[AMDGPU] Add an experimental buffer fat pointer address space.

[sheredom]

Summary

Add an experimental buffer fat pointer address space that is currently unhandled in the backend. This commit reserves address space 7 as a non-integral pointer repsenting the 160-bit fat pointer (128-bit buffer descriptor + 32-bit offset) that is heavily used in graphics workloads using the AMDGPU backend.

[arsenm]

I think this will be accepted right now as a no-op addrspacecast to any of the global-like address spaces

docs/AMDGPUUsage.rst

294–295

Might as well reserve more for 256-bit descriptors. We'll probably need several of these eventually

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

297

Shouldn't this add p7:128?

lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

257

This probably shouldn't be included now, at least without a test

lib/Target/AMDGPU/SIISelLowering.cpp

1050

Ditto

[sheredom]

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

297

It'd need to be p7:160 - but I'm entirely unsure whether LLVM will drop a lung on a non-power of 2 pointer size.

lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

257

It's needed for us to be able to run the middle end optimizations with fat pointers in the module though. I'll work out how to test this.

lib/Target/AMDGPU/SIISelLowering.cpp

1050

TLI calls into this (which is why it is needed). I can work out some pass that queries TLI and do a test though.

[sheredom]

Add a test case that triggers the target transform info code path.

[sheredom]

docs/AMDGPUUsage.rst

294–295

I'd rather not do that now - we'll need at least 3 (1 for image descriptors, 1 for structured buffer descriptors, 1 for samplers) more and I don't want to go through all the steps of adding them everywhere when we've got no immediate need for them.

[arsenm]

docs/AMDGPUUsage.rst

299–300

I don't understand why you would blend these. You just need the 128-bit pointer, and then the intrinsic accessing it will have a 32-bit offset operand that isn't part of the pointer

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

297

Why does it need to be 160? It should be 128 like the descriptor

[sheredom]

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

297

So for non-swizzable buffer descriptors we want to model it using normal LLVM load/store/atomic instructions, so that we have no intrinsics required for them at all. To model this we need a 160-bit pointer for the 128-bit descriptor + 32-bit offset. This is super important because it means these 160-bit pointers partake in all the normal load/store optimizations without us having to have special cases for whatever new intrinsics we'd have to introduce.

We're trying our best to avoid the need for any new intrinsics (if at all possible, it won't always be possible though).

[arsenm]

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

297

I still don't see how the offset is part of the pointer itself. You could always use an offset of 0, so it would be a matter of changing the GEP index type to be different from the bit width of the pointer, and an optimization during codegen to fold it in

[sheredom]

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

297

So if we explicitly laid out the pointer as a p7:128, what would happen if you stored that pointer into memory (say an alloca)? This is fine if you are storing the original pointer (just the 128-bit memory buffer descriptor), but if you had GEP'ed into this pointer, when you store the pointer into memory you are losing the offset the information as part of that store. It seems dangerous to me that we'd pretend the pointer is 128-bit when actually for all intermediate uses of the pointer it'll contain 160-bits of valid information.

I think I'd rather just leave it as non-integral which solves all these issues perfectly well and also has the added benefit of restricting the usages of the pointer from things we don't want to support.

[arsenm]

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

297

I don't follow this. Of course it's invalid to store a different value and expect a different one to be there afterwards. You seem to be implying getelementptr is invalid to use at all. The getelementptr is producing a new 128-bit value, it isn't packing the 32-bit offset into some merge value

[sheredom]

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

297

So your pitch is that GEP produces a new 128-bit value, where we've modified the base addr + the num_records within the GEP to record the new addr + upper bound?

This is really not what we want when we actually want to consume the fat pointer into an MUBUF instruction, we really want the original descriptor to be unmodified and the offset to be passed in a separate VGPR though.

[nhaehnle]

lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp

65

It may be more pragmatic to have NoAlias with Constant 32-bit.

The intention and current practice is for Constant 32-bit to be used with descriptor tables, and those really shouldn't ever alias buffer fat pointers.

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

297

Definitely agree with @sheredom here, as the way I would think about these pointers is similar to "pointers + bounds" you could do on a CPU.

Two very immediate practical problems that would come up with 128 bits are:

• No good way to support non-uniform GEPs. In practice, we'll want to create a buffer fat pointer from a uniform 128-bit descriptor using some trivial amdgcn intrinsic, but GEPs will very often be non-uniform. In order to be able to use MUBUF instructions, we need the 128+32 bit representation. (Of course, we'll also need a fallback for the case where the descriptor *isn't* uniform, but in actual practice it will be uniform almost always)
• Inability to support GEPs with negative offsets (because the required bounds-check information is lost)

[sheredom]

lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp

65

Should I then actually be changing all the Constant 32-bit aliasing rules to only alias with itself do you think?

