https://github.com/arsenm updated 
https://github.com/llvm/llvm-project/pull/150869

>From 78b0865aef1217571e4127067ff19a7a4d533e3a Mon Sep 17 00:00:00 2001
From: Matt Arsenault <matthew.arsena...@amd.com>
Date: Sun, 27 Jul 2025 23:26:20 +0900
Subject: [PATCH] RuntimeLibcalls: Add bitset for available libcalls

This is a step towards separating the set of available libcalls
from the lowering decision of which call to use. Libcall recognition
now directly checks availability instead of indirectly checking through
the lowering table.
---
 llvm/include/llvm/IR/RuntimeLibcalls.h        | 64 +++++++++++++++++++
 llvm/lib/IR/RuntimeLibcalls.cpp               |  8 +--
 .../RuntimeLibcallEmitter-calling-conv.td     | 22 +++++++
 .../RuntimeLibcallEmitter-conflict-warning.td | 15 +++++
 llvm/test/TableGen/RuntimeLibcallEmitter.td   | 27 ++++++++
 .../TableGen/Basic/RuntimeLibcallsEmitter.cpp | 36 ++++++++++-
 6 files changed, 164 insertions(+), 8 deletions(-)

diff --git a/llvm/include/llvm/IR/RuntimeLibcalls.h 
b/llvm/include/llvm/IR/RuntimeLibcalls.h
index ad3bb4eca1c56..1fc217b10d49d 100644
--- a/llvm/include/llvm/IR/RuntimeLibcalls.h
+++ b/llvm/include/llvm/IR/RuntimeLibcalls.h
@@ -53,8 +53,64 @@ static inline auto libcall_impls() {
   return enum_seq(static_cast<RTLIB::LibcallImpl>(1), RTLIB::NumLibcallImpls);
 }
 
+/// Manage a bitset representing the list of available libcalls for a module.
+///
+/// Most of this exists because std::bitset cannot be statically constructed in
+/// a size large enough before c++23
+class LibcallImplBitset {
+private:
+  using BitWord = uint64_t;
+  static constexpr unsigned BitWordSize = sizeof(BitWord) * CHAR_BIT;
+  static constexpr size_t NumArrayElts =
+      divideCeil(RTLIB::NumLibcallImpls, BitWordSize);
+  using Storage = BitWord[NumArrayElts];
+
+  Storage Bits = {};
+
+  /// Get bitmask for \p Impl in its Bits element.
+  static constexpr BitWord getBitmask(RTLIB::LibcallImpl Impl) {
+    unsigned Idx = static_cast<unsigned>(Impl);
+    return BitWord(1) << (Idx % BitWordSize);
+  }
+
+  /// Get index of array element of Bits for \p Impl
+  static constexpr unsigned getArrayIdx(RTLIB::LibcallImpl Impl) {
+    return static_cast<unsigned>(Impl) / BitWordSize;
+  }
+
+public:
+  constexpr LibcallImplBitset() = default;
+  constexpr LibcallImplBitset(const Storage &Src) {
+    for (size_t I = 0; I != NumArrayElts; ++I)
+      Bits[I] = Src[I];
+  }
+
+  /// Check if a LibcallImpl is available.
+  constexpr bool test(RTLIB::LibcallImpl Impl) const {
+    BitWord Mask = getBitmask(Impl);
+    return (Bits[getArrayIdx(Impl)] & Mask) != 0;
+  }
+
+  /// Mark a LibcallImpl as available
+  void set(RTLIB::LibcallImpl Impl) {
+    assert(Impl != RTLIB::Unsupported && "cannot enable unsupported libcall");
+    Bits[getArrayIdx(Impl)] |= getBitmask(Impl);
+  }
+
+  /// Mark a LibcallImpl as unavailable
+  void unset(RTLIB::LibcallImpl Impl) {
+    assert(Impl != RTLIB::Unsupported && "cannot enable unsupported libcall");
+    Bits[getArrayIdx(Impl)] &= ~getBitmask(Impl);
+  }
+};
+
 /// A simple container for information about the supported runtime calls.
 struct RuntimeLibcallsInfo {
+private:
+  /// Bitset of libcalls a module may emit a call to.
+  LibcallImplBitset AvailableLibcallImpls;
+
+public:
   explicit RuntimeLibcallsInfo(
       const Triple &TT,
       ExceptionHandling ExceptionModel = ExceptionHandling::None,
@@ -132,6 +188,14 @@ struct RuntimeLibcallsInfo {
     return ImplToLibcall[Impl];
   }
 
