[DirectX] Fix GEP flattening with 0-indexed GEPs on global variables (#149211)

Fixes #149179 

The issue is that `Builder.CreateGEP` does not return a GEP Instruction
or GEP ContantExpr when the pointer operand is a global variable and all
indices are constant zeroes.

This PR ensures that a GEP instruction is created if `Builder.CreateGEP`
did not return a GEP.

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Target/DirectX/DXILFlattenArrays.cpp b/llvm/lib/Target/DirectX/DXILFlattenArrays.cpp
index ce43645..f0e2e78 100644
--- a/llvm/lib/Target/DirectX/DXILFlattenArrays.cpp
+++ b/llvm/lib/Target/DirectX/DXILFlattenArrays.cpp
@@ -343,6 +343,16 @@ bool DXILFlattenArraysVisitor::visitGetElementPtrInst(GetElementPtrInst &GEP) {
         Info.RootFlattenedArrayType, Info.RootPointerOperand,
         {ZeroIndex, FlattenedIndex}, GEP.getName(), GEP.getNoWrapFlags());
 
+    // If the pointer operand is a global variable and all indices are 0,
+    // IRBuilder::CreateGEP will return the global variable instead of creating
+    // a GEP instruction or GEP ConstantExpr. In this case we have to create and
+    // insert our own GEP instruction.
+    if (!isa<GEPOperator>(NewGEP))
+      NewGEP = GetElementPtrInst::Create(
+          Info.RootFlattenedArrayType, Info.RootPointerOperand,
+          {ZeroIndex, FlattenedIndex}, GEP.getNoWrapFlags(), GEP.getName(),
+          Builder.GetInsertPoint());
+
     // Replace the current GEP with the new GEP. Store GEPInfo into the map
     // for later use in case this GEP was not the end of the chain
     GEPChainInfoMap.insert({cast<GEPOperator>(NewGEP), std::move(Info)});