[DirectX] Replace bitfield version in ProgramHeader.

[python3kgae]

Avoid using bitfield in dxbc::ProgramHeader.
It could potentially be read incorrectly on any host depending on the compiler.

From C++17's [class.bit]

Bit-fields are packed into some addressable allocation unit. [ Note: Bit-fields straddle allocation units on some machines and not on others. Bit-fields are assigned right-to-left on some machines, left-to-right on others.  — end note ]

For #91793

[llvmbot]

@llvm/pr-subscribers-llvm-binary-utilities
@llvm/pr-subscribers-mc
@llvm/pr-subscribers-objectyaml

@llvm/pr-subscribers-backend-directx

Author: Xiang Li (python3kgae)

Changes

Avoid using bitfield in dxbc::ProgramHeader.
It could potentially be read incorrectly on any host depending on the compiler.

From [C++17's [class.bit]](https://timsong-cpp.github.io/cppwp/n4659/class.bit#1)
> Bit-fields are packed into some addressable allocation unit. [ Note: Bit-fields straddle allocation units on some machines and not on others. Bit-fields are assigned right-to-left on some machines, left-to-right on others.  — end note ]

For #91793

---

Full diff: https://github.com/llvm/llvm-project/pull/91797.diff

4 Files Affected:

• (modified) llvm/include/llvm/BinaryFormat/DXContainer.h (+6-2)
• (modified) llvm/lib/MC/MCDXContainerWriter.cpp (+5-3)
• (modified) llvm/lib/ObjectYAML/DXContainerEmitter.cpp (+2-2)
• (modified) llvm/unittests/Object/DXContainerTest.cpp (+43)

diff --git a/llvm/include/llvm/BinaryFormat/DXContainer.h b/llvm/include/llvm/BinaryFormat/DXContainer.h
index e8d03f806715f..847d8103e6811 100644
--- a/llvm/include/llvm/BinaryFormat/DXContainer.h
+++ b/llvm/include/llvm/BinaryFormat/DXContainer.h
@@ -119,8 +119,7 @@ struct BitcodeHeader {
 };
 
 struct ProgramHeader {
-  uint8_t MinorVersion : 4;
-  uint8_t MajorVersion : 4;
+  uint8_t Version;
   uint8_t Unused;
   uint16_t ShaderKind;
   uint32_t Size; // Size in uint32_t words including this header.
@@ -131,6 +130,11 @@ struct ProgramHeader {
     sys::swapByteOrder(Size);
     Bitcode.swapBytes();
   }
+  uint8_t getMajorVersion() { return Version >> 4; }
+  uint8_t getMinorVersion() { return Version & 0xF; }
+  static uint8_t getVersion(uint8_t Major, uint8_t Minor) {
+    return (Major << 4) | Minor;
+  }
 };
 
 static_assert(sizeof(ProgramHeader) == 24, "ProgramHeader Size incorrect!");
diff --git a/llvm/lib/MC/MCDXContainerWriter.cpp b/llvm/lib/MC/MCDXContainerWriter.cpp
index 0580dc7e42826..ff64c6e538ac6 100644
--- a/llvm/lib/MC/MCDXContainerWriter.cpp
+++ b/llvm/lib/MC/MCDXContainerWriter.cpp
@@ -117,9 +117,11 @@ uint64_t DXContainerObjectWriter::writeObject(MCAssembler &Asm,
 