[nhaehnle]

LGTM, whether you do the NoAlias between constant 32-bit and buffer address space here or separately. (Making NoAlias between constant 32-bit and the other existing address spaces should definitely be a different patch.)

lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp

65

I think that's a good idea, but it should be a separate patch.

[arsenm]

lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp

65

It also seems to me like it should be renamed, since there's a more specific purpose in mind than just a 32-bit pointer for constant

[sheredom]

lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp

65

Agreed on both your comments, I'll submit that change as a separate patch.

docs/AMDGPUUsage.rst

 The memory space names used in the table, aside from the region memory space, is
 from the OpenCL standard.
 
 LLVM Address Space number is used throughout LLVM (for example, in LLVM IR).
 
   .. table:: Address Space Mapping
      :name: amdgpu-address-space-mapping-table
 
-     ================== =================
+     ================== =================================
      LLVM Address Space Memory Space
-     ================== =================
+     ================== =================================
      0                  Generic (Flat)
      1                  Global
      2                  Region (GDS)
      3                  Local (group/LDS)
      4                  Constant
      5                  Private (Scratch)
      6                  Constant 32-bit
-     ================== =================
+     7                  Buffer Fat Pointer (experimental)
+     ================== =================================
+
+The buffer fat pointer is an experimental address space that is currently
+unsupported in the backend. It exposes a non-integral pointer that is in future
+intended to support the modelling of 128-bit buffer descriptors + a 32-bit
+offset into the buffer descriptor (in total encapsulating a 160-bit 'pointer'),
+allowing us to use normal LLVM load/store/atomic operations to model the buffer
+descriptors used heavily in graphics workloads targeting the backend.
 
 .. _amdgpu-memory-scopes:
 
 Memory Scopes
 -------------
 
 This section provides LLVM memory synchronization scopes supported by the AMDGPU
 backend memory model when the target triple OS is ``amdhsa`` (see

lib/Target/AMDGPU/AMDGPU.h

 /// various memory regions on the hardware. On the CPU
 /// all of the address spaces point to the same memory,
 /// however on the GPU, each address space points to
 /// a separate piece of memory that is unique from other
 /// memory locations.
 namespace AMDGPUAS {
   enum : unsigned {
     // The maximum value for flat, generic, local, private, constant and region.
-    MAX_AMDGPU_ADDRESS = 6,
+    MAX_AMDGPU_ADDRESS = 7,
 
     FLAT_ADDRESS = 0,     ///< Address space for flat memory.
     GLOBAL_ADDRESS = 1,   ///< Address space for global memory (RAT0, VTX0).
     REGION_ADDRESS = 2,   ///< Address space for region memory. (GDS)
 
-    CONSTANT_ADDRESS = 4, ///< Address space for constant memory (VTX2)
+    CONSTANT_ADDRESS = 4, ///< Address space for constant memory (VTX2).
     LOCAL_ADDRESS = 3,    ///< Address space for local memory.
     PRIVATE_ADDRESS = 5,  ///< Address space for private memory.
 
-    CONSTANT_ADDRESS_32BIT = 6, ///< Address space for 32-bit constant memory
+    CONSTANT_ADDRESS_32BIT = 6, ///< Address space for 32-bit constant memory.
 
-    /// Address space for direct addressible parameter memory (CONST0)
+    BUFFER_FAT_POINTER = 7, ///< Address space for 160-bit buffer fat pointers.
+
+    /// Address space for direct addressible parameter memory (CONST0).
     PARAM_D_ADDRESS = 6,
-    /// Address space for indirect addressible parameter memory (VTX1)
+    /// Address space for indirect addressible parameter memory (VTX1).
     PARAM_I_ADDRESS = 7,
 
