[llvm-branch-commits] [llvm] [Bitcode] Add ByteCast encoding and decoding (PR #221885)

[Anshil Gandhi via llvm-branch-commits]

https://github.com/gandhi56 created https://github.com/llvm/llvm-project/pull/221885

## Summary
- Add `CAST_BYTECAST` to the bitcode cast opcode table.
- Wire reader and writer mappings so `bytecast` instructions round-trip in bitcode.
- Add `llvm/test/Bitcode/bytecast.ll` round-trip coverage.

## Test plan
- [x] `ninja check-llvm` passes on this branch
- [x] `llvm/test/Bitcode/bytecast.ll` passes

Depends on #221884


Made with [Cursor](https://cursor.com)

>From 537d8fab6a47a715b3ca9532e8d983468836d80e Mon Sep 17 00:00:00 2001
From: Anshil Gandhi <Anshil.Gandhi at amd.com>
Date: Mon, 7 Sep 2026 07:45:48 -0500
Subject: [PATCH] [Bitcode] Add ByteCast encoding and decoding

Add CAST_BYTECAST to the bitcode cast opcode table and wire it through the
reader and writer so bytecast instructions round-trip in bitcode.

Co-authored-by: Cursor <cursoragent at cursor.com>
---
 llvm/include/llvm/Bitcode/LLVMBitCodes.h  |  1 +
 llvm/lib/Bitcode/Reader/BitcodeReader.cpp |  1 +
 llvm/lib/Bitcode/Writer/BitcodeWriter.cpp |  1 +
 llvm/test/Bitcode/bytecast.ll             | 72 +++++++++++++++++++++++
 4 files changed, 75 insertions(+)
 create mode 100644 llvm/test/Bitcode/bytecast.ll

diff --git a/llvm/include/llvm/Bitcode/LLVMBitCodes.h b/llvm/include/llvm/Bitcode/LLVMBitCodes.h
index 4099b57f5482e..1422912900d2e 100644
--- a/llvm/include/llvm/Bitcode/LLVMBitCodes.h
+++ b/llvm/include/llvm/Bitcode/LLVMBitCodes.h
@@ -471,6 +471,7 @@ enum CastOpcodes {
   CAST_BITCAST = 11,
   CAST_ADDRSPACECAST = 12,
   CAST_PTRTOADDR = 13,
+  CAST_BYTECAST = 14,
 };
 
 /// UnaryOpcodes - These are values used in the bitcode files to encode which
diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 35e71c93c0109..888dbd8f8929b 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -1339,6 +1339,7 @@ static int getDecodedCastOpcode(unsigned Val) {
   case bitc::CAST_PTRTOINT: return Instruction::PtrToInt;
   case bitc::CAST_INTTOPTR: return Instruction::IntToPtr;
   case bitc::CAST_BITCAST : return Instruction::BitCast;
+  case bitc::CAST_BYTECAST : return Instruction::ByteCast;
   case bitc::CAST_ADDRSPACECAST: return Instruction::AddrSpaceCast;
   }
 }
diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index 859e073b91cc5..01816f2c7f685 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -661,6 +661,7 @@ static unsigned getEncodedCastOpcode(unsigned Opcode) {
   case Instruction::PtrToInt: return bitc::CAST_PTRTOINT;
   case Instruction::IntToPtr: return bitc::CAST_INTTOPTR;
   case Instruction::BitCast : return bitc::CAST_BITCAST;
+  case Instruction::ByteCast: return bitc::CAST_BYTECAST;
   case Instruction::AddrSpaceCast: return bitc::CAST_ADDRSPACECAST;
   }
 }
diff --git a/llvm/test/Bitcode/bytecast.ll b/llvm/test/Bitcode/bytecast.ll
new file mode 100644
index 0000000000000..ef97e289085e7
--- /dev/null
+++ b/llvm/test/Bitcode/bytecast.ll
@@ -0,0 +1,72 @@
+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
+ at g = global i64 0
+
+; CHECK: @ce.ptr.to.byte = global b64 bytecast (ptr @g to b64)
+ at ce.ptr.to.byte = global b64 bytecast (ptr @g to b64)
+
+define b64 @int_to_byte(i64 %i) {
+  ; CHECK: %b = bytecast i64 %i to b64
+  %b = bytecast i64 %i to b64
+  ret b64 %b
+}
+
+define i64 @byte_to_int(b64 %b) {
+  ; CHECK: %i = bytecast b64 %b to i64
+  %i = bytecast b64 %b to i64
+  ret i64 %i
+}
+
+define double @byte_to_fp(b64 %b) {
+  ; CHECK: %d = bytecast b64 %b to double
+  %d = bytecast b64 %b to double
+  ret double %d
+}
+
+define b64 @fp_to_byte(double %d) {
+  ; CHECK: %b = bytecast double %d to b64
+  %b = bytecast double %d to b64
+  ret b64 %b
+}
+
+define ptr @byte_to_ptr(b64 %b) {
+  ; CHECK: %p = bytecast b64 %b to ptr
+  %p = bytecast b64 %b to ptr
+  ret ptr %p
+}
+
+define b64 @ptr_to_byte(ptr %p) {
+  ; CHECK: %b = bytecast ptr %p to b64
+  %b = bytecast ptr %p to b64
+  ret b64 %b
+}
+
+define ptr addrspace(1) @byte_to_ptr_as1(b64 %b) {
+  ; CHECK: %p = bytecast b64 %b to ptr addrspace(1)
+  %p = bytecast b64 %b to ptr addrspace(1)
+  ret ptr addrspace(1) %p
+}
+
+define <4 x i8> @byte_vector_to_int_vector(<4 x b8> %b) {
+  ; CHECK: %i = bytecast <4 x b8> %b to <4 x i8>
+  %i = bytecast <4 x b8> %b to <4 x i8>
+  ret <4 x i8> %i
+}
+
+define i64 @byte_vector_to_scalar(<2 x b32> %b) {
+  ; CHECK: %i = bytecast <2 x b32> %b to i64
+  %i = bytecast <2 x b32> %b to i64
+  ret i64 %i
+}
+
+define <2 x b32> @scalar_to_byte_vector(i64 %i) {
+  ; CHECK: %b = bytecast i64 %i to <2 x b32>
+  %b = bytecast i64 %i to <2 x b32>
+  ret <2 x b32> %b
+}
+
+define b8 @byte_to_byte(b8 %b) {
+  ; CHECK: %b2 = bytecast b8 %b to b8
+  %b2 = bytecast b8 %b to b8
+  ret b8 %b2
+}