[AMDGPU][LowerBufferFatPointers] Handle addrspacecast null to p7

Some application code operating on generic pointers (that then gete
initialized to buffer fat pointers) may perform tests against nullptr.
After address space inference, this results in comparisons against
`addrspacecast (ptr null to ptr addrspace(7))`, which were crashing.

However, while general casts to ptr addrspace(7) from generic pointers
aren't supposted, it is possible to cast null pointers to the
all-zerose bufer resource and 0 offset, which this patch adds.

It also adds a TODO for casting _out_ of buffer resources, which isn't
implemented here but could be.

Author: krzysz00

llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp

@@ -1980,6 +1980,8 @@ PtrParts SplitPtrStructs::visitIntToPtrInst(IntToPtrInst &IP) {
 }
 
 PtrParts SplitPtrStructs::visitAddrSpaceCastInst(AddrSpaceCastInst &I) {
+  // TODO(krzysz00): handle casts from ptr addrspace(7) to global pointers
+  // by computing the effective address.
   if (!isSplitFatPtr(I.getType()))
     return {nullptr, nullptr};
   IRB.SetInsertPoint(&I);
@@ -1990,11 +1992,24 @@ PtrParts SplitPtrStructs::visitAddrSpaceCastInst(AddrSpaceCastInst &I) {
     SplitUsers.insert(&I);
     return {Rsrc, Off};
   }
-  if (I.getSrcAddressSpace() != AMDGPUAS::BUFFER_RESOURCE)
-    report_fatal_error("Only buffer resources (addrspace 8) can be cast to "
-                       "buffer fat pointers (addrspace 7)");
-  Type *OffTy = cast<StructType>(I.getType())->getElementType(1);
+
+  auto *ResTy = cast<StructType>(I.getType());
+  Type *RsrcTy = ResTy->getElementType(0);
+  Type *OffTy = ResTy->getElementType(1);
   Value *ZeroOff = Constant::getNullValue(OffTy);
+
+  // Special case for null pointers, which can be created by address space
+  // propagation.
+  auto *InConst = dyn_cast<Constant>(In);
+  if (InConst && InConst->isNullValue()) {
+    Value *NullRsrc = Constant::getNullValue(RsrcTy);
+    SplitUsers.insert(&I);
+    return {NullRsrc, ZeroOff};
+  }
+
+  if (I.getSrcAddressSpace() != AMDGPUAS::BUFFER_RESOURCE)
+    report_fatal_error("Only buffer resources (addrspace 8) and null pointers "
+                       "can be cast to buffer fat pointers (addrspace 7)");
   SplitUsers.insert(&I);
   return {In, ZeroOff};
 }

llvm/test/CodeGen/AMDGPU/lower-buffer-fat-pointers-pointer-ops.ll

@@ -332,6 +332,38 @@ define <2 x ptr addrspace(7)> @addrspacecast_vec(<2 x ptr addrspace(8)> %buf) {
   ret <2 x ptr addrspace(7)> %ret
 }
 
+define ptr addrspace(7) @addrspacecast_null() {
+; CHECK-LABEL: define { ptr addrspace(8), i32 } @addrspacecast_null
+; CHECK-SAME: () #[[ATTR0]] {
+; CHECK-NEXT:    ret { ptr addrspace(8), i32 } zeroinitializer
+;
+  %ret = addrspacecast ptr null to ptr addrspace(7)
+  ret ptr addrspace(7) %ret
+}
+
+define <2 x ptr addrspace(7)> @addrspacecast_null_vec() {
+; CHECK-LABEL: define { <2 x ptr addrspace(8)>, <2 x i32> } @addrspacecast_null_vec
+; CHECK-SAME: () #[[ATTR0]] {
+; CHECK-NEXT:    ret { <2 x ptr addrspace(8)>, <2 x i32> } zeroinitializer
+;
+  %ret = addrspacecast <2 x ptr> zeroinitializer to <2 x ptr addrspace(7)>
+  ret <2 x ptr addrspace(7)> %ret
+}
+
+define i1 @test_null(ptr addrspace(7) %p) {
+; CHECK-LABEL: define i1 @test_null
+; CHECK-SAME: ({ ptr addrspace(8), i32 } [[P:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    [[P_RSRC:%.*]] = extractvalue { ptr addrspace(8), i32 } [[P]], 0
+; CHECK-NEXT:    [[P_OFF:%.*]] = extractvalue { ptr addrspace(8), i32 } [[P]], 1
+; CHECK-NEXT:    [[IS_NULL_RSRC:%.*]] = icmp eq ptr addrspace(8) [[P_RSRC]], null
+; CHECK-NEXT:    [[IS_NULL_OFF:%.*]] = icmp eq i32 [[P_OFF]], 0
+; CHECK-NEXT:    [[IS_NULL:%.*]] = and i1 [[IS_NULL_RSRC]], [[IS_NULL_OFF]]
+; CHECK-NEXT:    ret i1 [[IS_NULL]]
+;
+  %is.null = icmp eq ptr addrspace(7) %p, addrspacecast (ptr null to ptr addrspace(7))
+  ret i1 %is.null
+}
+
 declare ptr addrspace(7) @llvm.amdgcn.make.buffer.rsrc.p7.p1(ptr addrspace(1), i16, i32, i32)
 
 define ptr addrspace(7) @make_buffer_rsrc(ptr addrspace(1) %buf, i16 %stride, i32 %numRecords, i32 %flags) {