       const Triple &TT = Asm.getContext().getTargetTriple();
       VersionTuple Version = TT.getOSVersion();
-      Header.MajorVersion = static_cast<uint8_t>(Version.getMajor());
-      if (Version.getMinor())
-        Header.MinorVersion = static_cast<uint8_t>(*Version.getMinor());
+      uint8_t MajorVersion = static_cast<uint8_t>(Version.getMajor());
+      uint8_t MinorVersion =
+          static_cast<uint8_t>(Version.getMinor().value_or(0));
+      Header.Version =
+          dxbc::ProgramHeader::getVersion(MajorVersion, MinorVersion);
       if (TT.hasEnvironment())
         Header.ShaderKind =
             static_cast<uint16_t>(TT.getEnvironment() - Triple::Pixel);
diff --git a/llvm/lib/ObjectYAML/DXContainerEmitter.cpp b/llvm/lib/ObjectYAML/DXContainerEmitter.cpp
index f3a518df31750..175f1a12f9314 100644
--- a/llvm/lib/ObjectYAML/DXContainerEmitter.cpp
+++ b/llvm/lib/ObjectYAML/DXContainerEmitter.cpp
@@ -131,8 +131,8 @@ void DXContainerWriter::writeParts(raw_ostream &OS) {
       if (!P.Program)
         continue;
       dxbc::ProgramHeader Header;
-      Header.MajorVersion = P.Program->MajorVersion;
-      Header.MinorVersion = P.Program->MinorVersion;
+      Header.Version = dxbc::ProgramHeader::getVersion(P.Program->MajorVersion,
+                                                       P.Program->MinorVersion);
       Header.Unused = 0;
       Header.ShaderKind = P.Program->ShaderKind;
       memcpy(Header.Bitcode.Magic, "DXIL", 4);
diff --git a/llvm/unittests/Object/DXContainerTest.cpp b/llvm/unittests/Object/DXContainerTest.cpp
index da640225617d4..115de47a3cefd 100644
--- a/llvm/unittests/Object/DXContainerTest.cpp
+++ b/llvm/unittests/Object/DXContainerTest.cpp
@@ -174,6 +174,49 @@ TEST(DXCFile, ParseEmptyParts) {
   }
 }
 
+// This test verify DXIL part are correctly parsed.
+// This test is based on the binary output constructed from this yaml.
+// --- !dxcontainer
+// Header:
+//   Hash:            [ 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
+//                      0x0, 0x0, 0x0, 0x0, 0x0, 0x0 ]
+//   Version:
+//     Major:           1
+//     Minor:           0
+//   PartCount:       1
+// Parts:
+//   - Name:            DXIL
+//     Size:            28
+//     Program:
+//       MajorVersion:    6
+//       MinorVersion:    5
+//       ShaderKind:      5
+//       Size:            8
+//       DXILMajorVersion: 1
+//       DXILMinorVersion: 5
+//       DXILSize:        4
+//       DXIL:            [ 0x42, 0x43, 0xC0, 0xDE, ]
+// ...
+TEST(DXCFile, ParseDXILPart) {
+  uint8_t Buffer[] = {
+      0x44, 0x58, 0x42, 0x43, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00,
+      0x48, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x24, 0x00, 0x00, 0x00,
+      0x44, 0x58, 0x49, 0x4c, 0x1c, 0x00, 0x00, 0x00, 0x65, 0x00, 0x05, 0x00,
+      0x08, 0x00, 0x00, 0x00, 0x44, 0x58, 0x49, 0x4c, 0x05, 0x01, 0x00, 0x00,
+      0x10, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x42, 0x43, 0xc0, 0xde};
+  DXContainer C =
+      llvm::cantFail(DXContainer::create(getMemoryBuffer<116>(Buffer)));
+  EXPECT_EQ(C.getHeader().PartCount, 1u);
+  const std::optional<object::DXContainer::DXILData> &DXIL = C.getDXIL();
+  EXPECT_TRUE(DXIL.has_value());
+  dxbc::ProgramHeader Header = DXIL->first;
+  EXPECT_EQ(Header.getMajorVersion(), 6u);
+  EXPECT_EQ(Header.getMinorVersion(), 5u);
+  EXPECT_EQ(Header.ShaderKind, 5u);
+  EXPECT_EQ(Header.Size, 8u);
+}
+
 static Expected<DXContainer>
 generateDXContainer(StringRef Yaml, SmallVectorImpl<char> &BinaryData) {
   DXContainerYAML::Object Obj;

[coopp]

Looks fine to me. Good idea putting the major and minor version into a single value instead of using bitfields. Bitfields to make it more readable, but I understand the byte swapping problems you are encountering.