInitElems <- constInit
+      , Just foldedConstInitElems <-
+          traverse (foldLoadRelConstInitElem global) constInitElems
+      = Just ( L.globalSym global
+             , L.Array (genericLength constInitElems) ptrToI8Type
+             , L.ValArray ptrToI8Type foldedConstInitElems
+             )
+    foldLoadRelConstInit (L.ValConstExpr (L.ConstConv L.BitCast tv _)) =
+      foldLoadRelConstInit (L.typedValue tv)
+    foldLoadRelConstInit _ =
+      Nothing
+
+    -- Check that an element of a constant initializer is of the form
+    -- `trunc(ptrtoint x - ptrtoint p)`, and if so, return `Just x`. Otherwise,
+    -- return Nothing.
+    foldLoadRelConstInitElem :: L.Global -> L.Value -> Maybe L.Value
+    foldLoadRelConstInitElem global constInitElem
+      | L.ValConstExpr
+          (L.ConstConv L.Trunc
+            (L.Typed { L.typedValue =
+              L.ValConstExpr
+                (L.ConstArith
+                  (L.Sub _ _)
+                  (L.Typed { L.typedValue =
+                    L.ValConstExpr (L.ConstConv L.PtrToInt x _) })
+                  (L.ValConstExpr (L.ConstConv L.PtrToInt p _))) })
+            _truncTy) <- constInitElem
+      , L.ValSymbol pSym <- L.typedValue p
+      , L.globalSym global == pSym
+      = Just (L.ValConstExpr (L.ConstConv L.BitCast x ptrToI8Type))
+
+      | otherwise
+      = Nothing
+
+    -- Type type i8*.
+    ptrToI8Type :: L.Type
+    ptrToI8Type = L.PtrTo $ L.PrimType $ L.Integer 8
+
+{-
+Note [Undoing LLVM's relative table lookup conversion pass]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Clang 14.0.0+ include a `rel-lookup-table-converter` optimization pass that is
+enabled with -O1 or greater. This optimization usually applies to code that
+looks like table lookups. For instance, this pass would take this C code:
+
+  const char *F(int tag) {
+    static const char *const table[] = {
+      "A",
+      "B",
+    };
+
+    return table[tag];
+  }
+
+And optimize it to LLVM bitcode that looks like this:
+
+  @reltable.F = internal unnamed_addr constant [2 x i32] [i32 trunc (i64 sub (i64 ptrtoint ([2 x i8]* @.str to i64), i64 ptrtoint ([2 x i32]* @reltable.F to i64)) to i32), i32 trunc (i64 sub (i64 ptrtoint ([2 x i8]* @.str.1 to i64), i64 ptrtoint ([2 x i32]* @reltable.F to i64)) to i32)], align 4
+  @.str = private unnamed_addr constant [2 x i8] c"A\00", align 1
+  @.str.1 = private unnamed_addr constant [2 x i8] c"B\00", align 1
+
+  define dso_local i8* @F(i32 noundef %0) local_unnamed_addr #0 {
+    %2 = sext i32 %0 to i64
+    %3 = shl i64 %2, 2
+    %4 = call i8* @llvm.load.relative.i64(i8* bitcast ([2 x i32]* @reltable.F to i8*), i64 %3)
+    ret i8* %4
+  }
+
+There are several remarkable things about this LLVM bitcode:
+
+* The definition of @F is backed up a relative lookup table @reltable.F.
+  Invoking @F is tantamount to looking up a value in the table by using the
+  special @llvm.load.relative.i* intrinsic, which is described here:
+  https://releases.llvm.org/17.0.1/docs/LangRef.html#llvm-load-relative-intrinsic
+
+* The definition of @reltable.F itself is quite unorthodox. Conceptually, it is
+  an array of strings (@.str and @.str1), but where each element of the array
+  contains the relative offset of the string to the table itself. As a result,
+  it is not an array of pointers, but rather an array of i32s!
+
+* Each i32 in the array consists of the address of each string represented as an
+  integer (obtained via ptrtoint) subtracted from the address of the table,
+  followed by a trunc to ensure the result fits in an i32. (One weird result of
+  this encoding is that @reltable.F is defined recursively in terms of itself.)
+
+This optimization pass is handy for Clang's purposes, as it allows Clang to
+produce more efficient assembly code. Unfortunately, this encoding is quite
+problematic for crucible-llvm. The problem ultimately lies in the fact that we
+are performing pointer arithmetic on pointers from completely different
+allocation regions (e.g., subtracting @reltable.F from @.str), which
+crucible-llvm has no ability to reason about. (This optimization is also
+problematic for CHERI, which tracks pointer provenance in a similar way—see
+https://github.com/CTSRD-CHERI/llvm-project/issues/572).
+
+What's more, we don't have a reliable way of avoiding this optimization, as
+Clang's optimization pass manager doesn't provide a way to disable individual
+passes via command-line arguments. We could tell users to downgrade from -O1
+from -O0, but this would be a pretty severe workaround.
+
+Our solution is to manually "undo" the optimization ourselves. That is, we
+replace the definition of @reltable.F with bitcode that looks like this:
+
+  @reltable.F = internal unnamed_addr constant [2 x i8*] [i8* bitcast ([2 x i8]* @.str to i8*), i8* bitcast ([2 x i8]* @.str.1 to i8*)]
+
+This avoids any problematic uses of pointer arithmetic altogether. Here is how
+we do this:
+
+1. When processing global definitions in an LLVM module, we identify the names
+   of all globals that are passed as the first argument to
+   @llvm.load.relative.i*. We'll refer to these as "reltable-like" globals.
+
+   This check assumes that the globals are passed directly to
+   @llvm.load.relative.i*, rather than going through any intermediate
+   variables. This is likely a safe assumption to make, considering that
+   Clang's -O1 settings will usually optimize away any such intermediate
+   variables.
+
+2. For each reltable-like global, we check that the global has a constant
+   initializer of type [<N> x i32] where each element is of the form
+   `trunc (ptrtoint x - ptrtoint p)`. This is somewhat fragile, but the
+   documentation for llvm.load.relative.i* implies that LLVM itself checks
+   for code that looks like this, so we follow suit.
+
+3. For each element in the constant initializer array, we turn
+   `trunc (ptrtoint x - ptrtoint p)` into `bitcast x to i8*`. Note that the
+   this changes its type from `i32` to `i8*`.
+
+4. When translating a global definition to Crucible, we check if the global
+   is reltable-like. If so, we replace its constant initializer with the
+   `bitcast`ed version. We must also make sure that the global is translated
+   at type `[<N> x i8*]` rather than `[<N> x i32]`.
+
+   Furthermore, we must also make sure not to use the original global's
+   alignment, as the `bitcast`ed version will almost certainly have different
+   alignment requirements. We rely on crucible-llvm's alignment inference to
+   figure out what the new alignment should be.
+
+5. In the override for llvm.load.relative.i*, we make sure to adjust the second
+   argument (the pointer offset). This is because LLVM assumes that the offset
+   is for something of type `[<N> x i32]`, so an offset value of 4 (four bytes)
+   refers to the first element, an offset value of 8 refers to the second
+   element, and so on. On the other hand, something of type `[<N> x i8*]` will
+   likely require different offsets, since the size of a pointer may be greater
+   than four bytes (e.g., it is eight bytes on 64-bit architectures).
+
+   To account for this difference, we divide the offset value by 4 and then
+   multiply it by the number of bytes in the size of a pointer.
+
+It is worth emphasizing that this is a very ad hoc workaround. At the same time,
+it is likely the best we can do without substantially changing how crucible-llvm
+tracks pointer provenance.
+-}