+  bool isAvailable(RTLIB::LibcallImpl Impl) const {
+    return AvailableLibcallImpls.test(Impl);
+  }
+
+  void setAvailable(RTLIB::LibcallImpl Impl) {
+    AvailableLibcallImpls.set(Impl);
+  }
+
   /// Check if this is valid libcall for the current module, otherwise
   /// RTLIB::Unsupported.
   RTLIB::LibcallImpl getSupportedLibcallImpl(StringRef FuncName) const;
diff --git a/llvm/lib/IR/RuntimeLibcalls.cpp b/llvm/lib/IR/RuntimeLibcalls.cpp
index 8c90c52141dc7..569f63f28db77 100644
--- a/llvm/lib/IR/RuntimeLibcalls.cpp
+++ b/llvm/lib/IR/RuntimeLibcalls.cpp
@@ -114,12 +114,8 @@ RuntimeLibcallsInfo::getSupportedLibcallImpl(StringRef 
FuncName) const {
   for (auto I = Range.begin(); I != Range.end(); ++I) {
     RTLIB::LibcallImpl Impl =
         static_cast<RTLIB::LibcallImpl>(I - RuntimeLibcallNameOffsets.begin());
-
-    // FIXME: This should not depend on looking up ImplToLibcall, only the list
-    // of libcalls for the module.
-    RTLIB::LibcallImpl Recognized = LibcallImpls[ImplToLibcall[Impl]];
-    if (Recognized != RTLIB::Unsupported)
-      return Recognized;
+    if (isAvailable(Impl))
+      return Impl;
   }
 