     // Do not re-order the CONSTANT_BUFFER_* enums.  Several places depend on
     // this order to be able to dynamically index a constant buffer, for
     // example:
     //
     // ConstantBufferAS = CONSTANT_BUFFER_0 + CBIdx
 

lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp

 ImmutablePass *llvm::createAMDGPUExternalAAWrapperPass() {
   return new AMDGPUExternalAAWrapper();
 }
 
 void AMDGPUAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesAll();
 }
 
-// These arrays are indexed by address space value enum elements 0 ... to 6
-static const AliasResult ASAliasRules[7][7] = {
-  /*                    Flat       Global    Region    Group     Constant  Private   Constant 32-bit */
-  /* Flat     */        {MayAlias, MayAlias, MayAlias, MayAlias, MayAlias, MayAlias, MayAlias},
-  /* Global   */        {MayAlias, MayAlias, NoAlias , NoAlias , MayAlias, NoAlias , MayAlias},
-  /* Region   */        {MayAlias, NoAlias , NoAlias , NoAlias,  MayAlias, NoAlias , MayAlias},
-  /* Group    */        {MayAlias, NoAlias , NoAlias , MayAlias, NoAlias , NoAlias , NoAlias},
-  /* Constant */        {MayAlias, MayAlias, MayAlias, NoAlias , NoAlias,  NoAlias , MayAlias},
-  /* Private  */        {MayAlias, NoAlias , NoAlias , NoAlias , NoAlias , MayAlias, NoAlias},
-  /* Constant 32-bit */ {MayAlias, MayAlias, MayAlias, NoAlias , MayAlias, NoAlias , NoAlias}
+// These arrays are indexed by address space value enum elements 0 ... to 7
+static const AliasResult ASAliasRules[8][8] = {
+  /*                    Flat       Global    Region    Group     Constant  Private   Constant 32-bit  Buffer Fat Ptr */
+  /* Flat     */        {MayAlias, MayAlias, MayAlias, MayAlias, MayAlias, MayAlias, MayAlias,        MayAlias},
+  /* Global   */        {MayAlias, MayAlias, NoAlias , NoAlias , MayAlias, NoAlias , MayAlias,        MayAlias},
+  /* Region   */        {MayAlias, NoAlias , NoAlias , NoAlias , MayAlias, NoAlias , MayAlias,        NoAlias},
+  /* Group    */        {MayAlias, NoAlias , NoAlias , MayAlias, NoAlias , NoAlias , NoAlias ,        NoAlias},
+  /* Constant */        {MayAlias, MayAlias, MayAlias, NoAlias , NoAlias , NoAlias , MayAlias,        MayAlias},
+  /* Private  */        {MayAlias, NoAlias , NoAlias , NoAlias , NoAlias , MayAlias, NoAlias ,        NoAlias},
+  /* Constant 32-bit */ {MayAlias, MayAlias, MayAlias, NoAlias , MayAlias, NoAlias , NoAlias ,        MayAlias},
+  /* Buffer Fat Ptr  */ {MayAlias, MayAlias, NoAlias , NoAlias , MayAlias, NoAlias , MayAlias,        MayAlias}
 };
 
 static AliasResult getAliasResult(unsigned AS1, unsigned AS2) {
-  static_assert(AMDGPUAS::MAX_AMDGPU_ADDRESS <= 6, "Addr space out of range");
+  static_assert(AMDGPUAS::MAX_AMDGPU_ADDRESS <= 7, "Addr space out of range");
 
   if (AS1 > AMDGPUAS::MAX_AMDGPU_ADDRESS || AS2 > AMDGPUAS::MAX_AMDGPU_ADDRESS)
     return MayAlias;
 
   return ASAliasRules[AS1][AS2];
 }
 
 AliasResult AMDGPUAAResult::alias(const MemoryLocation &LocA,

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

 static StringRef computeDataLayout(const Triple &TT) {
   if (TT.getArch() == Triple::r600) {
     // 32-bit pointers.
       return "e-p:32:32-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128"
              "-v192:256-v256:256-v512:512-v1024:1024-v2048:2048-n32:64-S32-A5";
   }
 
   // 32-bit private, local, and region pointers. 64-bit global, constant and
-  // flat.
+  // flat, non-integral buffer fat pointers.
     return "e-p:64:64-p1:64:64-p2:32:32-p3:32:32-p4:64:64-p5:32:32-p6:32:32"
          "-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128"
-         "-v192:256-v256:256-v512:512-v1024:1024-v2048:2048-n32:64-S32-A5";
+         "-v192:256-v256:256-v512:512-v1024:1024-v2048:2048-n32:64-S32-A5"
+         "-ni:7";
 }
 
