AMDGPU: Always emit amdgpu-flat-work-group-size

[arsenm]

Summary

The backend default maximum should be the hardware maximum, so the
frontend should set the implementation defined default maximum.

[b-sumner]

lib/CodeGen/TargetInfo.cpp

7947

Theoretically, shouldn't the minimum be 1?

[arsenm]

lib/CodeGen/TargetInfo.cpp

7947

That's what I thought, but the backend is defaulting to 2 * wave size now

[yaxunl]

lib/CodeGen/TargetInfo.cpp

7947

I don't get it. This attribute indicates the possible workgroup size range this kernel may be run with, right? It only depends on how user execute the kernel. How is it related to backend defaults?

[arsenm]

lib/CodeGen/TargetInfo.cpp

7947

The backend currently assumes 128,256 by default as the bounds. I want to make this a frontend decision, and make the backend assumption the most conservative default

[arsenm]

ping

[yaxunl]

My concern is that this essentially forcing user to add amdgpu_flat_work_group_size attribute to all kernels that are executed outside of (128,256). Potentially this can cause lots of regressions for existing OpenCL apps. I am not sure if it is feasible to force all OpenCL apps to make this change. Should we do some tests before making this change?

[b-sumner]

We need to communicate with anyone generating IR to ensure this is being generated before we change the default. clang is only one of those generators. This change will also need to be documented in the usage document.

[arsenm]

In D62739#1536390, @yaxunl wrote:

My concern is that this essentially forcing user to add amdgpu_flat_work_group_size attribute to all kernels that are executed outside of (128,256). Potentially this can cause lots of regressions for existing OpenCL apps. I am not sure if it is feasible to force all OpenCL apps to make this change. Should we do some tests before making this change?

This is already the case. This is just moving it to the frontend. There's no user observable change from this patch

[arsenm]

In D62739#1536428, @b-sumner wrote:

We need to communicate with anyone generating IR to ensure this is being generated before we change the default. clang is only one of those generators. This change will also need to be documented in the usage document.

The planned change is to make the backend more conservative, so it shouldn't break other frontends

[arsenm]

In D62739#1543437, @arsenm wrote:

In D62739#1536390, @yaxunl wrote:

My concern is that this essentially forcing user to add amdgpu_flat_work_group_size attribute to all kernels that are executed outside of (128,256). Potentially this can cause lots of regressions for existing OpenCL apps. I am not sure if it is feasible to force all OpenCL apps to make this change. Should we do some tests before making this change?

This is already the case. This is just moving it to the frontend. There's no user observable change from this patch

I do want to reduce the minimum bound to 1 at least (which is the default for graphics shaders), but that's a separate change

[b-sumner]

In D62739#1543438, @arsenm wrote:

In D62739#1536428, @b-sumner wrote:

We need to communicate with anyone generating IR to ensure this is being generated before we change the default. clang is only one of those generators. This change will also need to be documented in the usage document.

The planned change is to make the backend more conservative, so it shouldn't break other frontends

It may not break other frontends, but could cause substantial performance regressions. At a minimum the summary should clearly mention this possibility.

[arsenm]

In D62739#1543578, @b-sumner wrote:

In D62739#1543438, @arsenm wrote:

In D62739#1536428, @b-sumner wrote:

We need to communicate with anyone generating IR to ensure this is being generated before we change the default. clang is only one of those generators. This change will also need to be documented in the usage document.

The planned change is to make the backend more conservative, so it shouldn't break other frontends

It may not break other frontends, but could cause substantial performance regressions. At a minimum the summary should clearly mention this possibility.

The summary of the future backend patch will

[arsenm]

Rebase

[arsenm]

ping

[arsenm]

ping

[arsenm]

ping

[rampitec]

lib/CodeGen/TargetInfo.cpp

7947

I would agree that minimum should be 1. Backend's choice is also unclear, but emitting a false attribute is a separate issue.