   return RTLIB::Unsupported;
diff --git a/llvm/test/TableGen/RuntimeLibcallEmitter-calling-conv.td 
b/llvm/test/TableGen/RuntimeLibcallEmitter-calling-conv.td
index 49d5ecaa0e5c5..14c811da64910 100644
--- a/llvm/test/TableGen/RuntimeLibcallEmitter-calling-conv.td
+++ b/llvm/test/TableGen/RuntimeLibcallEmitter-calling-conv.td
@@ -48,12 +48,18 @@ def MSP430LibraryWithCondCC : SystemRuntimeLibrary<isMSP430,
 // CHECK-NEXT:     Entry = DefaultCC;
 // CHECK-NEXT:   }
 // CHECK-EMPTY:
+// CHECK-NEXT:   static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:     0x0000000000001a
+// CHECK-NEXT:   });
+// CHECK-NEXT:   AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT:    static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:        {RTLIB::MALLOC, RTLIB::malloc}, // malloc
 // CHECK-NEXT:    };
 // CHECK-EMPTY:
 // CHECK-NEXT:    for (const auto [Func, Impl] : LibraryCalls) {
 // CHECK-NEXT:      setLibcallImpl(Func, Impl);
+// CHECK-NEXT:      setAvailable(Impl);
 // CHECK-NEXT:    }
 // CHECK-EMPTY:
 // CHECK-NEXT:    static const LibcallImplPair LibraryCalls_AlwaysAvailable[] 
= {
@@ -63,6 +69,7 @@ def MSP430LibraryWithCondCC : SystemRuntimeLibrary<isMSP430,
 // CHECK-EMPTY:
 // CHECK-NEXT:    for (const auto [Func, Impl] : LibraryCalls_AlwaysAvailable) 
{
 // CHECK-NEXT:      setLibcallImpl(Func, Impl);
+// CHECK-NEXT:      setAvailable(Impl);
 // CHECK-NEXT:      setLibcallImplCallingConv(Impl, CallingConv::AVR_BUILTIN);
 // CHECK-NEXT:    }
 // CHECK-EMPTY:
@@ -70,12 +77,18 @@ def MSP430LibraryWithCondCC : SystemRuntimeLibrary<isMSP430,
 // CHECK-NEXT:  }
 // CHECK-EMPTY:
 // CHECK-NEXT: if (TT.getArch() == Triple::avr) {
+// CHECK-NEXT:   static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:     0x0000000000001a
+// CHECK-NEXT:   });
+// CHECK-NEXT:   AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT:   static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:       {RTLIB::MALLOC, RTLIB::malloc}, // malloc
 // CHECK-NEXT:   };
 // CHECK-EMPTY:
 // CHECK-NEXT:   for (const auto [Func, Impl] : LibraryCalls) {
 // CHECK-NEXT:     setLibcallImpl(Func, Impl);
+// CHECK-NEXT:     setAvailable(Impl);
 // CHECK-NEXT:   }
 // CHECK-EMPTY:
 // CHECK-NEXT:   static const LibcallImplPair LibraryCalls_AlwaysAvailable[] = 
{
@@ -85,6 +98,7 @@ def MSP430LibraryWithCondCC : SystemRuntimeLibrary<isMSP430,
 // CHECK-EMPTY:
 // CHECK-NEXT:   for (const auto [Func, Impl] : LibraryCalls_AlwaysAvailable) {
 // CHECK-NEXT:     setLibcallImpl(Func, Impl);
+// CHECK-NEXT:     setAvailable(Impl);
 // CHECK-NEXT:     setLibcallImplCallingConv(Impl, CallingConv::AVR_BUILTIN);
 // CHECK-NEXT:   }
 // CHECK-EMPTY:
@@ -92,12 +106,18 @@ def MSP430LibraryWithCondCC : 
SystemRuntimeLibrary<isMSP430,
 // CHECK-NEXT:  }
 // CHECK-EMPTY:
 // CHECK-NEXT:  if (TT.getArch() == Triple::msp430) {
+// CHECK-NEXT:    static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:      0x00000000000010
+// CHECK-NEXT:    });
+// CHECK-NEXT:    AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT:    static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:        {RTLIB::MALLOC, RTLIB::malloc}, // malloc
 // CHECK-NEXT:    };
 // CHECK-EMPTY:
 // CHECK-NEXT:    for (const auto [Func, Impl] : LibraryCalls) {
 // CHECK-NEXT:      setLibcallImpl(Func, Impl);
+// CHECK-NEXT:      setAvailable(Impl);
 // CHECK-NEXT:    }
 // CHECK-EMPTY:
 // CHECK-NEXT:    if ( isFoo() ) {
@@ -107,6 +127,7 @@ def MSP430LibraryWithCondCC : SystemRuntimeLibrary<isMSP430,
 // CHECK-EMPTY:
 // CHECK-NEXT:      for (const auto [Func, Impl] : LibraryCalls_anonymous_3) {
 // CHECK-NEXT:        setLibcallImpl(Func, Impl);
+// CHECK-NEXT:        setAvailable(Impl);
 // CHECK-NEXT:        setLibcallImplCallingConv(Impl, 
CallingConv::AVR_BUILTIN);
 // CHECK-NEXT:      }
 // CHECK-EMPTY:
@@ -119,6 +140,7 @@ def MSP430LibraryWithCondCC : SystemRuntimeLibrary<isMSP430,
 // CHECK-EMPTY:
 // CHECK-NEXT:      for (const auto [Func, Impl] : LibraryCalls_anonymous_5) {
 // CHECK-NEXT:        setLibcallImpl(Func, Impl);
+// CHECK-NEXT:        setAvailable(Impl);
 // CHECK-NEXT:        setLibcallImplCallingConv(Impl, 
CallingConv::MSP430_BUILTIN);
 // CHECK-NEXT:      }
 // CHECK-EMPTY:
diff --git a/llvm/test/TableGen/RuntimeLibcallEmitter-conflict-warning.td 
b/llvm/test/TableGen/RuntimeLibcallEmitter-conflict-warning.td
index 00c9d53846326..2a8739335a8dd 100644
--- a/llvm/test/TableGen/RuntimeLibcallEmitter-conflict-warning.td
+++ b/llvm/test/TableGen/RuntimeLibcallEmitter-conflict-warning.td
@@ -25,6 +25,11 @@ def dup1 : RuntimeLibcallImpl<ANOTHER_DUP>;
 // func_a and func_b both provide SOME_FUNC.
 
