RegAllocGreedy: Add dummy priority advisor for writing MIR tests (#121207)

I regularly struggle reproducing failures in greedy due to changes
in priority when resuming the allocation from MIR vs. a complete
compilation starting at IR. That is, the fix in
e0919b1 did not really fix the
problem of the instruction distance mattering.

Add a way to bypass all of the priority heuristics for MIR tests,
by prioritizing only by virtual register number. Could also
give this a more specific name, like PrioritizeLowVirtRegNumber

Author: arsenm

llvm/lib/CodeGen/RegAllocGreedy.cpp

@@ -376,6 +376,12 @@ unsigned DefaultPriorityAdvisor::getPriority(const LiveInterval &LI) const {
   return Prio;
 }
 
+unsigned DummyPriorityAdvisor::getPriority(const LiveInterval &LI) const {
+  // Prioritize by virtual register number, lowest first.
+  Register Reg = LI.reg();
+  return ~Reg.virtRegIndex();
+}
+
 const LiveInterval *RAGreedy::dequeue() { return dequeue(Queue); }
 
 const LiveInterval *RAGreedy::dequeue(PQueue &CurQueue) {

llvm/lib/CodeGen/RegAllocPriorityAdvisor.cpp

@@ -30,7 +30,10 @@ static cl::opt<RegAllocPriorityAdvisorAnalysis::AdvisorMode> Mode(
         clEnumValN(RegAllocPriorityAdvisorAnalysis::AdvisorMode::Release,
                    "release", "precompiled"),
         clEnumValN(RegAllocPriorityAdvisorAnalysis::AdvisorMode::Development,
-                   "development", "for training")));
+                   "development", "for training"),
+        clEnumValN(
+            RegAllocPriorityAdvisorAnalysis::AdvisorMode::Dummy, "dummy",
+            "prioritize low virtual register numbers for test and debug")));
 
 char RegAllocPriorityAdvisorAnalysis::ID = 0;
 INITIALIZE_PASS(RegAllocPriorityAdvisorAnalysis, "regalloc-priority",
@@ -67,6 +70,31 @@ class DefaultPriorityAdvisorAnalysis final
   }
   const bool NotAsRequested;
 };
+
+class DummyPriorityAdvisorAnalysis final
+    : public RegAllocPriorityAdvisorAnalysis {
+public:
+  DummyPriorityAdvisorAnalysis()
+      : RegAllocPriorityAdvisorAnalysis(AdvisorMode::Dummy) {}
+
+  // support for isa<> and dyn_cast.
+  static bool classof(const RegAllocPriorityAdvisorAnalysis *R) {
+    return R->getAdvisorMode() == AdvisorMode::Dummy;
+  }
+
+private:
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<SlotIndexesWrapperPass>();
+    RegAllocPriorityAdvisorAnalysis::getAnalysisUsage(AU);
+  }
+
+  std::unique_ptr<RegAllocPriorityAdvisor>
+  getAdvisor(const MachineFunction &MF, const RAGreedy &RA) override {
+    return std::make_unique<DummyPriorityAdvisor>(
+        MF, RA, &getAnalysis<SlotIndexesWrapperPass>().getSI());
+  }
+};
+
 } // namespace
 
 template <> Pass *llvm::callDefaultCtor<RegAllocPriorityAdvisorAnalysis>() {
@@ -75,6 +103,9 @@ template <> Pass *llvm::callDefaultCtor<RegAllocPriorityAdvisorAnalysis>() {
   case RegAllocPriorityAdvisorAnalysis::AdvisorMode::Default:
     Ret = new DefaultPriorityAdvisorAnalysis(/*NotAsRequested*/ false);
     break;
+  case RegAllocPriorityAdvisorAnalysis::AdvisorMode::Dummy:
+    Ret = new DummyPriorityAdvisorAnalysis();
+    break;
   case RegAllocPriorityAdvisorAnalysis::AdvisorMode::Development:
 #if defined(LLVM_HAVE_TFLITE)
     Ret = createDevelopmentModePriorityAdvisor();
@@ -97,6 +128,8 @@ StringRef RegAllocPriorityAdvisorAnalysis::getPassName() const {
     return "Release mode Regalloc Priority Advisor";
   case AdvisorMode::Development:
     return "Development mode Regalloc Priority Advisor";
+  case AdvisorMode::Dummy:
+    return "Dummy Regalloc Priority Advisor";
   }
   llvm_unreachable("Unknown advisor kind");
 }

llvm/lib/CodeGen/RegAllocPriorityAdvisor.h

@@ -56,9 +56,21 @@ class DefaultPriorityAdvisor : public RegAllocPriorityAdvisor {
   unsigned getPriority(const LiveInterval &LI) const override;
 };
 
+/// Stupid priority advisor which just enqueues in virtual register number
+/// order, for debug purposes only.
+class DummyPriorityAdvisor : public RegAllocPriorityAdvisor {
+public:
+  DummyPriorityAdvisor(const MachineFunction &MF, const RAGreedy &RA,
+                       SlotIndexes *const Indexes)
+      : RegAllocPriorityAdvisor(MF, RA, Indexes) {}
+
+private:
+  unsigned getPriority(const LiveInterval &LI) const override;
+};
+
 class RegAllocPriorityAdvisorAnalysis : public ImmutablePass {
 public:
-  enum class AdvisorMode : int { Default, Release, Development };
+  enum class AdvisorMode : int { Default, Release, Development, Dummy };
 
