[NVPTX] Avoid temp copy of byval kernel parameters.

[tra]

Summary

Avoid making a temporary copy of byval argument if all accesses are loads and therefore the pointer to the parameter can not escape.

This avoids excessive global memory accesses when each kernel makes its own copy.

[jlebar]

This is going to be so great. Thanks, Art.

llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp

165–166

Looking at https://llvm.org/docs/LangRef.html#load-instruction, there's a lot of additional metadata that might be on a load instruction. Do we need to copy it? (Is there no LLVM utility that says, "create a new load in a different address space?)

Also, should we disallow atomic loads or otherwise handle them?

179

Is it safe to write this recursively? (Stack overflow?)

189

same -- is recursion ok here?

205

Nit: run-on sentence. s/which/. This/

[asbirlea]

llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp

165–166

AFAIK there is no such utility.

AAMDNodes AATags;
LI->getAAMetadata(AATags);
NewLI->setAAMetadata(AATags);

Perhaps also set debug location, e.g. NewLI->setDebugLoc(DebugLoc());

[tra]

Do not clone LoadInst, just replace its operand.

[tra]

llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp

165–166

Good point. I do not even need to clone it -- it does not care about address space itself. Its pointer operand deals with it.

179

It could be a problem.

LLVM's manual does not mention anything about that, but the examples are all 'flat'.
https://llvm.org/docs/ProgrammersManual.html#basic-inspection-and-traversal-routines

Shouldn't be hard to rewrite it as a loop.

@asbirlea : is recursion OK here, or do I need to use a loop instead?

[tra]

Replaced recursive iteration with loop-based one.

[jlebar]

...in which we all remember why we like writing this recursively. :D

🚢

llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp

161

I think there was a clang-tidy warning at one point saying this should be lower-case c?

183

needed?

[tra]

Fixed few tests affected by the patch.

[tra]

llvm/test/CodeGen/NVPTX/lower-kernel-ptr-arg.ll

35 ↗

(On Diff #330375)

With a temporary copy of the argument, the alignment used to be inferred to be 8 based on the struct alignment and that allowed LLVM to generate 64-bit load. W/o a temporary copy, we're only looking at the load's alignment of 4, which probably evolved from the original 32-bit IR.
Making load alignment to be 8 makes the test work predictably regardless of whether we have a temp copy.

[asbirlea]

Updates look good.

llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp

 
 char NVPTXLowerArgs::ID = 1;
 
 INITIALIZE_PASS(NVPTXLowerArgs, "nvptx-lower-args",
                 "Lower arguments (NVPTX)", false, false)
 
 // =============================================================================
 // If the function had a byval struct ptr arg, say foo(%struct.x* byval %d),
+// and we can't guarantee that the only accesses are loads,
 // then add the following instructions to the first basic block:
 //
 // %temp = alloca %struct.x, align 8
 // %tempd = addrspacecast %struct.x* %d to %struct.x addrspace(101)*
 // %tv = load %struct.x addrspace(101)* %tempd
 // store %struct.x %tv, %struct.x* %temp, align 8
 //
 // The above code allocates some space in the stack and copies the incoming
 // struct from param space to local space.
 // Then replace all occurrences of %d by %temp.
+//
+// In case we know that all users are GEPs or Loads, replace them with the same
+// ones in parameter AS, so we can access them using ld.param.
 // =============================================================================
+
+// Replaces the \p OldUser instruction with the same in parameter AS.
+// Only Load and GEP are supported.
+static void ConvertToParamAS(Value *OldUser, Value *Param) {
+  Instruction *I = dyn_cast<Instruction>(OldUser);
+  assert(I && "OldUser must be an instruction");
+  if (auto *LI = dyn_cast<LoadInst>(I)) {
+    auto *NewLI = new LoadInst(LI->getType(), Param, LI->getName(),
+                               LI->isVolatile(), LI->getAlign(), I);
+    LI->replaceAllUsesWith(NewLI);
+    LI->eraseFromParent();
+    return;
+  }
+  if (auto *GEP = dyn_cast<GetElementPtrInst>(I)) {
+    SmallVector<Value *, 4> Indices(GEP->indices());
+    auto *NewGEP =
+        GetElementPtrInst::Create(nullptr, Param, Indices, GEP->getName(), GEP);
+    NewGEP->setIsInBounds(GEP->isInBounds());
+
+    SmallVector<User *, 16> UsersToUpdate(GEP->users());
+    llvm::for_each(UsersToUpdate,
+                   [NewGEP](Value *V) { ConvertToParamAS(V, NewGEP); });
+    GEP->eraseFromParent();
+    return;
+  }
+  llvm_unreachable("Only Load and GEP can be converted to param AS.");
+}
+
+static bool isALoadChain(Value *V) {
+  if (Instruction *I = dyn_cast<Instruction>(V)) {
+    if (isa<GetElementPtrInst>(I))
+      return llvm::all_of(I->users(), isALoadChain);
+    return isa<LoadInst>(I);
+  }
+  return false;
+};
+
 void NVPTXLowerArgs::handleByValParam(Argument *Arg) {
   Function *Func = Arg->getParent();
   Instruction *FirstInst = &(Func->getEntryBlock().front());
   PointerType *PType = dyn_cast<PointerType>(Arg->getType());
 
