[AMDGPU] Add Relocation Constant Support

[csyonghe]

Summary

This change adds amdgcn.reloc.constant intrinsic to the amdgpu backend, which will compile into a relocation entry in the resulting elf.

The intrinsics takes a MetadataNode (String) as its only argument, which specifies the symbol name of the relocation entry.

SelectionDAGBuilder::getValueImpl is changed to allow metadata operands passed through to ISel.

[arsenm]

It seems weird to me to expose relocation names so directly. Why isn't this just exposing whatever specific construct you need?

llvm/test/CodeGen/AMDGPU/amdgpu-reloc-const.ll

19

You can just use 0.0

[csyonghe]

This is suggested by @nhaehnle
I like this idea because it does not lock into any specific data structure prematurely in amdgcn, which the backend really does not care. In the end you want to control the name of the symbol exported in the elf.
This will enable us to use the intrinsic for many different purposes in llpc, and push the responsibility of naming a relocation symbol to llpc instead of in amdgpu.

[nhaehnle]

Thank you for doing this. And yes, the point of the generic mechanism is to decouple the frontend and backend a bit more, as the envisioned use case for this are very graphics-specific, and we will want to evolve it on the frontend side without having to juggle LLVM changes for each one individually.

Please augment the test with a run line that checks the ELF relocation entries as well? I.e. parsing the generated ELF (-filetype=obj) with llvm-readobj. There are existing tests in CodeGen/AMDGPU which do this that you can imitate.

llvm/include/llvm/IR/IntrinsicsAMDGPU.td

1990–1991

The comment is out of date.

llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp

1764–1780

Please either move this hunk into a separate patch or add a test case to this one that shows the effect.

Also, the ZERO_EXTEND handling still looks like it'd need proper type checks.

1844–1846

This should be unnecessary.

[csyonghe]

Revised per @nhaehnle's comments.

[csyonghe]

llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp

1764–1780

This change is necessary for correct selection of the offset-from-a-register version of the load instruction following a offset relocation. Otherwise a lot of unnecessary code will get generated.
Since this is in SelectSMRDOffset, the only type that can appear in the ZERO_EXTEND is int32, no? What kind of type check do you think is necessary here?

1844–1846

I want to handle the case where Addr is in the form of base+offset, where offset is not a constant. In this case, the register-offset version of the load instruction should be selected (see line 1765). Without this, that logic does not trigger.

[nhaehnle]

llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp

1764–1780

Okay, thank you for the explanation and the added test. However, relying on it being i32 is unsafe here. It's conceivable that there could be a zeroext from i16, for example. (Now in practice, that gets combined to an anyext followed by and today, but why wouldn't that potentially change at some point in the future?)

Simply checking that the operand of the ZERO_EXTEND is an i32 should be fine, I don't think you have to try to handle anything more general than that.

1844–1846

Ah of course. Thanks for the explanation.

[csyonghe]

Added operand type check for ZERO_EXT.

[nhaehnle]

Thanks, LGTM

llvm/include/llvm/IR/IntrinsicsAMDGPU.td

   [llvm_float_ty], [llvm_float_ty, llvm_float_ty],
   [IntrNoMem, IntrSpeculatable]
 >;
+
+// Represent a relocation constant.
+def int_amdgcn_reloc_constant : Intrinsic<
+  [llvm_i32_ty], [llvm_metadata_ty],
+  [IntrNoMem, IntrSpeculatable]
+>;
 }

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

     return RFV.getCopyFromRegs(DAG, FuncInfo, getCurSDLoc(), Chain, nullptr, V);
   }
 
+  if (const MetadataAsValue *MD = dyn_cast<MetadataAsValue>(V)) {
+    return DAG.getMDNode(cast<MDNode>(MD->getMetadata()));
+  }
   llvm_unreachable("Can't get register for value!");
 }
 

llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp

 
 bool AMDGPUDAGToDAGISel::SelectSMRDOffset(SDValue ByteOffsetNode,
                                           SDValue &Offset, bool &Imm) const {
-
-  // FIXME: Handle non-constant offsets.
   ConstantSDNode *C = dyn_cast<ConstantSDNode>(ByteOffsetNode);
-  if (!C)
+  if (!C) {
+    if (ByteOffsetNode.getValueType().isScalarInteger() &&
+        ByteOffsetNode.getValueType().getSizeInBits() == 32) {
+      Offset = ByteOffsetNode;
+      Imm = false;
+      return true;
+    }
+    if (ByteOffsetNode.getOpcode() == ISD::ZERO_EXTEND) {
+      if (ByteOffsetNode.getOperand(0).getValueType().getSizeInBits() == 32) {
+        Offset = ByteOffsetNode.getOperand(0);
+        Imm = false;
+        return true;
+      }
+    }
     return false;
+  }
 
