Rework the Option library to reduce dynamic relocations (#119198)

Apologies for the large change, I looked for ways to break this up and
all of the ones I saw added real complexity. This change focuses on the
option's prefixed names and the array of prefixes. These are present in
every option and the dominant source of dynamic relocations for PIE or
PIC users of LLVM and Clang tooling. In some cases, 100s or 1000s of
them for the Clang driver which has a huge number of options.

This PR addresses this by building a string table and a prefixes table
that can be referenced with indices rather than pointers that require
dynamic relocations. This removes almost 7k dynmaic relocations from the
`clang` binary, roughly 8% of the remaining dynmaic relocations outside
of vtables. For busy-boxing use cases where many different option tables
are linked into the same binary, the savings add up a bit more.

The string table is a straightforward mechanism, but the prefixes
required some subtlety. They are encoded in a Pascal-string fashion with
a size followed by a sequence of offsets. This works relatively well for
the small realistic prefixes arrays in use.

Lots of code has to change in order to land this though: both all the
option library code has to be updated to use the string table and
prefixes table, and all the users of the options library have to be
updated to correctly instantiate the objects.

Some follow-up patches in the works to provide an abstraction for this
style of code, and to start using the same technique for some of the
other strings here now that the infrastructure is in place.

Author: chandlerc

clang-tools-extra/clangd/CompileCommands.cpp

@@ -458,20 +458,6 @@ llvm::ArrayRef<ArgStripper::Rule> ArgStripper::rulesFor(llvm::StringRef Arg) {
       PrevAlias[Self] = T;
       NextAlias[T] = Self;
     };
-    // Also grab prefixes for each option, these are not fully exposed.
-    llvm::ArrayRef<llvm::StringLiteral> Prefixes[DriverID::LastOption];
-
-#define PREFIX(NAME, VALUE)                                                    \
-  static constexpr llvm::StringLiteral NAME##_init[] = VALUE;                  \
-  static constexpr llvm::ArrayRef<llvm::StringLiteral> NAME(                   \
-      NAME##_init, std::size(NAME##_init) - 1);
-#define OPTION(PREFIX, PREFIXED_NAME, ID, KIND, GROUP, ALIAS, ALIASARGS,       \
-               FLAGS, VISIBILITY, PARAM, HELPTEXT, HELPTEXTSFORVARIANTS,       \
-               METAVAR, VALUES)                                                \
-  Prefixes[DriverID::OPT_##ID] = PREFIX;
-#include "clang/Driver/Options.inc"
-#undef OPTION
-#undef PREFIX
 
     struct {
       DriverID ID;
@@ -498,7 +484,9 @@ llvm::ArrayRef<ArgStripper::Rule> ArgStripper::rulesFor(llvm::StringRef Arg) {
       llvm::SmallVector<Rule> Rules;
       // Iterate over each alias, to add rules for parsing it.
       for (unsigned A = ID; A != DriverID::OPT_INVALID; A = NextAlias[A]) {
-        if (!Prefixes[A].size()) // option groups.
+        llvm::SmallVector<llvm::StringRef, 4> Prefixes;
+        DriverTable.appendOptionPrefixes(A, Prefixes);
+        if (Prefixes.empty()) // option groups.
           continue;
         auto Opt = DriverTable.getOption(A);
         // Exclude - and -foo pseudo-options.
@@ -507,7 +495,7 @@ llvm::ArrayRef<ArgStripper::Rule> ArgStripper::rulesFor(llvm::StringRef Arg) {
         auto Modes = getModes(Opt);
         std::pair<unsigned, unsigned> ArgCount = getArgCount(Opt);
         // Iterate over each spelling of the alias, e.g. -foo vs --foo.
-        for (StringRef Prefix : Prefixes[A]) {
+        for (StringRef Prefix : Prefixes) {
           llvm::SmallString<64> Buf(Prefix);
           Buf.append(Opt.getName());
           llvm::StringRef Spelling = Result->try_emplace(Buf).first->getKey();

clang/lib/Driver/DriverOptions.cpp

@@ -14,24 +14,21 @@ using namespace clang::driver;
 using namespace clang::driver::options;
 using namespace llvm::opt;
 
+#define OPTTABLE_STR_TABLE_CODE
+#include "clang/Driver/Options.inc"
+#undef OPTTABLE_STR_TABLE_CODE
+
 #define OPTTABLE_VALUES_CODE
 #include "clang/Driver/Options.inc"
 #undef OPTTABLE_VALUES_CODE
 
-#define PREFIX(NAME, VALUE)                                                    \
-  static constexpr llvm::StringLiteral NAME##_init[] = VALUE;                  \
-  static constexpr llvm::ArrayRef<llvm::StringLiteral> NAME(                   \
-      NAME##_init, std::size(NAME##_init) - 1);
+#define OPTTABLE_PREFIXES_TABLE_CODE
 #include "clang/Driver/Options.inc"
-#undef PREFIX
+#undef OPTTABLE_PREFIXES_TABLE_CODE
 
-static constexpr const llvm::StringLiteral PrefixTable_init[] =
-#define PREFIX_UNION(VALUES) VALUES
+#define OPTTABLE_PREFIXES_UNION_CODE
 #include "clang/Driver/Options.inc"
-#undef PREFIX_UNION
-    ;
-static constexpr const llvm::ArrayRef<llvm::StringLiteral>
-    PrefixTable(PrefixTable_init, std::size(PrefixTable_init) - 1);
+#undef OPTTABLE_PREFIXES_UNION_CODE
 
 static constexpr OptTable::Info InfoTable[] = {
 #define OPTION(...) LLVM_CONSTRUCT_OPT_INFO(__VA_ARGS__),
@@ -43,7 +40,9 @@ namespace {
 
 class DriverOptTable : public PrecomputedOptTable {
 public:
-  DriverOptTable() : PrecomputedOptTable(InfoTable, PrefixTable) {}
+  DriverOptTable()
+      : PrecomputedOptTable(OptionStrTable, OptionPrefixesTable, InfoTable,
+                            OptionPrefixesUnion) {}
 };
 }