[PATCH] D139877: [RISCV]{InsertVSETVLI] Reverse order of block traversal in post pass [nfc]

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added reviewers: craig.topper, frasercrmck, kito-cheng, pcwang-thead.
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This unblocks a following change to be more sophisticated during post pass rewriting.

Review wise, I basically just want a second set of eyes.  This change should be straight forward, but since it took me an embarrassing number of attempts to get make check to pass.  Let's make sure I'm not missing yet another cornercase.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D139877

Files:
  llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp


Index: llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
+++ llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
@@ -1220,34 +1220,38 @@
 }
 
 void RISCVInsertVSETVLI::doLocalPostpass(MachineBasicBlock &MBB) {
-  MachineInstr *PrevMI = nullptr;
+  MachineInstr *NextMI = nullptr;
+  // We can have arbitrary code in successors, so VL and VTYPE
+  // must be considered demanded.
   DemandedFields Used;
+  Used.VL = true;
+  Used.demandVTYPE();
   SmallVector<MachineInstr*> ToDelete;
-  for (MachineInstr &MI : MBB) {
-    // Note: Must be *before* vsetvli handling to account for config cases
-    // which only change some subfields.
-    doUnion(Used, getDemanded(MI));
+  for (MachineInstr &MI : iterator_range(MBB.rbegin(), MBB.rend())) {
 
-    if (!isVectorConfigInstr(MI))
+    if (!isVectorConfigInstr(MI)) {
+      doUnion(Used, getDemanded(MI));
       continue;
+    }
+
+    Register VRegDef = MI.getOperand(0).getReg();
+    if (VRegDef != RISCV::X0 &&
+        !(VRegDef.isVirtual() && MRI->use_nodbg_empty(VRegDef)))
+      Used.VL = true;
 
-    if (PrevMI) {
+    if (NextMI) {
       if (!Used.VL && !Used.usedVTYPE()) {
-        ToDelete.push_back(PrevMI);
-        // fallthrough
-      } else if (canMutatePriorConfig(*PrevMI, MI, Used)) {
-        PrevMI->getOperand(2).setImm(MI.getOperand(2).getImm());
         ToDelete.push_back(&MI);
-        // Leave PrevMI unchanged
+        // Leave NextMI unchanged
         continue;
+      } else if (canMutatePriorConfig(MI, *NextMI, Used)) {
+        MI.getOperand(2).setImm(NextMI->getOperand(2).getImm());
+        ToDelete.push_back(NextMI);
+        // fallthrough
       }
     }
-    PrevMI = &MI;
+    NextMI = &MI;
     Used = getDemanded(MI);
-    Register VRegDef = MI.getOperand(0).getReg();
-    if (VRegDef != RISCV::X0 &&
-        !(VRegDef.isVirtual() && MRI->use_nodbg_empty(VRegDef)))
-      Used.VL = true;
   }
 
   for (auto *MI : ToDelete)


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