[AMDGPU] Support function attribute to override postRA scheduling direction

[harrisonGPU]

Allow postRA scheduling direction to be set via misched-postra-direction
function attribute, supporting topdown, bottomup, and bidirectional.

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

Author: Harrison Hao (harrisonGPU)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/147708.diff

3 Files Affected:

• (modified) llvm/include/llvm/CodeGen/MachineScheduler.h (+1)
• (modified) llvm/lib/CodeGen/MachineScheduler.cpp (+1-1)
• (modified) llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp (+17)

diff --git a/llvm/include/llvm/CodeGen/MachineScheduler.h b/llvm/include/llvm/CodeGen/MachineScheduler.h
index e7a7091acee64..1635eae758590 100644
--- a/llvm/include/llvm/CodeGen/MachineScheduler.h
+++ b/llvm/include/llvm/CodeGen/MachineScheduler.h
@@ -116,6 +116,7 @@ enum Direction {
 } // namespace MISched
 
 LLVM_ABI extern cl::opt<MISched::Direction> PreRADirection;
+LLVM_ABI extern cl::opt<MISched::Direction> PostRADirection;
 LLVM_ABI extern cl::opt<bool> VerifyScheduling;
 #ifndef NDEBUG
 extern cl::opt<bool> ViewMISchedDAGs;
diff --git a/llvm/lib/CodeGen/MachineScheduler.cpp b/llvm/lib/CodeGen/MachineScheduler.cpp
index 76cba2949af60..72e1fce07f33e 100644
--- a/llvm/lib/CodeGen/MachineScheduler.cpp
+++ b/llvm/lib/CodeGen/MachineScheduler.cpp
@@ -190,7 +190,7 @@ cl::opt<MISched::Direction> PreRADirection(
         clEnumValN(MISched::Bidirectional, "bidirectional",
                    "Force bidirectional pre reg-alloc list scheduling")));
 
-static cl::opt<MISched::Direction> PostRADirection(
+cl::opt<MISched::Direction> PostRADirection(
     "misched-postra-direction", cl::Hidden,
     cl::desc("Post reg-alloc list scheduling direction"),
     cl::init(MISched::Unspecified),
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp b/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
index c3536113e9bef..4c46e703b4604 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
@@ -1144,6 +1144,23 @@ GCNTargetMachine::createMachineScheduler(MachineSchedContext *C) const {
 
 ScheduleDAGInstrs *
 GCNTargetMachine::createPostMachineScheduler(MachineSchedContext *C) const {
+  Attribute PostRADirectionAttr =
+      C->MF->getFunction().getFnAttribute("misched-postra-direction");
+
+  if (PostRADirectionAttr.isValid()) {
+    StringRef PostRADirectionStr = PostRADirectionAttr.getValueAsString();
+    if (PostRADirectionStr == "topdown")
+      PostRADirection = MISched::TopDown;
+    else if (PostRADirectionStr == "bottomup")
+      PostRADirection = MISched::BottomUp;
+    else if (PostRADirectionStr == "bidirectional")
+      PostRADirection = MISched::Bidirectional;
+    else
+      report_fatal_error(
+          Twine("invalid value for 'misched-postra-direction' attribute: ") +
+          PostRADirectionStr);
+  }
+
   ScheduleDAGMI *DAG =
       new GCNPostScheduleDAGMILive(C, std::make_unique<PostGenericScheduler>(C),
                                    /*RemoveKillFlags=*/true);

[arsenm]

Missing tests and I thought there already was an attribute for this

[shiltian]

I thought there already was an attribute for this

I wonder what the reason of having a function attribute for this

[harrisonGPU]

I wonder what the reason of having a function attribute for this

We use function attributes here because in graphics shader compiler, performance tuning often needs to be done per shader stage—not across the whole pipeline.

[harrisonGPU]

Missing tests and I thought there already was an attribute for this

No, we don’t have this attribute. I’ve run into this issue, the existing option is a static one, so it applies to the entire pipeline, not to individual shader stages. Also, I’m not sure how to write a proper lit test for this , what exactly should the test check?