[SPIRV][SPIRVPrepareGlobals] Convert llvm.embedded.module from a 0-element array to a 1-element array

When compiling with -fembed-bitcode-marker, Clang inserts a placeholder
for the bitcode. This placeholder is a [0 x i8] array, which we cannot
represent in SPIRV.

For AMD flavoured SPIRV, we extend the llvm.embedded.module global to a
zeroinitialized 1-element array.

Author: jmmartinez

llvm/lib/Target/SPIRV/SPIRVPrepareGlobals.cpp

@@ -13,6 +13,8 @@
 
 #include "SPIRV.h"
 
+#include "llvm/IR/Module.h"
+
 using namespace llvm;
 
 namespace {
@@ -28,7 +30,30 @@ struct SPIRVPrepareGlobals : public ModulePass {
   bool runOnModule(Module &M) override;
 };
 
-bool SPIRVPrepareGlobals::runOnModule(Module &M) { return false; }
+bool tryExtendLLVMBitcodeMarker(GlobalVariable &Bitcode) {
+  assert(Bitcode.getName() == "llvm.embedded.module");
+
+  ArrayType *AT = cast<ArrayType>(Bitcode.getValueType());
+  if (AT->getNumElements() != 0)
+    return false;
+
+  ArrayType *AT1 = ArrayType::get(AT->getElementType(), 1);
+  Constant *OneEltInit = Constant::getNullValue(AT1);
+  Bitcode.replaceInitializer(OneEltInit);
+  return true;
+}
+
+bool SPIRVPrepareGlobals::runOnModule(Module &M) {
+  const bool IsAMD = M.getTargetTriple().getVendor() == Triple::AMD;
+  if (!IsAMD)
+    return false;
+
+  bool Changed = false;
+  if (GlobalVariable *Bitcode = M.getNamedGlobal("llvm.embedded.module"))
+    Changed |= tryExtendLLVMBitcodeMarker(*Bitcode);
+
+  return Changed;
+}
 char SPIRVPrepareGlobals::ID = 0;
 
 } // namespace

llvm/test/CodeGen/SPIRV/fembed-bitcode-marker.ll

@@ -1,5 +1,7 @@
+; Expanding the bitcode marker works only for AMD at the moment
 ; RUN: not llc -verify-machineinstrs -mtriple=spirv-unknown-unknown %s -o -
-; RUN: not llc -verify-machineinstrs -mtriple=spirv64-amd-amdhsa %s -o -
+; RUN: llc -verify-machineinstrs -mtriple=spirv64-amd-amdhsa %s -o - | FileCheck %s
+; RUN: %if spirv-tools %{ llc -mtriple=spirv64-amd-amdhsa %s -o - -filetype=obj | spirv-val %}
 ;
 ; Verify that we lower the embedded bitcode