Switch builtin strings to use string tables (#118734)

The Clang binary (and any binary linking Clang as a library), when built
using PIE, ends up with a pretty shocking number of dynamic relocations
to apply to the executable image: roughly 400k.

Each of these takes up binary space in the executable, and perhaps most
interestingly takes start-up time to apply the relocations.

The largest pattern I identified were the strings used to describe
target builtins. The addresses of these string literals were stored into
huge arrays, each one requiring a dynamic relocation. The way to avoid
this is to design the target builtins to use a single large table of
strings and offsets within the table for the individual strings. This
switches the builtin management to such a scheme.

This saves over 100k dynamic relocations by my measurement, an over 25%
reduction. Just looking at byte size improvements, using the `bloaty`
tool to compare a newly built `clang` binary to an old one:

```
    FILE SIZE        VM SIZE
 --------------  --------------
  +1.4%  +653Ki  +1.4%  +653Ki    .rodata
  +0.0%    +960  +0.0%    +960    .text
  +0.0%    +197  +0.0%    +197    .dynstr
  +0.0%    +184  +0.0%    +184    .eh_frame
  +0.0%     +96  +0.0%     +96    .dynsym
  +0.0%     +40  +0.0%     +40    .eh_frame_hdr
  +114%     +32  [ = ]       0    [Unmapped]
  +0.0%     +20  +0.0%     +20    .gnu.hash
  +0.0%      +8  +0.0%      +8    .gnu.version
  +0.9%      +7  +0.9%      +7    [LOAD #2 [R]]
  [ = ]       0 -75.4% -3.00Ki    .relro_padding
 -16.1%  -802Ki -16.1%  -802Ki    .data.rel.ro
 -27.3% -2.52Mi -27.3% -2.52Mi    .rela.dyn
  -1.6% -2.66Mi  -1.6% -2.66Mi    TOTAL
```

We get a 16% reduction in the `.data.rel.ro` section, and nearly 30%
reduction in `.rela.dyn` where those reloctaions are stored.

This is also visible in my benchmarking of binary start-up overhead at
least:

```
Benchmark 1: ./old_clang --version
  Time (mean ± σ):      17.6 ms ±   1.5 ms    [User: 4.1 ms, System: 13.3 ms]
  Range (min … max):    14.2 ms …  22.8 ms    162 runs

Benchmark 2: ./new_clang --version
  Time (mean ± σ):      15.5 ms ±   1.4 ms    [User: 3.6 ms, System: 11.8 ms]
  Range (min … max):    12.4 ms …  20.3 ms    216 runs

Summary
  './new_clang --version' ran
    1.13 ± 0.14 times faster than './old_clang --version'
```

We get about 2ms faster `--version` runs. While there is a lot of noise
in binary execution time, this delta is pretty consistent, and
represents over 10% improvement. This is particularly interesting to me
because for very short source files, repeatedly starting the `clang`
binary is actually the dominant cost. For example, `configure` scripts
running against the `clang` compiler are slow in large part because of
binary start up time, not the time to process the actual inputs to the
compiler.

----

This PR implements the string tables using `constexpr` code and the
existing macro system. I understand that the builtins are moving towards
a TableGen model, and if complete that would provide more options for
modeling this. Unfortunately, that migration isn't complete, and even
the parts that are migrated still rely on the ability to break out of
the TableGen model and directly expand an X-macro style `BUILTIN(...)`
textually. I looked at trying to complete the move to TableGen, but it
would both require the difficult migration of the remaining targets, and
solving some tricky problems with how to move away from any macro-based
expansion.

I was also able to find a reasonably clean and effective way of doing
this with the existing macros and some `constexpr` code that I think is
clean enough to be a pretty good intermediate state, and maybe give a
good target for the eventual TableGen solution. I was also able to
factor the macros into set of consistent patterns that avoids a
significant regression in overall boilerplate.

1 files changed, 81 insertions, 39 deletions

diff --git a/clang/lib/Basic/Builtins.cpp b/clang/lib/Basic/Builtins.cpp
index 25a6015..c991139 100644
--- a/clang/lib/Basic/Builtins.cpp
+++ b/clang/lib/Basic/Builtins.cpp
@@ -29,54 +29,93 @@ const char *HeaderDesc::getName() const {
   llvm_unreachable("Unknown HeaderDesc::HeaderID enum");
 }
 
