[PATCH] D88775: [AMDGPU] SIInsertSkips: Refactor early exit block creation

[Carl Ritson via Phabricator via llvm-commits]

critson created this revision.
critson added reviewers: nhaehnle, arsenm.
Herald added subscribers: llvm-commits, kerbowa, hiraditya, t-tye, tpr, dstuttard, yaxunl, jvesely, kzhuravl.
Herald added a project: LLVM.
critson requested review of this revision.
Herald added a subscriber: wdng.

Refactor exit block creation to a single call ensureEarlyExitBlock.
Add support for generating an early exit block which clears the
exec mask, but only add this instruction when required.
These changes are to facilitate adding more forms of early
termination for PS shaders in the near future.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D88775

Files:
  llvm/lib/Target/AMDGPU/SIInsertSkips.cpp


Index: llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
===================================================================
--- llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
+++ llvm/lib/Target/AMDGPU/SIInsertSkips.cpp
@@ -58,12 +58,13 @@
   MachineDominatorTree *MDT = nullptr;
 
   MachineBasicBlock *EarlyExitBlock = nullptr;
+  bool EarlyExitClearsExec = false;
 
   bool shouldSkip(const MachineBasicBlock &From,
                   const MachineBasicBlock &To) const;
 
   bool dominatesAllReachable(MachineBasicBlock &MBB);
-  void createEarlyExitBlock(MachineBasicBlock &MBB);
+  void ensureEarlyExitBlock(MachineBasicBlock &MBB, bool ClearExec);
   void skipIfDead(MachineBasicBlock &MBB, MachineBasicBlock::iterator I,
                   DebugLoc DL);
 
@@ -180,15 +181,26 @@
   BuildMI(MBB, I, DL, TII->get(AMDGPU::S_ENDPGM)).addImm(0);
 }
 
-void SIInsertSkips::createEarlyExitBlock(MachineBasicBlock &MBB) {
+void SIInsertSkips::ensureEarlyExitBlock(MachineBasicBlock &MBB,
+                                         bool ClearExec) {
   MachineFunction *MF = MBB.getParent();
   DebugLoc DL;
 
-  assert(!EarlyExitBlock);
-  EarlyExitBlock = MF->CreateMachineBasicBlock();
-  MF->insert(MF->end(), EarlyExitBlock);
+  if (!EarlyExitBlock) {
+    EarlyExitBlock = MF->CreateMachineBasicBlock();
+    MF->insert(MF->end(), EarlyExitBlock);
+    generatePsEndPgm(*EarlyExitBlock, EarlyExitBlock->end(), DL, TII);
+  }
 
-  generatePsEndPgm(*EarlyExitBlock, EarlyExitBlock->end(), DL, TII);
+  if (ClearExec && !EarlyExitClearsExec) {
+    const GCNSubtarget &ST = MF->getSubtarget<GCNSubtarget>();
+    unsigned Mov = ST.isWave32() ? AMDGPU::S_MOV_B32 : AMDGPU::S_MOV_B64;
+    Register Exec = ST.isWave32() ? AMDGPU::EXEC_LO : AMDGPU::EXEC;
+    auto ExitI = EarlyExitBlock->getFirstNonPHI();
+    assert(ExitI->getOpcode() == AMDGPU::EXP_DONE);
+    BuildMI(*EarlyExitBlock, ExitI, DL, TII->get(Mov), Exec).addImm(0);
+    EarlyExitClearsExec = true;
+  }
 }
 
 static void splitBlock(MachineBasicBlock &MBB, MachineInstr &MI,
@@ -233,11 +245,9 @@
   if (NoSuccessor) {
     generatePsEndPgm(MBB, I, DL, TII);
   } else {
-    if (!EarlyExitBlock) {
-      createEarlyExitBlock(MBB);
-      // Update next block pointer to reflect any new blocks
-      NextBBI = std::next(MBB.getIterator());
-    }
+    ensureEarlyExitBlock(MBB, false);
+    // Update next block pointer to reflect any new blocks
+    NextBBI = std::next(MBB.getIterator());
 
     MachineInstr *BranchMI =
         BuildMI(MBB, I, DL, TII->get(AMDGPU::S_CBRANCH_EXECZ))


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