AMDGPU: Track physical registers in SIWholeQuadMode

[nhaehnle]

Summary

There are cases where uniform branch conditions are computed in VGPRs, and
we didn't correctly mark those as WQM.

The stray change in basic-branch.ll is because invoking the LiveIntervals
analysis leads to the detection of a dead register that would otherwise not
be seen at -O0.

This is a candidate for the 3.9 branch, as it fixes a possible hang.

[arsenm]

lib/Target/AMDGPU/SIWholeQuadMode.cpp

132

Should this also preserve it? Recomputing it if not used is quite expensive. I've also seen crashes when not preserving it (although I'm not 100% sure if that counts as a bug somewhere else)

[nhaehnle]

lib/Target/AMDGPU/SIWholeQuadMode.cpp

132

As-is, this runs in SSA before Phi elimination, and other passes don't preserve LiveIntervals anyway.

I did it this way because I wanted to have a fix that uses the same logic in the 3.9 release branch as well as after my series of changes that moves SIWholeQuadMode to after machine scheduling.

If you're worried about the recomputation, we have two options, both of them a bit riskier:
(1) pull the entire WQM series into the 3.9 release branch
(2) replace the use of LiveIntervals in this patch by a manual backwards scan through MachineInstrs (or are there some helper functions to do this for us? I'm not aware, since the passes that need this info usually rely on LiveIntervals or similar).

Honestly, I'd prefer to accept the recomputation and have a conservative fix for the 3.9 release branch. On trunk, the recomputation will soon be gone anyway.

[arsenm]

LGTM

lib/Target/AMDGPU/SIWholeQuadMode.cpp

struct WorkItem {
   WorkItem(MachineInstr *MI) : MI(MI) {}
 };
 
 class SIWholeQuadMode : public MachineFunctionPass {
 private:
   const SIInstrInfo *TII;
   const SIRegisterInfo *TRI;
   MachineRegisterInfo *MRI;
+  LiveIntervals *LIS;
 
   DenseMap<const MachineInstr *, InstrInfo> Instructions;
   DenseMap<MachineBasicBlock *, BlockInfo> Blocks;
   SmallVector<const MachineInstr *, 2> ExecExports;
   SmallVector<MachineInstr *, 1> LiveMaskQueries;
 
+  void markInstruction(MachineInstr &MI, char Flag,
+                       std::vector<WorkItem> &Worklist);
   char scanInstructions(MachineFunction &MF, std::vector<WorkItem> &Worklist);
   void propagateInstruction(MachineInstr &MI, std::vector<WorkItem> &Worklist);
   void propagateBlock(MachineBasicBlock &MBB, std::vector<WorkItem> &Worklist);
   char analyzeFunction(MachineFunction &MF);
 
   void toExact(MachineBasicBlock &MBB, MachineBasicBlock::iterator Before,
                unsigned SaveWQM, unsigned LiveMaskReg);
   void toWQM(MachineBasicBlock &MBB, MachineBasicBlock::iterator Before,
public:
 
   bool runOnMachineFunction(MachineFunction &MF) override;
 
   const char *getPassName() const override {
     return "SI Whole Quad Mode";
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<LiveIntervals>();
     AU.setPreservesCFG();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 };
 
 } // End anonymous namespace
 
 char SIWholeQuadMode::ID = 0;
 
-INITIALIZE_PASS(SIWholeQuadMode, DEBUG_TYPE,
-                "SI Whole Quad Mode", false, false)
+INITIALIZE_PASS_BEGIN(SIWholeQuadMode, DEBUG_TYPE, "SI Whole Quad Mode", false,
+                      false)
+INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
+INITIALIZE_PASS_END(SIWholeQuadMode, DEBUG_TYPE, "SI Whole Quad Mode", false,
+                    false)
 
 char &llvm::SIWholeQuadModeID = SIWholeQuadMode::ID;
 
 FunctionPass *llvm::createSIWholeQuadModePass() {
   return new SIWholeQuadMode;
 }
 
+void SIWholeQuadMode::markInstruction(MachineInstr &MI, char Flag,
+                                      std::vector<WorkItem> &Worklist) {
+  InstrInfo &II = Instructions[&MI];
+
+  assert(Flag == StateWQM || Flag == StateExact);
+
+  // Ignore if the instruction is already marked. The typical case is that we
+  // mark an instruction WQM multiple times, but for atomics it can happen that
+  // Flag is StateWQM, but Needs is already set to StateExact. In this case,
+  // letting the atomic run in StateExact is correct as per the relevant specs.
+  if (II.Needs)
+    return;
+
+  II.Needs = Flag;
+  Worklist.push_back(&MI);
+}
+
 // Scan instructions to determine which ones require an Exact execmask and
 // which ones seed WQM requirements.
 char SIWholeQuadMode::scanInstructions(MachineFunction &MF,
                                        std::vector<WorkItem> &Worklist) {
   char GlobalFlags = 0;
   bool WQMOutputs = MF.getFunction()->hasFnAttribute("amdgpu-ps-wqm-outputs");
 
