[llvm] [AMDGPU] Register allocation anti-hints to reduce MFMA hazard NOPs (PR #156943)

[Matt Arsenault via llvm-commits]

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@@ -860,6 +867,56 @@ class MachineRegisterInfo {
     return RegAllocHints.inBounds(VReg) ? &RegAllocHints[VReg] : nullptr;
   }
 
+  /// setRegAllocationAntiHint - Add a register allocation anti-hint for the
+  /// specified virtual register. This tells the allocator to avoid allocating
+  /// VReg to the same physical register as AntiHintVReg (or overlapping ones).
+  void setRegAllocationAntiHint(Register VReg, Register AntiHintVReg) {
+    assert(VReg.isVirtual() && "Anti-hints are only for virtual registers");
+    assert(AntiHintVReg.isVirtual() && "Anti-hint target must be virtual");
+    AntiHintRegs.grow(Register::index2VirtReg(getNumVirtRegs()));
+    auto &AntiHints = AntiHintRegs[VReg];
+    // Avoid duplicates
+    if (llvm::find(AntiHints, AntiHintVReg) == AntiHints.end())
+      AntiHints.push_back(AntiHintVReg);
+  }
+
+  /// addRegAllocationAntiHint - Add multiple anti-hints at once.
+  void addRegAllocationAntiHints(Register VReg,
+                                 ArrayRef<Register> AntiHintVRegs) {
+    for (Register AntiHint : AntiHintVRegs)
+      setRegAllocationAntiHint(VReg, AntiHint);
+  }
+
+  /// clearRegAllocationAntiHints - Clear all anti-hints for a register.
+  void clearRegAllocationAntiHints(Register VReg) {
+    assert(VReg.isVirtual());
+    if (AntiHintRegs.inBounds(VReg))
+      AntiHintRegs[VReg].clear();
+  }
+
+  /// getRegAllocationAntiHints - Return the vector of anti-hints for VReg.
+  ArrayRef<Register> getRegAllocationAntiHints(Register VReg) const {
+    assert(VReg.isVirtual());
+    if (!AntiHintRegs.inBounds(VReg))
+      return ArrayRef<Register>();
+    return AntiHintRegs[VReg];
+  }
+
+  /// hasRegAllocationAntiHint - Check if VReg has AntiHintVReg as an anti-hint.
+  bool hasRegAllocationAntiHint(Register VReg, Register AntiHintVReg) const {
+    assert(VReg.isVirtual() && AntiHintVReg.isVirtual());
+    if (!AntiHintRegs.inBounds(VReg))
+      return false;
+    const auto &AntiHints = AntiHintRegs[VReg];
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arsenm wrote:

NO auto 

https://github.com/llvm/llvm-project/pull/156943