-static constexpr Builtin::Info BuiltinInfo[] = {
-    {"not a builtin function", nullptr, nullptr, nullptr, HeaderDesc::NO_HEADER,
-     ALL_LANGUAGES},
-#define BUILTIN(ID, TYPE, ATTRS)                                               \
-  {#ID, TYPE, ATTRS, nullptr, HeaderDesc::NO_HEADER, ALL_LANGUAGES},
-#define LANGBUILTIN(ID, TYPE, ATTRS, LANGS)                                    \
-  {#ID, TYPE, ATTRS, nullptr, HeaderDesc::NO_HEADER, LANGS},
-#define LIBBUILTIN(ID, TYPE, ATTRS, HEADER, LANGS)                             \
-  {#ID, TYPE, ATTRS, nullptr, HeaderDesc::HEADER, LANGS},
+static constexpr auto BuiltinStorage =
+    Builtin::Storage<Builtin::FirstTSBuiltin>::Make(
+        CLANG_BUILTIN_STR_TABLE("not a builtin function", "", "")
+#define BUILTIN CLANG_BUILTIN_STR_TABLE
 #include "clang/Basic/Builtins.inc"
-};
+            ,
+        {CLANG_BUILTIN_ENTRY("not a builtin function", "", "")
+#define BUILTIN CLANG_BUILTIN_ENTRY
+#define LANGBUILTIN CLANG_LANGBUILTIN_ENTRY
+#define LIBBUILTIN CLANG_LIBBUILTIN_ENTRY
+#include "clang/Basic/Builtins.inc"
+        });
 
-const Builtin::Info &Builtin::Context::getRecord(unsigned ID) const {
+std::pair<const char *, const Builtin::Info &>
+Builtin::Context::getStrTableAndInfo(unsigned ID) const {
   if (ID < Builtin::FirstTSBuiltin)
-    return BuiltinInfo[ID];
-  assert(((ID - Builtin::FirstTSBuiltin) <
-          (TSRecords.size() + AuxTSRecords.size())) &&
-         "Invalid builtin ID!");
+    return {BuiltinStorage.StringTable, BuiltinStorage.Infos[ID]};
+  assert(
+      ((ID - Builtin::FirstTSBuiltin) < (TSInfos.size() + AuxTSInfos.size())) &&
+      "Invalid builtin ID!");
   if (isAuxBuiltinID(ID))
-    return AuxTSRecords[getAuxBuiltinID(ID) - Builtin::FirstTSBuiltin];
-  return TSRecords[ID - Builtin::FirstTSBuiltin];
+    return {AuxTSStrTable,
+            AuxTSInfos[getAuxBuiltinID(ID) - Builtin::FirstTSBuiltin]};
+  return {TSStrTable, TSInfos[ID - Builtin::FirstTSBuiltin]};
+}
+
+static llvm::StringRef getStrFromTable(const char *StrTable, int Offset) {
+  return &StrTable[Offset];
+}
+
+/// Return the identifier name for the specified builtin,
+/// e.g. "__builtin_abs".
+llvm::StringRef Builtin::Context::getName(unsigned ID) const {
+  const auto &[StrTable, I] = getStrTableAndInfo(ID);
+  return getStrFromTable(StrTable, I.Offsets.Name);
+}
+
+const char *Builtin::Context::getTypeString(unsigned ID) const {
+  const auto &[StrTable, I] = getStrTableAndInfo(ID);
+  return getStrFromTable(StrTable, I.Offsets.Type).data();
+}
+
+const char *Builtin::Context::getAttributesString(unsigned ID) const {
+  const auto &[StrTable, I] = getStrTableAndInfo(ID);
+  return getStrFromTable(StrTable, I.Offsets.Attributes).data();
+}
+
+const char *Builtin::Context::getRequiredFeatures(unsigned ID) const {
+  const auto &[StrTable, I] = getStrTableAndInfo(ID);
+  return getStrFromTable(StrTable, I.Offsets.Features).data();
 }
 
 void Builtin::Context::InitializeTarget(const TargetInfo &Target,
                                         const TargetInfo *AuxTarget) {
-  assert(TSRecords.empty() && "Already initialized target?");
-  TSRecords = Target.getTargetBuiltins();
-  if (AuxTarget)
-    AuxTSRecords = AuxTarget->getTargetBuiltins();
+  assert(TSStrTable == nullptr && "Already initialized target?");
+  assert(TSInfos.empty() && "Already initialized target?");
+  std::tie(TSStrTable, TSInfos) = Target.getTargetBuiltinStorage();
+  if (AuxTarget) {
+    std::tie(AuxTSStrTable, AuxTSInfos) = AuxTarget->getTargetBuiltinStorage();
+  }
 }
 
 bool Builtin::Context::isBuiltinFunc(llvm::StringRef FuncName) {
   bool InStdNamespace = FuncName.consume_front("std-");
+  const char *StrTable = BuiltinStorage.StringTable;
   for (unsigned i = Builtin::NotBuiltin + 1; i != Builtin::FirstTSBuiltin;
        ++i) {
-    if (FuncName == BuiltinInfo[i].Name &&
-        (bool)strchr(BuiltinInfo[i].Attributes, 'z') == InStdNamespace)
-      return strchr(BuiltinInfo[i].Attributes, 'f') != nullptr;
+    const auto &I = BuiltinStorage.Infos[i];
+    if (FuncName == getStrFromTable(StrTable, I.Offsets.Name) &&
+        (bool)strchr(getStrFromTable(StrTable, I.Offsets.Attributes).data(),
+                     'z') == InStdNamespace)
+      return strchr(getStrFromTable(StrTable, I.Offsets.Attributes).data(),
+                    'f') != nullptr;
   }
 