 LLVM_READNONE
 static StringRef getGPUOrDefault(const Triple &TT, StringRef GPU) {
   if (!GPU.empty())
     return GPU;
 
   if (TT.getArch() == Triple::amdgcn)

lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

if (VecRegBitWidth > 128)
     return 128 / StoreSize;
 
   return VF;
 }
 
 unsigned GCNTTIImpl::getLoadStoreVecRegBitWidth(unsigned AddrSpace) const {
   if (AddrSpace == AMDGPUAS::GLOBAL_ADDRESS ||
       AddrSpace == AMDGPUAS::CONSTANT_ADDRESS ||
-      AddrSpace == AMDGPUAS::CONSTANT_ADDRESS_32BIT) {
+      AddrSpace == AMDGPUAS::CONSTANT_ADDRESS_32BIT ||
+      AddrSpace == AMDGPUAS::BUFFER_FAT_POINTER) {
     return 512;
   }
 
   if (AddrSpace == AMDGPUAS::FLAT_ADDRESS ||
       AddrSpace == AMDGPUAS::LOCAL_ADDRESS ||
       AddrSpace == AMDGPUAS::REGION_ADDRESS)
     return 128;
 

lib/Target/AMDGPU/SIISelLowering.cpp

bool SITargetLowering::isLegalAddressingMode(const DataLayout &DL,
   // No global is ever allowed as a base.
   if (AM.BaseGV)
     return false;
 
   if (AS == AMDGPUAS::GLOBAL_ADDRESS)
     return isLegalGlobalAddressingMode(AM);
 
   if (AS == AMDGPUAS::CONSTANT_ADDRESS ||
-      AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT) {
+      AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT ||
+      AS == AMDGPUAS::BUFFER_FAT_POINTER) {
     // If the offset isn't a multiple of 4, it probably isn't going to be
     // correctly aligned.
     // FIXME: Can we get the real alignment here?
     if (AM.BaseOffs % 4 != 0)
       return isLegalMUBUFAddressingMode(AM);
 
     // There are no SMRD extloads, so if we have to do a small type access we
     // will use a MUBUF load.

test/CodeGen/AMDGPU/amdgpu-alias-analysis.ll

 define void @test_1_999(i8 addrspace(1)* %p, i8 addrspace(999)* %p1) {
   ret void
 }
 
 ; CHECK: MayAlias:  i8 addrspace(1)* %p1, i8 addrspace(999)* %p
 define void @test_999_1(i8 addrspace(999)* %p, i8 addrspace(1)* %p1) {
   ret void
 }
+
+; CHECK: MayAlias:  i8 addrspace(7)* %p, i8* %p1
+define void @test_7_0(i8 addrspace(7)* %p, i8 addrspace(0)* %p1) {
+    ret void
+}
+
+; CHECK: MayAlias:  i8 addrspace(1)* %p1, i8 addrspace(7)* %p
+define void @test_7_1(i8 addrspace(7)* %p, i8 addrspace(1)* %p1) {
+    ret void
+}
+
+; CHECK: NoAlias:  i8 addrspace(2)* %p1, i8 addrspace(7)* %p
+define void @test_7_2(i8 addrspace(7)* %p, i8 addrspace(2)* %p1) {
+    ret void
+}
+
+; CHECK: NoAlias:  i8 addrspace(3)* %p1, i8 addrspace(7)* %p
+define void @test_7_3(i8 addrspace(7)* %p, i8 addrspace(3)* %p1) {
+    ret void
+}
+
+; CHECK: MayAlias:  i8 addrspace(4)* %p1, i8 addrspace(7)* %p
+define void @test_7_4(i8 addrspace(7)* %p, i8 addrspace(4)* %p1) {
+    ret void
+}
+
+; CHECK: NoAlias:  i8 addrspace(5)* %p1, i8 addrspace(7)* %p
+define void @test_7_5(i8 addrspace(7)* %p, i8 addrspace(5)* %p1) {
+    ret void
+}
+
+; CHECK: MayAlias:  i8 addrspace(6)* %p1, i8 addrspace(7)* %p
+define void @test_7_6(i8 addrspace(7)* %p, i8 addrspace(6)* %p1) {
+    ret void
+}
+
+; CHECK: MayAlias:  i8 addrspace(7)* %p, i8 addrspace(7)* %p1
+define void @test_7_7(i8 addrspace(7)* %p, i8 addrspace(7)* %p1) {
+    ret void
+}

test/CodeGen/AMDGPU/r600.amdgpu-alias-analysis.ll

 ; RUN: opt -mtriple=r600-- -O3 -aa-eval -print-all-alias-modref-info -disable-output < %s 2>&1 | FileCheck %s
 
-; CHECK: MayAlias: i8 addrspace(5)* %p, i8 addrspace(7)* %p1
-
-define amdgpu_kernel void @test(i8 addrspace(5)* %p, i8 addrspace(7)* %p1) {
+; CHECK: MayAlias: i8 addrspace(5)* %p, i8 addrspace(999)* %p1
+define amdgpu_kernel void @test(i8 addrspace(5)* %p, i8 addrspace(999)* %p1) {
   ret void
 }