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

[Krzysztof Drewniak via llvm-commits]

https://github.com/krzysz00 created https://github.com/llvm/llvm-project/pull/140775

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.

>From 1639578af2ba688424813ee272f06afddb9bfbd3 Mon Sep 17 00:00:00 2001
From: Krzysztof Drewniak <Krzysztof.Drewniak at amd.com>
Date: Tue, 20 May 2025 18:09:19 +0000
Subject: [PATCH] [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.
---
 .../AMDGPU/AMDGPULowerBufferFatPointers.cpp   | 23 ++++++++++---
 .../lower-buffer-fat-pointers-pointer-ops.ll  | 32 +++++++++++++++++++
 2 files changed, 51 insertions(+), 4 deletions(-)

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) {