[llvm] RuntimeLibcalls: Generate table of libcall name lengths (PR #153210)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm updated https://github.com/llvm/llvm-project/pull/153210

>From f0c374c169826c88d631f0f4b4f64ff7228e6cbc Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Tue, 12 Aug 2025 21:08:47 +0900
Subject: [PATCH] RuntimeLibcalls: Generate table of libcall name lengths

Avoids strlen when constructing the returned StringRef. We were emitting
these in the libcall name lookup anyway, so split out the offsets for
general use.

Currently emitted as a separate table, not sure if it would be better
to change the string offset table to store pairs of offset and width
instead.
---
 llvm/include/llvm/IR/RuntimeLibcalls.h        |  5 +++-
 llvm/test/TableGen/RuntimeLibcallEmitter.td   | 23 ++++++++++++----
 .../TableGen/Basic/RuntimeLibcallsEmitter.cpp | 27 ++++++++++++-------
 3 files changed, 39 insertions(+), 16 deletions(-)

diff --git a/llvm/include/llvm/IR/RuntimeLibcalls.h b/llvm/include/llvm/IR/RuntimeLibcalls.h
index 620774fd296e3..308be543de2bd 100644
--- a/llvm/include/llvm/IR/RuntimeLibcalls.h
+++ b/llvm/include/llvm/IR/RuntimeLibcalls.h
@@ -85,7 +85,9 @@ struct RuntimeLibcallsInfo {
   static StringRef getLibcallImplName(RTLIB::LibcallImpl CallImpl) {
     if (CallImpl == RTLIB::Unsupported)
       return StringRef();
-    return RuntimeLibcallImplNameTable[RuntimeLibcallNameOffsetTable[CallImpl]];
+    return StringRef(RuntimeLibcallImplNameTable.getCString(
+                         RuntimeLibcallNameOffsetTable[CallImpl]),
+                     RuntimeLibcallNameSizeTable[CallImpl]);
   }
 
   /// Return the lowering's selection of implementation call for \p Call
@@ -182,6 +184,7 @@ struct RuntimeLibcallsInfo {
   LLVM_ABI static const char RuntimeLibcallImplNameTableStorage[];
   LLVM_ABI static const StringTable RuntimeLibcallImplNameTable;
   LLVM_ABI static const uint16_t RuntimeLibcallNameOffsetTable[];
+  LLVM_ABI static const uint8_t RuntimeLibcallNameSizeTable[];
 
   /// Map from a concrete LibcallImpl implementation to its RTLIB::Libcall kind.
   LLVM_ABI static const RTLIB::Libcall ImplToLibcall[RTLIB::NumLibcallImpls];
diff --git a/llvm/test/TableGen/RuntimeLibcallEmitter.td b/llvm/test/TableGen/RuntimeLibcallEmitter.td
index 54ca3f97e2d4b..7c62402227f7d 100644
--- a/llvm/test/TableGen/RuntimeLibcallEmitter.td
+++ b/llvm/test/TableGen/RuntimeLibcallEmitter.td
@@ -137,6 +137,19 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add calloc, LibraryWithCondi
 // CHECK-NEXT:   54, // sqrtl
 // CHECK-NEXT:   54, // sqrtl
 // CHECK-NEXT: };
+// CHECK-EMPTY:
+// CHECK-NEXT: const uint8_t RTLIB::RuntimeLibcallsInfo::RuntimeLibcallNameSizeTable[] = {
+// CHECK-NEXT:   0,
+// CHECK-NEXT:   9,
+// CHECK-NEXT:   9,
+// CHECK-NEXT:   9,
+// CHECK-NEXT:   9,
+// CHECK-NEXT:   5,
+// CHECK-NEXT:   6,
+// CHECK-NEXT:   5,
+// CHECK-NEXT:   5,
+// CHECK-NEXT: };
+// CHECK-EMPTY:
 // CHECK-NEXT: const RTLIB::Libcall llvm::RTLIB::RuntimeLibcallsInfo::ImplToLibcall[RTLIB::NumLibcallImpls] = {
 // CHECK-NEXT: RTLIB::UNKNOWN_LIBCALL, // RTLIB::Unsupported
 // CHECK-NEXT: RTLIB::MEMCPY, // RTLIB::___memcpy
@@ -162,11 +175,11 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add calloc, LibraryWithCondi
 // CHECK-NEXT: }
 