 // CHECK: if (isTargetArchA()) {
+// CHECK-NEXT: static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:   0x00000000000018
+// CHECK-NEXT: });
+// CHECK-NEXT: AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT: static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:   {RTLIB::SOME_FUNC, RTLIB::func_b}, // func_b
 // CHECK-NEXT: };
@@ -35,6 +40,11 @@ def TheSystemLibraryA : SystemRuntimeLibrary<isTargetArchA,
 >;
 
 // CHECK: if (isTargetArchB()) {
+// CHECK-NEXT: static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:   0x00000000000058
+// CHECK-NEXT: });
+// CHECK-NEXT: AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT: static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:   {RTLIB::OTHER_FUNC, RTLIB::other_func}, // other_func
 // CHECK-NEXT:  {RTLIB::SOME_FUNC, RTLIB::func_a}, // func_a
@@ -46,6 +56,11 @@ def TheSystemLibraryB : SystemRuntimeLibrary<isTargetArchB,
 >;
 
 // CHECK: if (isTargetArchC()) {
+// CHECK-NEXT: static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:   0x0000000000007e
+// CHECK-NEXT: });
+// CHECK-NEXT: AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT: static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:   {RTLIB::ANOTHER_DUP, RTLIB::dup1}, // dup1
 // CHECK-NEXT:   {RTLIB::OTHER_FUNC, RTLIB::other_func}, // other_func
diff --git a/llvm/test/TableGen/RuntimeLibcallEmitter.td 
b/llvm/test/TableGen/RuntimeLibcallEmitter.td
index 642f8b85a89c6..97a632e8e75c7 100644
--- a/llvm/test/TableGen/RuntimeLibcallEmitter.td
+++ b/llvm/test/TableGen/RuntimeLibcallEmitter.td
@@ -157,6 +157,11 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-NEXT:  };
 // CHECK-EMPTY:
 // CHECK-NEXT: if (TT.getArch() == Triple::blah) {
+// CHECK-NEXT:     static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:       0x000000000000e0
+// CHECK-NEXT:     });
+// CHECK-NEXT:     AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT:     static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:         {RTLIB::BZERO, RTLIB::bzero}, // bzero
 // CHECK-NEXT:         {RTLIB::CALLOC, RTLIB::calloc}, // calloc
@@ -165,6 +170,7 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-EMPTY:
 // CHECK-NEXT:     for (const auto [Func, Impl] : LibraryCalls) {
 // CHECK-NEXT:       setLibcallImpl(Func, Impl);
+// CHECK-NEXT:       setAvailable(Impl);
 // CHECK-NEXT:     }
 // CHECK-EMPTY:
 // CHECK-NEXT:    if (TT.hasCompilerRT()) {
@@ -175,6 +181,7 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-EMPTY:
 // CHECK-NEXT:      for (const auto [Func, Impl] : LibraryCalls_hasCompilerRT) 
{
 // CHECK-NEXT:        setLibcallImpl(Func, Impl);
+// CHECK-NEXT:        setAvailable(Impl);
 // CHECK-NEXT:      }
 // CHECK-EMPTY:
 // CHECK-NEXT:    }
@@ -186,6 +193,7 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-EMPTY:
 // CHECK-NEXT:      for (const auto [Func, Impl] : LibraryCalls_isBarOS) {
 // CHECK-NEXT:        setLibcallImpl(Func, Impl);
+// CHECK-NEXT:        setAvailable(Impl);
 // CHECK-NEXT:      }
 // CHECK-EMPTY:
 // CHECK-NEXT:    }
@@ -194,6 +202,11 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-NEXT: }
