[AMDGPU] Support function attribute to override postRA scheduling direction (#147708)

This patch adds support for controlling the post-RA machine scheduler
direction
(topdown, bottomup, bidirectional) on a per-function basis using the 
"amdgpu-post-ra-direction" function attribute.

Author: harrisonGPU

llvm/lib/Target/AMDGPU/GCNSubtarget.cpp

@@ -340,6 +340,43 @@ void GCNSubtarget::overrideSchedPolicy(MachineSchedPolicy &Policy,
     Policy.ShouldTrackLaneMasks = true;
 }
 
+void GCNSubtarget::overridePostRASchedPolicy(MachineSchedPolicy &Policy,
+                                             const SchedRegion &Region) const {
+  const Function &F = Region.RegionBegin->getMF()->getFunction();
+  Attribute PostRADirectionAttr = F.getFnAttribute("amdgpu-post-ra-direction");
+  if (!PostRADirectionAttr.isValid())
+    return;
+
+  StringRef PostRADirectionStr = PostRADirectionAttr.getValueAsString();
+  if (PostRADirectionStr == "topdown") {
+    Policy.OnlyTopDown = true;
+    Policy.OnlyBottomUp = false;
+  } else if (PostRADirectionStr == "bottomup") {
+    Policy.OnlyTopDown = false;
+    Policy.OnlyBottomUp = true;
+  } else if (PostRADirectionStr == "bidirectional") {
+    Policy.OnlyTopDown = false;
+    Policy.OnlyBottomUp = false;
+  } else {
+    DiagnosticInfoOptimizationFailure Diag(
+        F, F.getSubprogram(), "invalid value for postRA direction attribute");
+    F.getContext().diagnose(Diag);
+  }
+
+  LLVM_DEBUG({
+    const char *DirStr = "default";
+    if (Policy.OnlyTopDown && !Policy.OnlyBottomUp)
+      DirStr = "topdown";
+    else if (!Policy.OnlyTopDown && Policy.OnlyBottomUp)
+      DirStr = "bottomup";
+    else if (!Policy.OnlyTopDown && !Policy.OnlyBottomUp)
+      DirStr = "bidirectional";
+
+    dbgs() << "Post-MI-sched direction (" << F.getName() << "): " << DirStr
+           << '\n';
+  });
+}
+
 void GCNSubtarget::mirFileLoaded(MachineFunction &MF) const {
   if (isWave32()) {
     // Fix implicit $vcc operands after MIParser has verified that they match

llvm/lib/Target/AMDGPU/GCNSubtarget.h

@@ -1041,6 +1041,9 @@ class GCNSubtarget final : public AMDGPUGenSubtargetInfo,
   void overrideSchedPolicy(MachineSchedPolicy &Policy,
                            const SchedRegion &Region) const override;
 
+  void overridePostRASchedPolicy(MachineSchedPolicy &Policy,
+                                 const SchedRegion &Region) const override;
+
   void mirFileLoaded(MachineFunction &MF) const override;
 
   unsigned getMaxNumUserSGPRs() const {

llvm/test/CodeGen/AMDGPU/postra-sched-attribute.ll

@@ -0,0 +1,34 @@
+; REQUIRES: asserts
+
+; RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx1100 -debug-only=gcn-subtarget < %s 2>&1 | FileCheck %s
+; RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx1100 < %s 2>&1 | FileCheck -check-prefixes=WARNING %s
+
+; CHECK: Post-MI-sched direction (postra-sched-topdown): topdown
+define float @postra-sched-topdown(float %input) nounwind #0 {
+  %x = fadd float %input, 1.000000e+00
+  ret float %x
+}
+
+; CHECK: Post-MI-sched direction (postra-sched-bottomup): bottomup
+define float @postra-sched-bottomup(float %input) nounwind #1 {
+  %x = fsub float %input, 1.000000e+00
+  ret float %x
+}
+
+; CHECK: Post-MI-sched direction (postra-sched-bidirectional): bidirectional
+define float @postra-sched-bidirectional(float %input) nounwind #2 {
+  %x = fadd float %input, 1.000000e+00
+  ret float %x
+}
+
+; CHECK: Post-MI-sched direction (postra-sched-warning): topdown
+; WARNING: invalid value for postRA direction attribute
+define float @postra-sched-warning(float %input) nounwind #3 {
+  %x = fsub float %input, 1.000000e+00
+  ret float %x
+}
+
+attributes #0 = {"amdgpu-post-ra-direction"="topdown"}
+attributes #1 = {"amdgpu-post-ra-direction"="bottomup"}
+attributes #2 = {"amdgpu-post-ra-direction"="bidirectional"}
+attributes #3 = {"amdgpu-post-ra-direction"="warning"}