   for (auto BI = MF.begin(), BE = MF.end(); BI != BE; ++BI) {
for (auto II = MBB.begin(), IE = MBB.end(); II != IE; ++II) {
             }
           }
         }
 
         if (!Flags)
           continue;
       }
 
-      Instructions[&MI].Needs = Flags;
-      Worklist.push_back(&MI);
+      markInstruction(MI, Flags, Worklist);
       GlobalFlags |= Flags;
     }
 
     if (WQMOutputs && MBB.succ_empty()) {
       // This is a prolog shader. Make sure we go back to exact mode at the end.
       Blocks[&MBB].OutNeeds = StateExact;
       Worklist.push_back(&MBB);
       GlobalFlags |= StateExact;
void SIWholeQuadMode::propagateInstruction(MachineInstr &MI,
   assert(II.Needs != (StateWQM | StateExact));
   if (II.Needs != StateWQM)
     return;
 
   for (const MachineOperand &Use : MI.uses()) {
     if (!Use.isReg() || !Use.isUse())
       continue;
 
-    // At this point, physical registers appear as inputs or outputs
-    // and following them makes no sense (and would in fact be incorrect
-    // when the same VGPR is used as both an output and an input that leads
-    // to a NeedsWQM instruction).
-    //
-    // Note: VCC appears e.g. in 64-bit addition with carry - theoretically we
-    // have to trace this, in practice it happens for 64-bit computations like
-    // pointers where both dwords are followed already anyway.
-    if (!TargetRegisterInfo::isVirtualRegister(Use.getReg()))
+    unsigned Reg = Use.getReg();
+
+    // Handle physical registers that we need to track; this is mostly relevant
+    // for VCC, which can appear as the (implicit) input of a uniform branch,
+    // e.g. when a loop counter is stored in a VGPR.
+    if (!TargetRegisterInfo::isVirtualRegister(Reg)) {
+      if (Reg == AMDGPU::EXEC)
         continue;
 
-    for (MachineInstr &DefMI : MRI->def_instructions(Use.getReg())) {
-      InstrInfo &DefII = Instructions[&DefMI];
+      for (MCRegUnitIterator RegUnit(Reg, TRI); RegUnit.isValid(); ++RegUnit) {
+        LiveRange &LR = LIS->getRegUnit(*RegUnit);
+        const VNInfo *Value = LR.Query(LIS->getInstructionIndex(MI)).valueIn();
+        if (!Value)
+          continue;
 
-      // Obviously skip if DefMI is already flagged as NeedWQM.
-      //
-      // The instruction might also be flagged as NeedExact. This happens when
-      // the result of an atomic is used in a WQM computation. In this case,
-      // the atomic must not run for helper pixels and the WQM result is
-      // undefined.
-      if (DefII.Needs != 0)
+        // Since we're in machine SSA, we do not need to track physical
+        // registers across basic blocks.
+        if (Value->isPHIDef())
           continue;
 
-      DefII.Needs = StateWQM;
-      Worklist.push_back(&DefMI);
+        markInstruction(*LIS->getInstructionFromIndex(Value->def), StateWQM,
+                        Worklist);
       }
+
+      continue;
+    }
+
+    for (MachineInstr &DefMI : MRI->def_instructions(Use.getReg()))
+      markInstruction(DefMI, StateWQM, Worklist);
   }
 }
 
 void SIWholeQuadMode::propagateBlock(MachineBasicBlock &MBB,
                                      std::vector<WorkItem>& Worklist) {
   BlockInfo BI = Blocks[&MBB]; // Make a copy to prevent dangling references.
 
   // Propagate through instructions
bool SIWholeQuadMode::runOnMachineFunction(MachineFunction &MF) {
   ExecExports.clear();
   LiveMaskQueries.clear();
 
   const SISubtarget &ST = MF.getSubtarget<SISubtarget>();
 
   TII = ST.getInstrInfo();
   TRI = &TII->getRegisterInfo();
   MRI = &MF.getRegInfo();
+  LIS = &getAnalysis<LiveIntervals>();
 
   char GlobalFlags = analyzeFunction(MF);
   if (!(GlobalFlags & StateWQM)) {
     lowerLiveMaskQueries(AMDGPU::EXEC);
     return !LiveMaskQueries.empty();
   }
 