   assert(PType && "Expecting pointer type in handleByValParam");
 
   Type *StructType = PType->getElementType();
+
+  if (llvm::all_of(Arg->users(), isALoadChain)) {
+    // Replace all loads with the loads in param AS which allows loading the Arg
+    // directly from parameter AS, without making a temporary copy.
+    SmallVector<User *, 16> UsersToUpdate(Arg->users());
+    Value *ArgInParamAS = new AddrSpaceCastInst(
+        Arg, PointerType::get(StructType, ADDRESS_SPACE_PARAM), Arg->getName(),
+        FirstInst);
+    llvm::for_each(UsersToUpdate, [ArgInParamAS](Value *V) {
+      ConvertToParamAS(V, ArgInParamAS);
+    });
+    return;
+  }
+
+  // Otherwise we have to create a temporary copy.
   const DataLayout &DL = Func->getParent()->getDataLayout();
   unsigned AS = DL.getAllocaAddrSpace();
   AllocaInst *AllocA = new AllocaInst(StructType, AS, Arg->getName(), FirstInst);
   // Set the alignment to alignment of the byval parameter. This is because,
   // later load/stores assume that alignment, and we are going to replace
   // the use of the byval parameter with this alloca instruction.
   AllocA->setAlignment(Func->getParamAlign(Arg->getArgNo())
                            .getValueOr(DL.getPrefTypeAlign(StructType)));

