[AMDGPU][LowerBufferFatPointers] Handle addrspacecast null to p7

[krzysz00]

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.

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

Author: Krzysztof Drewniak (krzysz00)

Changes

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 --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) {

[shiltian]

The reason we have addrspacecast (ptr null to ptr addrspace(7)) thingy is ConstantPointerNull doesn't really mean a nullptr; otherwise this could be folded to just ptr addrspace(7) null and then everything just pans out. I'm working on changing the semantics of ConstantPointerNull but current run into multiple issues. Hopefully I can get it through. :-D

For this particular case, a ptr addrspace(7) null does mean a nullptr though.

[github-actions]

⚠️ undef deprecator found issues in your code. ⚠️

You can test this locally with the following command:

git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 'HEAD~1' HEAD llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp llvm/test/CodeGen/AMDGPU/lower-buffer-fat-pointers-pointer-ops.ll

The following files introduce new uses of undef:

• llvm/lib/Target/AMDGPU/AMDGPULowerBufferFatPointers.cpp
• llvm/test/CodeGen/AMDGPU/lower-buffer-fat-pointers-pointer-ops.ll

Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields undef. You should use poison values for placeholders instead.

In tests, avoid using undef and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:

define void @fn() {
  ...
  br i1 undef, ...
}

Please use the following instead:

define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}

Please refer to the Undefined Behavior Manual for more information.

[krzysz00]

Landing over the objections of the undef lint since this is a test that actually handles undef