   // Store a copy of the original live mask when required

test/CodeGen/AMDGPU/basic-branch.ll

 ; RUN: llc -O0 -march=amdgcn -verify-machineinstrs < %s | FileCheck -check-prefix=GCNNOOPT -check-prefix=GCN %s
 ; RUN: llc -O0 -march=amdgcn -mcpu=tonga -verify-machineinstrs < %s | FileCheck -check-prefix=GCNNOOPT -check-prefix=GCN %s
 ; RUN: llc -march=amdgcn -verify-machineinstrs < %s | FileCheck -check-prefix=GCNOPT -check-prefix=GCN %s
 ; RUN: llc -march=amdgcn -mcpu=tonga -verify-machineinstrs < %s | FileCheck -check-prefix=GCNOPT -check-prefix=GCN %s
 
 ; GCN-LABEL: {{^}}test_branch:
 ; GCNNOOPT: v_writelane_b32
 ; GCNNOOPT: v_writelane_b32
-; GCNNOOPT: v_writelane_b32
 ; GCN: s_cbranch_scc1 [[END:BB[0-9]+_[0-9]+]]
 
 ; GCN: ; BB#1
 ; GCNNOOPT: v_readlane_b32
 ; GCNNOOPT: v_readlane_b32
 ; GCN: buffer_store_dword
 ; GCN: s_endpgm
 

test/CodeGen/AMDGPU/wqm.ll

 ; CHECK: v_add_f32_e32 v0,
 ; CHECK: s_and_b64 exec, exec, [[ORIG]]
 define amdgpu_ps float @test_prolog_1(float %a, float %b) #4 {
 main_body:
   %s = fadd float %a, %b
   ret float %s
 }
 
+; CHECK-LABEL: {{^}}test_loop_vcc:
+; CHECK-NEXT: ; %entry
+; CHECK-NEXT: s_mov_b64 [[LIVE:s\[[0-9]+:[0-9]+\]]], exec
+; CHECK: s_wqm_b64 exec, exec
+; CHECK: s_and_b64 exec, exec, [[LIVE]]
+; CHECK: image_store
+; CHECK: s_wqm_b64 exec, exec
+; CHECK: v_mov_b32_e32 [[CTR:v[0-9]+]], -2
+; CHECK: s_branch [[LOOPHDR:BB[0-9]+_[0-9]+]]
+
+; CHECK: [[LOOPHDR]]: ; %loop
+; CHECK: v_add_i32_e32 [[CTR]], vcc, 2, [[CTR]]
+; CHECK: v_cmp_lt_i32_e32 vcc, 7, [[CTR]]
+; CHECK: s_cbranch_vccz
+; CHECK: ; %break
+
+; CHECK: ; return
+define amdgpu_ps <4 x float> @test_loop_vcc(<4 x float> %in) nounwind {
+entry:
+  call void @llvm.amdgcn.image.store.v4i32(<4 x float> %in, <4 x i32> undef, <8 x i32> undef, i32 15, i1 0, i1 0, i1 0, i1 0)
+  br label %loop
+
+loop:
+  %ctr.iv = phi i32 [ 0, %entry ], [ %ctr.next, %body ]
+  %c.iv = phi <4 x float> [ %in, %entry ], [ %c.next, %body ]
+  %cc = icmp sgt i32 %ctr.iv, 7
+  br i1 %cc, label %break, label %body
+
+body:
+  %c.i = bitcast <4 x float> %c.iv to <4 x i32>
+  %c.next = call <4 x float> @llvm.SI.image.sample.v4i32(<4 x i32> %c.i, <8 x i32> undef, <4 x i32> undef, i32 15, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0)
+  %ctr.next = add i32 %ctr.iv, 2
+  br label %loop
+
+break:
+  ret <4 x float> %c.iv
+}
+
 declare void @llvm.amdgcn.image.store.v4i32(<4 x float>, <4 x i32>, <8 x i32>, i32, i1, i1, i1, i1) #1
 
 declare <4 x float> @llvm.amdgcn.image.load.v4i32(<4 x i32>, <8 x i32>, i32, i1, i1, i1, i1) #2
 
 declare <4 x float> @llvm.SI.image.sample.i32(i32, <8 x i32>, <4 x i32>, i32, i32, i32, i32, i32, i32, i32, i32) #3
 declare <4 x float> @llvm.SI.image.sample.v4i32(<4 x i32>, <8 x i32>, <4 x i32>, i32, i32, i32, i32, i32, i32, i32, i32) #3
 
 declare void @llvm.AMDGPU.kill(float)
 declare void @llvm.SI.export(i32, i32, i32, i32, i32, float, float, float, float)
 
 attributes #1 = { nounwind }
 attributes #2 = { nounwind readonly }
 attributes #3 = { nounwind readnone }
 attributes #4 = { "amdgpu-ps-wqm-outputs" }