[arsenm]

Make lower bound 1, although this is a behavior change

[rampitec]

LGTM

[arsenm]

r370101

lib/CodeGen/TargetInfo.cpp

void AMDGPUTargetCodeGenInfo::setTargetAttributes(
   if (!FD)
     return;
 
   llvm::Function *F = cast<llvm::Function>(GV);
 
   const auto *ReqdWGS = M.getLangOpts().OpenCL ?
     FD->getAttr<ReqdWorkGroupSizeAttr>() : nullptr;
 
-  if (((M.getLangOpts().OpenCL && FD->hasAttr<OpenCLKernelAttr>()) ||
+
+  const bool IsOpenCLKernel = M.getLangOpts().OpenCL &&
+                              FD->hasAttr<OpenCLKernelAttr>();
+  if ((IsOpenCLKernel ||
        (M.getLangOpts().HIP && FD->hasAttr<CUDAGlobalAttr>())) &&
       (M.getTriple().getOS() == llvm::Triple::AMDHSA))
     F->addFnAttr("amdgpu-implicitarg-num-bytes", "56");
 
   const auto *FlatWGS = FD->getAttr<AMDGPUFlatWorkGroupSizeAttr>();
   if (ReqdWGS || FlatWGS) {
     unsigned Min = 0;
     unsigned Max = 0;
     if (FlatWGS) {
if (ReqdWGS || FlatWGS) {
 
     if (Min != 0) {
       assert(Min <= Max && "Min must be less than or equal Max");
 
       std::string AttrVal = llvm::utostr(Min) + "," + llvm::utostr(Max);
       F->addFnAttr("amdgpu-flat-work-group-size", AttrVal);
     } else
       assert(Max == 0 && "Max must be zero");
+  } else if (IsOpenCLKernel) {
+    // By default, restrict the maximum size to 256.
+    F->addFnAttr("amdgpu-flat-work-group-size", "1,256");
   }
 
   if (const auto *Attr = FD->getAttr<AMDGPUWavesPerEUAttr>()) {
     unsigned Min =
         Attr->getMin()->EvaluateKnownConstInt(M.getContext()).getExtValue();
     unsigned Max = Attr->getMax() ? Attr->getMax()
                                         ->EvaluateKnownConstInt(M.getContext())
                                         .getExtValue()

test/CodeGenOpenCL/amdgpu-attrs.cl

 kernel void reqd_work_group_size_32_2_1_flat_work_group_size_16_128() {
 // CHECK: define amdgpu_kernel void @reqd_work_group_size_32_2_1_flat_work_group_size_16_128() [[FLAT_WORK_GROUP_SIZE_16_128:#[0-9]+]]
 }
 
 void a_function() {
 // CHECK: define void @a_function() [[A_FUNCTION:#[0-9]+]]
 }
 
+kernel void default_kernel() {
+// CHECK: define amdgpu_kernel void @default_kernel() [[DEFAULT_KERNEL_ATTRS:#[0-9]+]]
+}
+
 
 // Make sure this is silently accepted on other targets.
 // X86-NOT: "amdgpu-flat-work-group-size"
 // X86-NOT: "amdgpu-waves-per-eu"
 // X86-NOT: "amdgpu-num-vgpr"
 // X86-NOT: "amdgpu-num-sgpr"
 // X86-NOT: "amdgpu-implicitarg-num-bytes"
 // NONAMDHSA-NOT: "amdgpu-implicitarg-num-bytes"
 
 // CHECK-NOT: "amdgpu-flat-work-group-size"="0,0"
 // CHECK-NOT: "amdgpu-waves-per-eu"="0"
 // CHECK-NOT: "amdgpu-waves-per-eu"="0,0"
 // CHECK-NOT: "amdgpu-num-sgpr"="0"
 // CHECK-NOT: "amdgpu-num-vgpr"="0"
 
-// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" 
-// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_64_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="64,64" "amdgpu-implicitarg-num-bytes"="56" 
-// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_16_128]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="16,128" "amdgpu-implicitarg-num-bytes"="56" 
-// CHECK-DAG: attributes [[WAVES_PER_EU_2]] = { convergent noinline nounwind optnone "amdgpu-implicitarg-num-bytes"="56" "amdgpu-waves-per-eu"="2"
-// CHECK-DAG: attributes [[WAVES_PER_EU_2_4]] = { convergent noinline nounwind optnone "amdgpu-implicitarg-num-bytes"="56" "amdgpu-waves-per-eu"="2,4"
-// CHECK-DAG: attributes [[NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" 
-// CHECK-DAG: attributes [[NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64" 
+// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56"
+// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_64_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="64,64" "amdgpu-implicitarg-num-bytes"="56"
+// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_16_128]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="16,128" "amdgpu-implicitarg-num-bytes"="56"
+
+// CHECK-DAG: attributes [[WAVES_PER_EU_2]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256"  "amdgpu-implicitarg-num-bytes"="56" "amdgpu-waves-per-eu"="2"
+
+// CHECK-DAG: attributes [[WAVES_PER_EU_2_4]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-waves-per-eu"="2,4"
+// CHECK-DAG: attributes [[NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32"
+// CHECK-DAG: attributes [[NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64"
 
 // CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_WAVES_PER_EU_2]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-waves-per-eu"="2"
 // CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_WAVES_PER_EU_2_4]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-waves-per-eu"="2,4"
-// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" 
-// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64" 
-// CHECK-DAG: attributes [[WAVES_PER_EU_2_NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-waves-per-eu"="2"
-// CHECK-DAG: attributes [[WAVES_PER_EU_2_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64" "amdgpu-waves-per-eu"="2"
-// CHECK-DAG: attributes [[WAVES_PER_EU_2_4_NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-waves-per-eu"="2,4"
-// CHECK-DAG: attributes [[WAVES_PER_EU_2_4_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64" "amdgpu-waves-per-eu"="2,4"
-// CHECK-DAG: attributes [[NUM_SGPR_32_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-num-vgpr"="64" 
+// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32"
+// CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64"
+// CHECK-DAG: attributes [[WAVES_PER_EU_2_NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-waves-per-eu"="2"
+// CHECK-DAG: attributes [[WAVES_PER_EU_2_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64" "amdgpu-waves-per-eu"="2"
+// CHECK-DAG: attributes [[WAVES_PER_EU_2_4_NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-waves-per-eu"="2,4"
+// CHECK-DAG: attributes [[WAVES_PER_EU_2_4_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64" "amdgpu-waves-per-eu"="2,4"
+// CHECK-DAG: attributes [[NUM_SGPR_32_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-num-vgpr"="64"
 
 // CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_WAVES_PER_EU_2_NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-waves-per-eu"="2"
 // CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_WAVES_PER_EU_2_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64" "amdgpu-waves-per-eu"="2"
 // CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_WAVES_PER_EU_2_4_NUM_SGPR_32]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-waves-per-eu"="2,4"
 // CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_WAVES_PER_EU_2_4_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-vgpr"="64" "amdgpu-waves-per-eu"="2,4"
 
 // CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_WAVES_PER_EU_2_NUM_SGPR_32_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-num-vgpr"="64" "amdgpu-waves-per-eu"="2"
 // CHECK-DAG: attributes [[FLAT_WORK_GROUP_SIZE_32_64_WAVES_PER_EU_2_4_NUM_SGPR_32_NUM_VGPR_64]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="32,64" "amdgpu-implicitarg-num-bytes"="56" "amdgpu-num-sgpr"="32" "amdgpu-num-vgpr"="64" "amdgpu-waves-per-eu"="2,4"
 
 // CHECK-DAG: attributes [[A_FUNCTION]] = { convergent noinline nounwind optnone "correctly-rounded-divide-sqrt-fp-math"="false"
+// CHECK-DAG: attributes [[DEFAULT_KERNEL_ATTRS]] = { convergent noinline nounwind optnone "amdgpu-flat-work-group-size"="1,256" "amdgpu-implicitarg-num-bytes"="56"