 // CHECK-EMPTY:
 // CHECK-NEXT: if (TT.getArch() == Triple::buzz) {
+// CHECK-NEXT:    static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:      0x00000000000118
+// CHECK-NEXT:    });
+// CHECK-NEXT:    AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT:    static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:        {RTLIB::SHL_I32, RTLIB::__ashlsi3}, // __ashlsi3
 // CHECK-NEXT:        {RTLIB::SQRT_F80, RTLIB::sqrtl_f80}, // sqrtl
@@ -202,12 +215,18 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-EMPTY:
 // CHECK-NEXT:    for (const auto [Func, Impl] : LibraryCalls) {
 // CHECK-NEXT:      setLibcallImpl(Func, Impl);
+// CHECK-NEXT:      setAvailable(Impl);
 // CHECK-NEXT:    }
 // CHECK-EMPTY:
 // CHECK-NEXT:   return;
 // CHECK-NEXT: }
 // CHECK-EMPTY:
 // CHECK-NEXT: if (TT.getArch() == Triple::foo) {
+// CHECK-NEXT:    static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:      0x000000000000a0
+// CHECK-NEXT:    });
+// CHECK-NEXT:    AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT:    static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:        {RTLIB::BZERO, RTLIB::bzero}, // bzero
 // CHECK-NEXT:        {RTLIB::SQRT_F128, RTLIB::sqrtl_f128}, // sqrtl
@@ -215,6 +234,7 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-EMPTY:
 // CHECK-NEXT:    for (const auto [Func, Impl] : LibraryCalls) {
 // CHECK-NEXT:      setLibcallImpl(Func, Impl);
+// CHECK-NEXT:      setAvailable(Impl);
 // CHECK-NEXT:    }
 // CHECK-EMPTY:
 // CHECK-NEXT:    if (TT.getOS() == Triple::bar) {
@@ -224,6 +244,7 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-EMPTY:
 // CHECK-NEXT:      for (const auto [Func, Impl] : LibraryCalls_isBarOS) {
 // CHECK-NEXT:        setLibcallImpl(Func, Impl);
+// CHECK-NEXT:        setAvailable(Impl);
 // CHECK-NEXT:      }
 // CHECK-EMPTY:
 // CHECK-NEXT:    }
@@ -232,6 +253,11 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-NEXT:  }
 // CHECK-EMPTY:
 // CHECK-NEXT: if (TT.getArch() == Triple::simple) {
+// CHECK-NEXT:    static constexpr LibcallImplBitset SystemAvailableImpls({
+// CHECK-NEXT:      0x00000000000158
+// CHECK-NEXT:    });
+// CHECK-NEXT:    AvailableLibcallImpls = SystemAvailableImpls;
+// CHECK-EMPTY:
 // CHECK-NEXT:    static const LibcallImplPair LibraryCalls[] = {
 // CHECK-NEXT:        {RTLIB::CALLOC, RTLIB::calloc}, // calloc
 // CHECK-NEXT:        {RTLIB::SHL_I32, RTLIB::__ashlsi3}, // __ashlsi3
@@ -241,6 +267,7 @@ def BlahLibrary : SystemRuntimeLibrary<isBlahArch, (add 
calloc, LibraryWithCondi
 // CHECK-EMPTY:
 // CHECK-NEXT:    for (const auto [Func, Impl] : LibraryCalls) {
 // CHECK-NEXT:      setLibcallImpl(Func, Impl);
+// CHECK-NEXT:      setAvailable(Impl);
 // CHECK-NEXT:    }
 // CHECK-EMPTY:
 // CHECK-NEXT:   return;
diff --git a/llvm/utils/TableGen/Basic/RuntimeLibcallsEmitter.cpp 
b/llvm/utils/TableGen/Basic/RuntimeLibcallsEmitter.cpp
index 412431b96d030..de361161a8bfd 100644
--- a/llvm/utils/TableGen/Basic/RuntimeLibcallsEmitter.cpp
+++ b/llvm/utils/TableGen/Basic/RuntimeLibcallsEmitter.cpp
@@ -8,6 +8,7 @@
 
