[llvm] [SPIRV] Improve Logical SPIR-V Pointer Access and GEP Legalization (PR #169076)

[Nathan Gauër via llvm-commits]

================
@@ -1569,16 +1569,60 @@ Instruction *SPIRVEmitIntrinsics::visitSwitchInst(SwitchInst &I) {
   return BrI;
 }
 
-Instruction *SPIRVEmitIntrinsics::visitGetElementPtrInst(GetElementPtrInst &I) {
-  if (I.getSourceElementType() == IntegerType::getInt8Ty(CurrF->getContext()) &&
-      TM->getSubtargetImpl()->isLogicalSPIRV()) {
-    Instruction *Result = buildLogicalAccessChainFromGEP(I);
-    if (Result)
-      return Result;
+static bool isFirstIndexZero(const GetElementPtrInst *GEP) {
+  if (GEP->getNumIndices() == 0)
+    return false;
+  if (const auto *CI = dyn_cast<ConstantInt>(GEP->getOperand(1))) {
+    return CI->getZExtValue() == 0;
   }
+  return false;
+}
 
+Instruction *SPIRVEmitIntrinsics::visitGetElementPtrInst(GetElementPtrInst &I) {
   IRBuilder<> B(I.getParent());
   B.SetInsertPoint(&I);
+
+  if (TM->getSubtargetImpl()->isLogicalSPIRV() && !isFirstIndexZero(&I)) {
+    // Logical SPIR-V cannot use the OpPtrAccessChain instruction. If the first
+    // index of the GEP is not 0, then we need to try to adjust it.
+    //
+    // If the GEP is doing byte addressing, try to rebuild the full access chain
+    // from the type of the pointer.
+    if (I.getSourceElementType() ==
+        IntegerType::getInt8Ty(CurrF->getContext())) {
+      Instruction *Result = buildLogicalAccessChainFromGEP(I);
+      if (Result) {
+        return Result;
+      }
----------------
Keenuts wrote:

return everytime? If the instruction allocation failed, seemed like a fallthrough is not wanted anyway

https://github.com/llvm/llvm-project/pull/169076