llvm/test/CodeGen/NVPTX/lower-byval-args.ll

+; RUN: llc < %s -mcpu=sm_20 | FileCheck %s
+
+target triple = "nvptx64-nvidia-cuda"
+
+%struct.ham = type { [4 x i32] }
+
+; // Verify that load with static offset into parameter is done directly.
+; CHECK-LABEL: .visible .entry static_offset
+; CHECK: ld.param.u64    [[result_addr:%rd[0-9]+]], [{{.*}}_param_0]
+; CHECK: mov.b64         %[[param_addr:rd[0-9]+]], {{.*}}_param_1
+; CHECK: mov.u64         %[[param_addr1:rd[0-9]+]], %[[param_addr]]
+; CHECK: cvta.to.global.u64 [[result_addr_g:%rd[0-9]+]], [[result_addr]]
+; CHECK: ld.param.u32    [[value:%r[0-9]+]], [%[[param_addr1]]+12];
+; CHECK  st.global.u32   [[[result_addr_g]]], [[value]];
+; Function Attrs: nofree norecurse nounwind willreturn mustprogress
+define dso_local void @static_offset(i32* nocapture %arg, %struct.ham* nocapture readonly byval(%struct.ham) align 4 %arg1, i32 %arg2) local_unnamed_addr #0 {
+bb:
+  %tmp = icmp eq i32 %arg2, 3
+  br i1 %tmp, label %bb3, label %bb6
+
+bb3:                                              ; preds = %bb
+  %tmp4 = getelementptr inbounds %struct.ham, %struct.ham* %arg1, i64 0, i32 0, i64 3
+  %tmp5 = load i32, i32* %tmp4, align 4
+  store i32 %tmp5, i32* %arg, align 4
+  br label %bb6
+
+bb6:                                              ; preds = %bb3, %bb
+  ret void
+}
+
+; // Verify that load with dynamic offset into parameter is also done directly.
+; CHECK-LABEL: .visible .entry dynamic_offset
+; CHECK: ld.param.u64    [[result_addr:%rd[0-9]+]], [{{.*}}_param_0]
+; CHECK: mov.b64         %[[param_addr:rd[0-9]+]], {{.*}}_param_1
+; CHECK: mov.u64         %[[param_addr1:rd[0-9]+]], %[[param_addr]]
+; CHECK: cvta.to.global.u64 [[result_addr_g:%rd[0-9]+]], [[result_addr]]
+; CHECK: add.s64         %[[param_w_offset:rd[0-9]+]], %[[param_addr1]],
+; CHECK: ld.param.u32    [[value:%r[0-9]+]], [%[[param_w_offset]]];
+; CHECK  st.global.u32   [[[result_addr_g]]], [[value]];
+
+; Function Attrs: nofree norecurse nounwind willreturn mustprogress
+define dso_local void @dynamic_offset(i32* nocapture %arg, %struct.ham* nocapture readonly byval(%struct.ham) align 4 %arg1, i32 %arg2) local_unnamed_addr #0 {
+bb:
+  %tmp = sext i32 %arg2 to i64
+  %tmp3 = getelementptr inbounds %struct.ham, %struct.ham* %arg1, i64 0, i32 0, i64 %tmp
+  %tmp4 = load i32, i32* %tmp3, align 4
+  store i32 %tmp4, i32* %arg, align 4
+  ret void
+}
+
+; Verify that if the pointer escapes, then we do fall back onto using a temp copy.
+; CHECK-LABEL: .visible .entry pointer_escapes
+; CHECK: .local .align 8 .b8     __local_depot{{.*}}
+; CHECK: ld.param.u64    [[result_addr:%rd[0-9]+]], [{{.*}}_param_0]
+; CHECK: add.u64         %[[copy_addr:rd[0-9]+]], %SPL, 0;
+; CHECK-DAG: ld.param.u32    %{{.*}}, [pointer_escapes_param_1+12];
+; CHECK-DAG: ld.param.u32    %{{.*}}, [pointer_escapes_param_1+8];
+; CHECK-DAG: ld.param.u32    %{{.*}}, [pointer_escapes_param_1+4];
+; CHECK-DAG: ld.param.u32    %{{.*}}, [pointer_escapes_param_1];
+; CHECK-DAG: st.local.u32    [%[[copy_addr]]+12],
+; CHECK-DAG: st.local.u32    [%[[copy_addr]]+8],
+; CHECK-DAG: st.local.u32    [%[[copy_addr]]+4],
+; CHECK-DAG: st.local.u32    [%[[copy_addr]]],
+; CHECK: cvta.to.global.u64 [[result_addr_g:%rd[0-9]+]], [[result_addr]]
+; CHECK: add.s64         %[[copy_w_offset:rd[0-9]+]], %[[copy_addr]],
+; CHECK: ld.local.u32    [[value:%r[0-9]+]], [%[[copy_w_offset]]];
+; CHECK  st.global.u32   [[[result_addr_g]]], [[value]];
+
+; Function Attrs: convergent norecurse nounwind mustprogress
+define dso_local void @pointer_escapes(i32* nocapture %arg, %struct.ham* byval(%struct.ham) align 4 %arg1, i32 %arg2) local_unnamed_addr #1 {
+bb:
+  %tmp = sext i32 %arg2 to i64
+  %tmp3 = getelementptr inbounds %struct.ham, %struct.ham* %arg1, i64 0, i32 0, i64 %tmp
+  %tmp4 = load i32, i32* %tmp3, align 4
+  store i32 %tmp4, i32* %arg, align 4
+  %tmp5 = call i32* @escape(i32* nonnull %tmp3) #3
+  ret void
+}
+
+; Function Attrs: convergent nounwind
+declare dso_local i32* @escape(i32*) local_unnamed_addr
+
+
+!llvm.module.flags = !{!0, !1, !2}
+!nvvm.annotations = !{!3, !4, !5}
+
+!0 = !{i32 2, !"SDK Version", [2 x i32] [i32 9, i32 1]}
+!1 = !{i32 1, !"wchar_size", i32 4}
+!2 = !{i32 4, !"nvvm-reflect-ftz", i32 0}
+!3 = !{void (i32*, %struct.ham*, i32)* @static_offset, !"kernel", i32 1}
+!4 = !{void (i32*, %struct.ham*, i32)* @dynamic_offset, !"kernel", i32 1}
+!5 = !{void (i32*, %struct.ham*, i32)* @pointer_escapes, !"kernel", i32 1}