[clang] [llvm] [BPF] introduce `__attribute__((bpf_fastcall))` (PR #101228)

[via llvm-commits]

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@@ -596,6 +600,90 @@ bool BPFMIPreEmitPeephole::adjustBranch() {
   return Changed;
 }
 
+static const unsigned CallerSavedRegs[] = {BPF::R0, BPF::R1, BPF::R2,
+                                           BPF::R3, BPF::R4, BPF::R5};
+
+struct BPFFastCall {
+  MachineInstr *MI;
+  unsigned LiveCallerSavedRegs;
+};
+
+static void collectBPFFastCalls(const TargetRegisterInfo *TRI,
+                                LivePhysRegs &LiveRegs, MachineBasicBlock &BB,
+                                SmallVectorImpl<BPFFastCall> &Calls) {
+  LiveRegs.init(*TRI);
+  LiveRegs.addLiveOuts(BB);
+  Calls.clear();
+  for (MachineInstr &MI : llvm::reverse(BB)) {
+    unsigned LiveCallerSavedRegs;
+    if (!MI.isCall())
+      goto NextInsn;
+    LiveCallerSavedRegs = 0;
+    for (MCRegister R : CallerSavedRegs) {
+      bool DoSpillFill = !MI.definesRegister(R, TRI) && LiveRegs.contains(R);
+      if (!DoSpillFill)
+        continue;
+      LiveCallerSavedRegs |= 1 << R;
+    }
+    if (LiveCallerSavedRegs)
+      Calls.push_back({&MI, LiveCallerSavedRegs});
+  NextInsn:
+    LiveRegs.stepBackward(MI);
+  }
+}
+
+static int64_t computeMinFixedObjOffset(MachineFrameInfo &MFI,
+                                        unsigned SlotSize) {
+  int64_t MinFixedObjOffset = 0;
+  // Same logic as in X86FrameLowering::adjustFrameForMsvcCxxEh()
+  for (int I = MFI.getObjectIndexBegin(); I < MFI.getObjectIndexEnd(); ++I) {
+    if (MFI.isDeadObjectIndex(I))
+      continue;
+    MinFixedObjOffset = std::min(MinFixedObjOffset, MFI.getObjectOffset(I));
+  }
+  MinFixedObjOffset -=
+      (SlotSize + MinFixedObjOffset % SlotSize) & (SlotSize - 1);
+  return MinFixedObjOffset;
+}
+
+bool BPFMIPreEmitPeephole::insertMissingCallerSavedSpills() {
+  MachineFrameInfo &MFI = MF->getFrameInfo();
+  SmallVector<BPFFastCall, 8> Calls;
+  LivePhysRegs LiveRegs;
+  const unsigned SlotSize = 8;
+  int64_t MinFixedObjOffset = computeMinFixedObjOffset(MFI, SlotSize);
+  bool Changed = false;
+  for (MachineBasicBlock &BB : *MF) {
+    collectBPFFastCalls(TRI, LiveRegs, BB, Calls);
+    Changed |= !Calls.empty();
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yonghong-song wrote:

Calls.empty() does not mean changed unless there are actual fill/spill, right?

https://github.com/llvm/llvm-project/pull/101228