 // CHECK: iota_range<RTLIB::LibcallImpl> RTLIB::RuntimeLibcallsInfo::lookupLibcallImplNameImpl(StringRef Name) {
-// CHECK: static constexpr std::pair<uint16_t, uint16_t> HashTableNameToEnum[16] = {
-// CHECK: {2, 9}, // 0x000000705301b8, ___memset
-// CHECK: {0, 0},
-// CHECK: {6, 6}, // 0x0000001417a2af, calloc
-// CHECK: {0, 0},
+// CHECK: static constexpr uint16_t HashTableNameToEnum[16] = {
+// CHECK: 2, // 0x000000705301b8, ___memset
+// CHECK: 0,
+// CHECK: 6, // 0x0000001417a2af, calloc
+// CHECK: 0,
 // CHECK: };
 
 // CHECK: unsigned Idx = (hash(Name) % 8) * 2;
diff --git a/llvm/utils/TableGen/Basic/RuntimeLibcallsEmitter.cpp b/llvm/utils/TableGen/Basic/RuntimeLibcallsEmitter.cpp
index 775cef22db0b6..c305e6323ca9d 100644
--- a/llvm/utils/TableGen/Basic/RuntimeLibcallsEmitter.cpp
+++ b/llvm/utils/TableGen/Basic/RuntimeLibcallsEmitter.cpp
@@ -459,17 +459,14 @@ void RuntimeLibcallEmitter::emitNameMatchHashTable(
   OS << "iota_range<RTLIB::LibcallImpl> RTLIB::RuntimeLibcallsInfo::"
         "lookupLibcallImplNameImpl(StringRef Name) {\n";
 
-  // Emit pair of RTLIB::LibcallImpl, size of the string name. It's important to
-  // avoid strlen on the string table entries.
-  OS << "  static constexpr std::pair<uint16_t, uint16_t> HashTableNameToEnum["
-     << Lookup.size() << "] = {\n";
+  // Emit RTLIB::LibcallImpl values
+  OS << "  static constexpr uint16_t HashTableNameToEnum[" << Lookup.size()
+     << "] = {\n";
 
   for (auto [FuncName, Hash, TableVal] : Lookup) {
-    OS << "    {" << TableVal << ", " << FuncName.size() << "},";
-
-    if (TableVal != 0) {
+    OS << "    " << TableVal << ',';
+    if (TableVal != 0)
       OS << " // " << format_hex(Hash, 16) << ", " << FuncName;
-    }
 
     OS << '\n';
   }
@@ -480,11 +477,12 @@ void RuntimeLibcallEmitter::emitNameMatchHashTable(
      << ";\n\n"
         "  for (int I = 0; I != "
      << Collisions << R"(; ++I) {
-    auto [Entry, StringSize] = HashTableNameToEnum[Idx + I];
+    const uint16_t Entry = HashTableNameToEnum[Idx + I];
     const uint16_t StrOffset = RuntimeLibcallNameOffsetTable[Entry];
+    const uint8_t StrSize = RuntimeLibcallNameSizeTable[Entry];
     StringRef Str(
       &RTLIB::RuntimeLibcallsInfo::RuntimeLibcallImplNameTableStorage[StrOffset],
-      StringSize);
+      StrSize);
     if (Str == Name)
       return libcallImplNameHit(Entry, StrOffset);
   }
@@ -520,6 +518,15 @@ const uint16_t RTLIB::RuntimeLibcallsInfo::RuntimeLibcallNameOffsetTable[] = {
   }
   OS << "};\n";
 
+  OS << R"(
+const uint8_t RTLIB::RuntimeLibcallsInfo::RuntimeLibcallNameSizeTable[] = {
+)";
+
+  OS << "  0,\n";
+  for (const RuntimeLibcallImpl &LibCallImpl : RuntimeLibcallImplDefList)
+    OS << "  " << LibCallImpl.getLibcallFuncName().size() << ",\n";
+  OS << "};\n\n";
+
   // Emit the reverse mapping from implementation libraries to RTLIB::Libcall
   OS << "const RTLIB::Libcall llvm::RTLIB::RuntimeLibcallsInfo::"
         "ImplToLibcall[RTLIB::NumLibcallImpls] = {\n"