   SDLoc SL(ByteOffsetNode);
   GCNSubtarget::Generation Gen = Subtarget->getGeneration();

   // wraparound, because s_load instructions perform the addition in 64 bits.
   if ((Addr.getValueType() != MVT::i32 ||
        Addr->getFlags().hasNoUnsignedWrap()) &&
-      CurDAG->isBaseWithConstantOffset(Addr)) {
+      (CurDAG->isBaseWithConstantOffset(Addr) ||
+       Addr.getOpcode() == ISD::ADD)) {
     SDValue N0 = Addr.getOperand(0);
     SDValue N1 = Addr.getOperand(1);
 

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

                                 DAG.getConstant(1, SL, MVT::i32));
     return DAG.getSetCC(SL, MVT::i1, SrcHi, Aperture, ISD::SETEQ);
   }
+  case Intrinsic::amdgcn_reloc_constant: {
+    Module *M = const_cast<Module *>(MF.getFunction().getParent());
+    const MDNode *Metadata = cast<MDNodeSDNode>(Op.getOperand(1))->getMD();
+    auto SymbolName = cast<MDString>(Metadata->getOperand(0))->getString();
+    auto RelocSymbol = cast<GlobalVariable>(
+        M->getOrInsertGlobal(SymbolName, Type::getInt32Ty(M->getContext())));
+    SDValue GA = DAG.getTargetGlobalAddress(RelocSymbol, DL, MVT::i32, 0,
+                                            SIInstrInfo::MO_ABS32_LO);
+    return {DAG.getMachineNode(AMDGPU::S_MOV_B32, DL, MVT::i32, GA), 0};
+  }
   default:
     if (const AMDGPU::ImageDimIntrinsicInfo *ImageDimIntr =
             AMDGPU::getImageDimIntrinsicInfo(IntrinsicID))

llvm/test/CodeGen/AMDGPU/amdgcn-load-offset-from-reg.ll

+; Test that DAG->DAG ISel is able to pick up the S_LOAD_DWORDX4_SGPR instruction that fetches the offset
+; from a register.
+
+; RUN: llc -march=amdgcn -verify-machineinstrs -stop-after=amdgpu-isel -o - %s | FileCheck -check-prefix=GCN %s
+
+; GCN: %[[OFFSET:[0-9]+]]:sreg_32 = S_MOV_B32 target-flags(amdgpu-abs32-lo) @DescriptorBuffer
+; GCN: %{{[0-9]+}}:sgpr_128 = S_LOAD_DWORDX4_SGPR killed %{{[0-9]+}}, killed %[[OFFSET]], 0, 0 :: (invariant load 16 from %ir.13, addrspace 4)
+
+define amdgpu_cs void @test_load_zext(i32 inreg %0, i32 inreg %1, i32 inreg %resNode0, i32 inreg %resNode1, <3 x i32> inreg %2, i32 inreg %3, <3 x i32> %4) local_unnamed_addr #2 {
+.entry:
+  %5 = call i64 @llvm.amdgcn.s.getpc() #3
+  %6 = bitcast i64 %5 to <2 x i32>
+  %7 = insertelement <2 x i32> %6, i32 %resNode0, i32 0
+  %8 = bitcast <2 x i32> %7 to i64
+  %9 = inttoptr i64 %8 to [4294967295 x i8] addrspace(4)*
+  %10 = call i32 @llvm.amdgcn.reloc.constant(metadata !4)
+  %11 = zext i32 %10 to i64
+  %12 = getelementptr [4294967295 x i8], [4294967295 x i8] addrspace(4)* %9, i64 0, i64 %11
+  %13 = bitcast i8 addrspace(4)* %12 to <4 x i32> addrspace(4)*, !amdgpu.uniform !5
+  %14 = load <4 x i32>, <4 x i32> addrspace(4)* %13, align 16, !invariant.load !5
+  %15 = call <4 x i32> @llvm.amdgcn.s.buffer.load.v4i32(<4 x i32> %14, i32 0, i32 0)
+  call void @llvm.amdgcn.raw.buffer.store.v4i32(<4 x i32> %15, <4 x i32> %14, i32 0, i32 0, i32 0)
+  ret void
+}
+
+declare <4 x float> @llvm.amdgcn.buffer.load.v4f32(<4 x i32>, i32, i32, i1, i1) #0
+; Function Attrs: nounwind writeonly
+declare void @llvm.amdgcn.raw.buffer.store.v4i32(<4 x i32>, <4 x i32>, i32, i32, i32 immarg) #1
+
+; Function Attrs: nounwind readnone speculatable
+declare i32 @llvm.amdgcn.reloc.constant(metadata) #3
+
+; Function Attrs: nounwind readnone speculatable
+declare i64 @llvm.amdgcn.s.getpc() #3
+
+; Function Attrs: nounwind readnone
+declare <4 x i32> @llvm.amdgcn.s.buffer.load.v4i32(<4 x i32>, i32, i32 immarg) #1
+
+attributes #0 = { argmemonly nounwind willreturn }
+attributes #1 = { nounwind readnone }
+attributes #2 = { nounwind "amdgpu-unroll-threshold"="700" }
+attributes #3 = { nounwind readnone speculatable }
+attributes #4 = { nounwind writeonly }
+
+!llpc.compute.mode = !{!0}
+!llpc.options = !{!1}
+!llpc.options.CS = !{!2}
+!llpc.user.data.nodes = !{!3, !4, !5, !6}
+!amdgpu.pal.metadata.msgpack = !{!7}
+
+!0 = !{i32 2, i32 3, i32 1}
+!1 = !{i32 245227952, i32 996822128, i32 2024708198, i32 497230408}
+!2 = !{i32 1381820427, i32 1742110173, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 64}
+!3 = !{!"DescriptorTableVaPtr", i32 0, i32 1, i32 1}
+!4 = !{!"DescriptorBuffer", i32 4, i32 8, i32 0, i32 0}
+!5 = !{!"DescriptorTableVaPtr", i32 1, i32 1, i32 1}
+!6 = !{!"DescriptorBuffer", i32 4, i32 8, i32 1, i32 0}
+!7 = !{!"\82\B0amdpal.pipelines\91\88\A4.api\A6Vulkan\B0.hardware_stages\81\A3.cs\82\AB.sgpr_limith\AB.vgpr_limit\CD\01\00\B7.internal_pipeline_hash\92\CF;jLp\0E\9D\E1\B0\CF\1D\A3\22Hx\AE\98f\AA.registers\88\CD.\07\02\CD.\08\03\CD.\09\01\CD.\12\CE\00,\00\00\CD.\13\CD\0F\88\CD.@\CE\10\00\00\00\CD.B\00\CD.C\01\A8.shaders\81\A8.compute\82\B0.api_shader_hash\92\CFg\D6}\DDR\\\E8\0B\00\B1.hardware_mapping\91\A3.cs\B0.spill_threshold\CE\FF\FF\FF\FF\A5.type\A2Cs\B0.user_data_limit\02\AEamdpal.version\92\02\03"}
+!8 = !{i32 5}
+!9 = !{!"doff_0_0_b"}
+!10 = !{}
+!11 = !{!"doff_1_0_b"}

