[DirectX] Move triple/DL compat to bitcode writer

Move our handling of setting up a compatible triple for DXIL to the
bitcode writer itself, and do the same for the datalayout. This avoids
us needing to temporarily change the triple just to change it back, and
it also allows us to make the datalayout truly compatible, including the
`i8:32` alignment that isn't accepted by modern LLVM's DataLayout.

Author: bogner

llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp

@@ -1165,12 +1165,15 @@ void DXILBitcodeWriter::writeValueSymbolTableForwardDecl() {}
 /// Returns the bit offset to backpatch with the location of the real VST.
 void DXILBitcodeWriter::writeModuleInfo() {
   // Emit various pieces of data attached to a module.
-  if (!M.getTargetTriple().empty())
-    writeStringRecord(Stream, bitc::MODULE_CODE_TRIPLE,
-                      M.getTargetTriple().str(), 0 /*TODO*/);
-  const std::string &DL = M.getDataLayoutStr();
-  if (!DL.empty())
-    writeStringRecord(Stream, bitc::MODULE_CODE_DATALAYOUT, DL, 0 /*TODO*/);
+
+  // We need to hardcode a triple and datalayout that's compatible with the
+  // historical DXIL triple and datalayout from DXC.
+  StringRef Triple = "dxil-ms-dx";
+  StringRef DL = "e-m:e-p:32:32-i1:8-i8:8-i16:32-i32:32-i64:64-"
+                 "f16:32-f32:32-f64:64-n8:16:32:64";
+  writeStringRecord(Stream, bitc::MODULE_CODE_TRIPLE, Triple, 0 /*TODO*/);
+  writeStringRecord(Stream, bitc::MODULE_CODE_DATALAYOUT, DL, 0 /*TODO*/);
+
   if (!M.getModuleInlineAsm().empty())
     writeStringRecord(Stream, bitc::MODULE_CODE_ASM, M.getModuleInlineAsm(),
                       0 /*TODO*/);

llvm/lib/Target/DirectX/DXILWriter/DXILWriterPass.cpp

@@ -149,11 +149,6 @@ class EmbedDXILPass : public llvm::ModulePass {
     std::string Data;
     llvm::raw_string_ostream OS(Data);
 
-    Triple OriginalTriple = M.getTargetTriple();
-    // Set to DXIL triple when write to bitcode.
-    // Only the output bitcode need to be DXIL triple.
-    M.setTargetTriple(Triple("dxil-ms-dx"));
-
     // Perform late legalization of lifetime intrinsics that would otherwise
     // fail the Module Verifier if performed in an earlier pass
     legalizeLifetimeIntrinsics(M);
@@ -165,9 +160,6 @@ class EmbedDXILPass : public llvm::ModulePass {
     // not-so-legal legalizations
     removeLifetimeIntrinsics(M);
 
-    // Recover triple.
-    M.setTargetTriple(OriginalTriple);
-
     Constant *ModuleConstant =
         ConstantDataArray::get(M.getContext(), arrayRefFromStringRef(Data));
     auto *GV = new llvm::GlobalVariable(M, ModuleConstant->getType(), true,