   return false;
 }
 
 /// Is this builtin supported according to the given language options?
-static bool builtinIsSupported(const Builtin::Info &BuiltinInfo,
+static bool builtinIsSupported(const char *StrTable,
+                               const Builtin::Info &BuiltinInfo,
                                const LangOptions &LangOpts) {
+  auto AttributesStr =
+      getStrFromTable(StrTable, BuiltinInfo.Offsets.Attributes);
+
   /* Builtins Unsupported */
-  if (LangOpts.NoBuiltin && strchr(BuiltinInfo.Attributes, 'f') != nullptr)
+  if (LangOpts.NoBuiltin && strchr(AttributesStr.data(), 'f') != nullptr)
     return false;
   /* CorBuiltins Unsupported */
   if (!LangOpts.Coroutines && (BuiltinInfo.Langs & COR_LANG))
@@ -123,7 +162,7 @@ static bool builtinIsSupported(const Builtin::Info &BuiltinInfo,
   if (!LangOpts.CPlusPlus && BuiltinInfo.Langs == CXX_LANG)
     return false;
   /* consteval Unsupported */
-  if (!LangOpts.CPlusPlus20 && strchr(BuiltinInfo.Attributes, 'G') != nullptr)
+  if (!LangOpts.CPlusPlus20 && strchr(AttributesStr.data(), 'G') != nullptr)
     return false;
   return true;
 }
@@ -134,20 +173,23 @@ static bool builtinIsSupported(const Builtin::Info &BuiltinInfo,
 void Builtin::Context::initializeBuiltins(IdentifierTable &Table,
                                           const LangOptions& LangOpts) {
   // Step #1: mark all target-independent builtins with their ID's.
-  for (unsigned i = Builtin::NotBuiltin+1; i != Builtin::FirstTSBuiltin; ++i)
-    if (builtinIsSupported(BuiltinInfo[i], LangOpts)) {
-      Table.get(BuiltinInfo[i].Name).setBuiltinID(i);
+  for (const auto &&[Index, I] :
+       llvm::enumerate(llvm::ArrayRef(BuiltinStorage.Infos).drop_front()))
+    if (builtinIsSupported(BuiltinStorage.StringTable, I, LangOpts)) {
+      Table.get(getStrFromTable(BuiltinStorage.StringTable, I.Offsets.Name))
+          .setBuiltinID(Index + 1);
     }
 
   // Step #2: Register target-specific builtins.
-  for (unsigned i = 0, e = TSRecords.size(); i != e; ++i)
-    if (builtinIsSupported(TSRecords[i], LangOpts))
-      Table.get(TSRecords[i].Name).setBuiltinID(i + Builtin::FirstTSBuiltin);
+  for (const auto &&[Index, I] : llvm::enumerate(TSInfos))
+    if (builtinIsSupported(TSStrTable, I, LangOpts))
+      Table.get(getStrFromTable(TSStrTable, I.Offsets.Name))
+          .setBuiltinID(Index + Builtin::FirstTSBuiltin);
 
   // Step #3: Register target-specific builtins for AuxTarget.
-  for (unsigned i = 0, e = AuxTSRecords.size(); i != e; ++i)
-    Table.get(AuxTSRecords[i].Name)
-        .setBuiltinID(i + Builtin::FirstTSBuiltin + TSRecords.size());
+  for (const auto &&[Index, I] : llvm::enumerate(AuxTSInfos))
+    Table.get(getStrFromTable(AuxTSStrTable, I.Offsets.Name))
+        .setBuiltinID(Index + Builtin::FirstTSBuiltin + TSInfos.size());
 
   // Step #4: Unregister any builtins specified by -fno-builtin-foo.
   for (llvm::StringRef Name : LangOpts.NoBuiltinFuncs) {
@@ -164,7 +206,7 @@ void Builtin::Context::initializeBuiltins(IdentifierTable &Table,
 }
 
 unsigned Builtin::Context::getRequiredVectorWidth(unsigned ID) const {
-  const char *WidthPos = ::strchr(getRecord(ID).Attributes, 'V');
+  const char *WidthPos = ::strchr(getAttributesString(ID), 'V');
   if (!WidthPos)
     return 0;
 
@@ -187,7 +229,7 @@ bool Builtin::Context::isLike(unsigned ID, unsigned &FormatIdx,
   assert(::toupper(Fmt[0]) == Fmt[1] &&
          "Format string is not in the form \"xX\"");
 
-  const char *Like = ::strpbrk(getRecord(ID).Attributes, Fmt);
+  const char *Like = ::strpbrk(getAttributesString(ID), Fmt);
   if (!Like)
     return false;
 
@@ -214,7 +256,7 @@ bool Builtin::Context::isScanfLike(unsigned ID, unsigned &FormatIdx,
 
 bool Builtin::Context::performsCallback(unsigned ID,
                                         SmallVectorImpl<int> &Encoding) const {
-  const char *CalleePos = ::strchr(getRecord(ID).Attributes, 'C');
+  const char *CalleePos = ::strchr(getAttributesString(ID), 'C');
   if (!CalleePos)
     return false;