[PATCH] D112385: [ELF] Support 128-bit bitmask in oneof(RelExpr)

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
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Taken from Chih-Mao Chen's D100835 <https://reviews.llvm.org/D100835>.

RelExpr has 64 bits now and needs the extension to support new members
(`R_PLT_GOTPLT` for `R_X86_64_PLTOFF64` support).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D112385

Files:
  lld/ELF/Relocations.cpp


Index: lld/ELF/Relocations.cpp
===================================================================
--- lld/ELF/Relocations.cpp
+++ lld/ELF/Relocations.cpp
@@ -125,35 +125,28 @@
 }
 
 namespace {
-// Build a bitmask with one bit set for each RelExpr.
-//
-// Constexpr function arguments can't be used in static asserts, so we
-// use template arguments to build the mask.
-// But function template partial specializations don't exist (needed
-// for base case of the recursion), so we need a dummy struct.
-template <RelExpr... Exprs> struct RelExprMaskBuilder {
-  static inline uint64_t build() { return 0; }
-};
+// Build a bitmask with one bit set for each 64 subset of RelExpr.
+constexpr uint64_t buildMask() { return 0; }
 
-// Specialization for recursive case.
-template <RelExpr Head, RelExpr... Tail>
-struct RelExprMaskBuilder<Head, Tail...> {
-  static inline uint64_t build() {
-    static_assert(0 <= Head && Head < 64,
-                  "RelExpr is too large for 64-bit mask!");
-    return (uint64_t(1) << Head) | RelExprMaskBuilder<Tail...>::build();
-  }
-};
+template <typename... Tails>
+constexpr uint64_t buildMask(int head, Tails... tails) {
+  return (0 <= head && head < 64 ? (uint64_t(1) << head) : 0) |
+         buildMask(tails...);
+}
 } // namespace
 
 // Return true if `Expr` is one of `Exprs`.
-// There are fewer than 64 RelExpr's, so we can represent any set of
-// RelExpr's as a constant bit mask and test for membership with a
-// couple cheap bitwise operations.
+// There are more than 64 but less than 128 RelExprs, so we divide the set of
+// exprs into [0, 64) and [64, 128) and represent each range as a constant
+// 64-bit mask. Then we decide which mask to test depending on the value of
+// expr and use a simple shift and bitwise-and to test for membership.
 template <RelExpr... Exprs> bool oneof(RelExpr expr) {
-  assert(0 <= expr && (int)expr < 64 &&
-         "RelExpr is too large for 64-bit mask!");
-  return (uint64_t(1) << expr) & RelExprMaskBuilder<Exprs...>::build();
+  assert(0 <= expr && (int)expr < 128 &&
+         "RelExpr is too large for 128-bit mask!");
+
+  if (expr >= 64)
+    return (uint64_t(1) << (expr - 64)) & buildMask((Exprs - 64)...);
+  return (uint64_t(1) << expr) & buildMask(Exprs...);
 }
 
 static RelType getMipsPairType(RelType type, bool isLocal) {


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