   RegAllocPriorityAdvisorAnalysis(AdvisorMode Mode)
       : ImmutablePass(ID), Mode(Mode){};

llvm/test/CodeGen/AMDGPU/dummy-regalloc-priority-advisor.mir

@@ -0,0 +1,54 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -start-before=greedy,2 -stress-regalloc=4 -stop-after=virtregrewriter,2 -regalloc-enable-priority-advisor=default -o - %s | FileCheck -check-prefixes=CHECK,DEFAULT %s
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -start-before=greedy,2 -stress-regalloc=4 -stop-after=virtregrewriter,2 -regalloc-enable-priority-advisor=dummy -o - %s | FileCheck -check-prefixes=CHECK,DUMMY %s
+
+# Check that the regalloc-enable-priority-advisor=dummy option works
+# and the result is different from the default. Ordinarily %1 would be
+# prioritized higher than %0 due to the register class priority
+
+---
+name:            foo
+tracksRegLiveness: true
+machineFunctionInfo:
+  scratchRSrcReg:  '$sgpr0_sgpr1_sgpr2_sgpr3'
+  frameOffsetReg:  '$sgpr33'
+  stackPtrOffsetReg: '$sgpr32'
+registers:
+  - { id: 0, class: vgpr_32 }
+  - { id: 1, class: vreg_128 }
+  - { id: 2, class: vgpr_32 }
+body:             |
+  bb.0:
+    liveins: $vgpr0, $vgpr1
+
+    ; DEFAULT-LABEL: name: foo
+    ; DEFAULT: liveins: $vgpr0, $vgpr1
+    ; DEFAULT-NEXT: {{  $}}
+    ; DEFAULT-NEXT: SI_SPILL_V128_SAVE $vgpr1_vgpr2_vgpr3_vgpr4, %stack.0, $sgpr32, 0, implicit $exec :: (store (s128) into %stack.0, align 4, addrspace 5)
+    ; DEFAULT-NEXT: SI_SPILL_V32_SAVE $vgpr0, %stack.1, $sgpr32, 0, implicit $exec :: (store (s32) into %stack.1, addrspace 5)
+    ; DEFAULT-NEXT: S_NOP 0, implicit-def $vgpr0_vgpr1_vgpr2_vgpr3
+    ; DEFAULT-NEXT: renamable $vgpr2_vgpr3_vgpr4_vgpr5 = SI_SPILL_V128_RESTORE %stack.0, $sgpr32, 0, implicit $exec :: (load (s128) from %stack.0, align 4, addrspace 5)
+    ; DEFAULT-NEXT: renamable $vgpr3 = SI_SPILL_V32_RESTORE %stack.1, $sgpr32, 0, implicit $exec :: (load (s32) from %stack.1, addrspace 5)
+    ; DEFAULT-NEXT: renamable $vgpr3 = V_ADD_U32_e32 killed $vgpr2, killed $vgpr3, implicit $exec
+    ; DEFAULT-NEXT: SI_RETURN implicit $vgpr3, implicit $vgpr0, implicit $vgpr1
+    ;
+    ; DUMMY-LABEL: name: foo
+    ; DUMMY: liveins: $vgpr0, $vgpr1
+    ; DUMMY-NEXT: {{  $}}
+    ; DUMMY-NEXT: SI_SPILL_V128_SAVE $vgpr1_vgpr2_vgpr3_vgpr4, %stack.1, $sgpr32, 0, implicit $exec :: (store (s128) into %stack.1, align 4, addrspace 5)
+    ; DUMMY-NEXT: SI_SPILL_V32_SAVE $vgpr0, %stack.0, $sgpr32, 0, implicit $exec :: (store (s32) into %stack.0, addrspace 5)
+    ; DUMMY-NEXT: S_NOP 0, implicit-def $vgpr0_vgpr1_vgpr2_vgpr3
+    ; DUMMY-NEXT: renamable $vgpr2 = SI_SPILL_V32_RESTORE %stack.0, $sgpr32, 0, implicit $exec :: (load (s32) from %stack.0, addrspace 5)
+    ; DUMMY-NEXT: renamable $vgpr3_vgpr4_vgpr5_vgpr6 = SI_SPILL_V128_RESTORE %stack.1, $sgpr32, 0, implicit $exec :: (load (s128) from %stack.1, align 4, addrspace 5)
+    ; DUMMY-NEXT: renamable $vgpr3 = V_ADD_U32_e32 killed $vgpr3, killed $vgpr2, implicit $exec
+    ; DUMMY-NEXT: SI_RETURN implicit $vgpr3, implicit $vgpr0, implicit $vgpr1
+    undef %1.sub0:vreg_128 = COPY $vgpr1
+    %0:vgpr_32 = COPY $vgpr0
+    S_NOP 0, implicit-def $vgpr0_vgpr1_vgpr2_vgpr3
+    %2:vgpr_32 = V_ADD_U32_e32 %1.sub0, %0, implicit $exec
+    $vgpr3 = COPY %2
+    SI_RETURN implicit $vgpr3, implicit $vgpr0, implicit $vgpr1
+
+...
+
+# CHECK: {{.*}}