llvm/test/CodeGen/AMDGPU/amdgpu-reloc-const.ll

+; RUN: llc -march=amdgcn -verify-machineinstrs < %s | FileCheck -check-prefix=GCN %s
+; RUN: llc -mtriple=amdgcn--amdpal -mcpu=gfx900 -filetype=obj -o %t.o < %s && llvm-readobj -relocations %t.o | FileCheck --check-prefix=ELF %s
+; GCN-LABEL: {{^}}ps_main:
+; GCN: v_mov_b32_{{.*}} v[[relocreg:[0-9]+]], doff_0_0_b@abs32@lo
+; GCN-NEXT: exp {{.*}} v[[relocreg]], {{.*}}
+; GCN-NEXT: s_endpgm
+; GCN-NEXT: .Lfunc_end
+
+; ELF: Relocations [
+; ELF-NEXT: Section (3) .rel.text {
+; ELF-NEXT: 0x{{[0-9]+}} R_AMDGPU_ABS32 doff_0_0_b {{.*}}
+
+define amdgpu_ps void @ps_main(i32 %arg, i32 inreg %arg1, i32 inreg %arg2) local_unnamed_addr #0 {
+  %rc = call i32 @llvm.amdgcn.reloc.constant(metadata !1)
+  %rcf = bitcast i32 %rc to float
+  call void @llvm.amdgcn.exp.f32(i32 immarg 40, i32 immarg 15, float %rcf, float undef, float undef, float undef, i1 immarg false, i1 immarg false) #0
+  ret void
+}
+
+; Function Attrs: inaccessiblememonly nounwind
+declare void @llvm.amdgcn.exp.f32(i32 immarg, i32 immarg, float, float, float, float, i1 immarg, i1 immarg) #1
+
+; Function Attrs: nounwind readnone speculatable
+declare i32 @llvm.amdgcn.reloc.constant(metadata) #2
+
+attributes #0 = { nounwind }
+attributes #1 = { inaccessiblememonly nounwind }
+attributes #2 = { nounwind readnone speculatable }
+
+!1 = !{!"doff_0_0_b"}