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/Format.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TableGen/Error.h"
@@ -402,6 +403,10 @@ void 
RuntimeLibcallEmitter::emitSystemRuntimeLibrarySetCalls(
         PredicateWithCC()); // No predicate or CC override first.
 
     DenseMap<PredicateWithCC, LibcallsWithCC> Pred2Funcs;
+
+    SmallVector<uint64_t, 32> BitsetValues(
+        divideCeil(RuntimeLibcallImplDefList.size(), 64));
+
     for (const Record *Elt : *Elements) {
       const RuntimeLibcallImpl *LibCallImpl = getRuntimeLibcallImpl(Elt);
       if (!LibCallImpl) {
@@ -410,16 +415,24 @@ void 
RuntimeLibcallEmitter::emitSystemRuntimeLibrarySetCalls(
         continue;
       }
 
+      size_t BitIdx = LibCallImpl->getEnumVal();
+      uint64_t BitmaskVal = uint64_t(1) << (BitIdx % 64);
+      size_t BitsetIdx = BitIdx / 64;
+
       auto It = Func2Preds.find(LibCallImpl);
       if (It == Func2Preds.end()) {
+        BitsetValues[BitsetIdx] |= BitmaskVal;
         Pred2Funcs[PredicateWithCC()].LibcallImpls.push_back(LibCallImpl);
         continue;
       }
 
       for (const Record *Pred : It->second.first) {
         const Record *CC = It->second.second;
-        PredicateWithCC Key(Pred, CC);
+        AvailabilityPredicate SubsetPredicate(Pred);
+        if (SubsetPredicate.isAlwaysAvailable())
+          BitsetValues[BitsetIdx] |= BitmaskVal;
 
+        PredicateWithCC Key(Pred, CC);
         auto &Entry = Pred2Funcs[Key];
         Entry.LibcallImpls.push_back(LibCallImpl);
         Entry.CallingConv = It->second.second;
@@ -427,6 +440,22 @@ void 
RuntimeLibcallEmitter::emitSystemRuntimeLibrarySetCalls(
       }
     }
 
+    OS << "    static constexpr LibcallImplBitset SystemAvailableImpls({\n"
+       << indent(6);
+
+    ListSeparator LS;
+    unsigned EntryCount = 0;
+    for (uint64_t Bits : BitsetValues) {
+      if (EntryCount++ == 4) {
+        EntryCount = 1;
+        OS << ",\n" << indent(6);
+      } else
+        OS << LS;
+      OS << format_hex(Bits, 16);
+    }
+    OS << "\n    });\n"
+          "    AvailableLibcallImpls = SystemAvailableImpls;\n\n";
+
     SmallVector<PredicateWithCC, 0> SortedPredicates =
         PredicateSorter.takeVector();
 
@@ -498,8 +527,11 @@ void 
RuntimeLibcallEmitter::emitSystemRuntimeLibrarySetCalls(
          << indent(IndentDepth + 2)
          << "for (const auto [Func, Impl] : LibraryCalls";
       SubsetPredicate.emitTableVariableNameSuffix(OS);
+
+      // TODO: setAvailable should be emitted without the uniquing above
       OS << ") {\n"
-         << indent(IndentDepth + 4) << "setLibcallImpl(Func, Impl);\n";
+         << indent(IndentDepth + 4) << "setLibcallImpl(Func, Impl);\n"
+         << indent(IndentDepth + 4) << "setAvailable(Impl);\n";
 
       if (FuncsWithCC.CallingConv) {
         StringRef CCEnum =

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