[llvm] [AMDGPU][LowerBufferFatPointers] Handle addrspacecast null to p7 (PR #140775)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Krzysztof Drewniak (krzysz00)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/140775.diff


2 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp (+19-4) 
- (modified) llvm/test/CodeGen/AMDGPU/lower-buffer-fat-pointers-pointer-ops.ll (+32) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp b/llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp
index 275b3a6f16e54..fe873508f46d2 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp
+++ b/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};
 }
diff --git a/llvm/test/CodeGen/AMDGPU/lower-buffer-fat-pointers-pointer-ops.ll b/llvm/test/CodeGen/AMDGPU/lower-buffer-fat-pointers-pointer-ops.ll
index fd5bf579cdeb2..cc5e2c833cc39 100644
--- a/llvm/test/CodeGen/AMDGPU/lower-buffer-fat-pointers-pointer-ops.ll
+++ b/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) {

``````````

</details>


https://github.com/llvm/llvm-project/pull/140775