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

[MaskRay]

Summary

Taken from Chih-Mao Chen's D100835.

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

Note: RelExpr needs to have at least a member >=64 to prevent
-Wtautological-constant-out-of-range-compare for if (expr >= 64).

[peter.smith]

LGTM. One possible alternative would be to order the RelExpr so that all the code we wanted to compare were 0 - 63, but this is more fragile. It may be possible for someone to come up with something better, but I guess that always can come in a follow up patch.

[arichardson]

LGTM - This generates very similar code to @theraven's version in CHERI LLVM (https://www.godbolt.org/z/vW7zsKobj) and is slightly simpler to understand.

[MaskRay]

Prefer static to anonymous namespace for functions.

Add a note that we need a RelExpr member to suppress -Wtautological-constant-compare

[theraven]

This looks cleaner and simpler than mine, which was a quick hack to get it compiling. Thanks!

lld/ELF/Relocations.cpp

void elf::reportRangeError(uint8_t *loc, int64_t v, int n, const Symbol &sym,
   std::string hint;
   if (!sym.getName().empty())
     hint = "; references " + lld::toString(sym) + getDefinedLocation(sym);
   errorOrWarn(errPlace.loc + msg + " is out of range: " + Twine(v) +
               " is not in [" + Twine(llvm::minIntN(n)) + ", " +
               Twine(llvm::maxIntN(n)) + "]" + hint);
 }
 
-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.
+static 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();
-  }
-};
-} // namespace
+template <typename... Tails>
+static constexpr uint64_t buildMask(int head, Tails... tails) {
+  return (0 <= head && head < 64 ? uint64_t(1) << head : 0) |
+         buildMask(tails...);
 
 // 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.
-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();
+// 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> static bool oneof(RelExpr expr) {
+  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) {
   switch (type) {
   case R_MIPS_HI16:
     return R_MIPS_LO16;
   case R_MIPS_GOT16:
     // In case of global symbol, the R_MIPS_GOT16 relocation does not