[llvm] [LLVM][Intrinsics] Allow IIT fixed encoding table to be 32-bit (PR #178531)

[Rahul Joshi via llvm-commits]

https://github.com/jurahul updated https://github.com/llvm/llvm-project/pull/178531

>From 472ad3bc583478a352c9e7f8aba32d1d7819c5e9 Mon Sep 17 00:00:00 2001
From: Rahul Joshi <rjoshi at nvidia.com>
Date: Wed, 28 Jan 2026 15:13:16 -0800
Subject: [PATCH] [LLVM][Intrinsics] Allow IIT fixed encoding table to be
 32-bit

---
 llvm/lib/IR/Intrinsics.cpp                     | 17 +++++++++++------
 llvm/utils/TableGen/Basic/IntrinsicEmitter.cpp | 11 ++++++-----
 2 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/llvm/lib/IR/Intrinsics.cpp b/llvm/lib/IR/Intrinsics.cpp
index 48abead0929dd..039e9f47cb7f4 100644
--- a/llvm/lib/IR/Intrinsics.cpp
+++ b/llvm/lib/IR/Intrinsics.cpp
@@ -438,21 +438,26 @@ DecodeIITType(unsigned &NextElt, ArrayRef<unsigned char> Infos,
 
 void Intrinsic::getIntrinsicInfoTableEntries(
     ID id, SmallVectorImpl<IITDescriptor> &T) {
-  static_assert(sizeof(IIT_Table[0]) == 2,
-                "Expect 16-bit entries in IIT_Table");
-  // Check to see if the intrinsic's type was expressible by the table.
-  uint16_t TableVal = IIT_Table[id - 1];
+  // Note that `FixedEncodingTy` is defined in IntrinsicImpl.inc.
+  constexpr unsigned FixedEncodingBits = sizeof(FixedEncodingTy) * CHAR_BIT;
+  constexpr unsigned MSBPosition = FixedEncodingBits - 1;
+  // Mask with all bits 1 except the most significant bit.
+  constexpr unsigned Mask = (1U << MSBPosition) - 1;
+
+  FixedEncodingTy TableVal = IIT_Table[id - 1];
 
   // Decode the TableVal into an array of IITValues.
   SmallVector<unsigned char> IITValues;
   ArrayRef<unsigned char> IITEntries;
   unsigned NextElt = 0;
-  if (TableVal >> 15) {
+  // Check to see if the intrinsic's type was inlined in the fixed encoding
+  // table.
+  if (TableVal >> MSBPosition) {
     // This is an offset into the IIT_LongEncodingTable.
     IITEntries = IIT_LongEncodingTable;
 
     // Strip sentinel bit.
-    NextElt = TableVal & 0x7fff;
+    NextElt = TableVal & Mask;
   } else {
     // If the entry was encoded into a single word in the table itself, decode
     // it from an array of nibbles to an array of bytes.
diff --git a/llvm/utils/TableGen/Basic/IntrinsicEmitter.cpp b/llvm/utils/TableGen/Basic/IntrinsicEmitter.cpp
index 9fed5920a019f..051bfa05b52a3 100644
--- a/llvm/utils/TableGen/Basic/IntrinsicEmitter.cpp
+++ b/llvm/utils/TableGen/Basic/IntrinsicEmitter.cpp
@@ -354,9 +354,9 @@ void IntrinsicEmitter::EmitGenerator(const CodeGenIntrinsicTable &Ints,
   using FixedEncodingTy =
       std::conditional_t<Use16BitFixedEncoding, uint16_t, uint32_t>;
   constexpr unsigned FixedEncodingBits = sizeof(FixedEncodingTy) * CHAR_BIT;
+  constexpr unsigned MSBPosition = FixedEncodingBits - 1;
   // Mask with all bits 1 except the most significant bit.
-  const unsigned Mask = (1U << (FixedEncodingBits - 1)) - 1;
-  const unsigned MSBPostion = FixedEncodingBits - 1;
+  constexpr unsigned Mask = (1U << MSBPosition) - 1;
   StringRef FixedEncodingTypeName =
       Use16BitFixedEncoding ? "uint16_t" : "uint32_t";
 
@@ -374,7 +374,7 @@ void IntrinsicEmitter::EmitGenerator(const CodeGenIntrinsicTable &Ints,
 
     // Check to see if we can encode it into a 16/32 bit word.
     std::optional<uint32_t> Result = encodePacked(TypeSig);
-    if (Result && (*Result & Mask) == Result) {
+    if (Result && (*Result & Mask) == *Result) {
       FixedEncodings.push_back(static_cast<FixedEncodingTy>(*Result));
       continue;
     }
@@ -389,7 +389,8 @@ void IntrinsicEmitter::EmitGenerator(const CodeGenIntrinsicTable &Ints,
 
   OS << formatv(R"(// Global intrinsic function declaration type table.
 #ifdef GET_INTRINSIC_GENERATOR_GLOBAL
-static constexpr {} IIT_Table[] = {{
+using FixedEncodingTy = {};
+static constexpr FixedEncodingTy IIT_Table[] = {{
   )",
                 FixedEncodingTypeName);
 
@@ -410,7 +411,7 @@ static constexpr {} IIT_Table[] = {{
 
     // Otherwise, emit the offset into the long encoding table.  We emit it this
     // way so that it is easier to read the offset in the .def file.
-    OS << formatv("(1U<<{}) | {}, ", MSBPostion, Offset);
+    OS << formatv("(1U<<{}) | {}, ", MSBPosition, Offset);
   }
 
   OS << "0\n};\n\n";