Translate AddrSpaceCast from generic null to OpGenericCastToPtr (#3411)

Some GPU backends materialize non-generic nulls via addrspacecast from
generic null, see
https://github.com/llvm/llvm-project/blob/14ed67411ce1/clang/lib/CodeGen/Targets/SPIR.cpp#L257

llvm-spirv should preserve the addrspacecast.

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@c4fd801eb1ee58e

Author: wenju-he

llvm-spirv/lib/SPIRV/SPIRVWriter.cpp

@@ -1673,7 +1673,8 @@ SPIRVValue *LLVMToSPIRVBase::transUnaryInst(UnaryInstruction *U,
       } else {
         BOC = OpCrossWorkgroupCastToPtrINTEL;
       }
-    } else if (isa<ConstantPointerNull>(Cast->getPointerOperand())) {
+    } else if (isa<ConstantPointerNull>(Cast->getPointerOperand()) &&
+               SrcAddrSpace != SPIRAS_Generic) {
       SPIRVType *TransTy = transScavengedType(U);
       return BM->addNullConstant(bcast<SPIRVTypePointer>(TransTy));
     } else {

llvm-spirv/test/SpecConstants/keep-tracked-const.ll

@@ -17,8 +17,8 @@ target triple = "spir64-unknown-unknown"
 ; CHECK: Constant [[#Int8]] [[#]] 1
 ; CHECK: Constant [[#Int8]] [[#]] 0
 
-; CHECK-DAG: Constant [[#Int32]] [[#]] 0
 ; CHECK-DAG: Constant [[#Int32]] [[#]] 1
+; CHECK-DAG: Constant [[#Int32]] [[#]] 0
 
 ; CHECK-LLVM: %conv17.i = sext i8 1 to i64
 

llvm-spirv/test/addrspacecast_generic_null.ll

@@ -0,0 +1,18 @@
+; RUN: llvm-spirv %s -o %t.spv
+; RUN: spirv-dis %t.spv | FileCheck %s
+
+target datalayout = "e-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-G1"
+target triple = "spir64-unknown-unknown"
+
+; Test that addrspacecast of generic null pointer generates GenericCastToPtr
+; instruction in SPIR-V.
+
+; CHECK: %_ptr_Workgroup_uchar = OpTypePointer Workgroup %uchar
+; CHECK: %[[NULL:[0-9]+]] = OpConstantNull %_ptr_Generic_uchar
+; CHECK: %[[CAST:[0-9]+]] = OpGenericCastToPtr %_ptr_Workgroup_uchar %[[NULL]]
+; CHECK: OpPtrEqual %bool %[[CAST]]
+
+define spir_kernel void @bar_generic_null(ptr addrspace(3) %arg) {
+  %expr = icmp eq ptr addrspace(3) addrspacecast (ptr addrspace(4) null to ptr addrspace(3)), %arg
+  ret void
+}