[llvm] Spill/restore FP/BP around instructions in which they are clobbered (PR #81048)

[Eli Friedman via llvm-commits]

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@@ -4178,3 +4178,158 @@ void X86FrameLowering::restoreWinEHStackPointersInParent(
                                   /*RestoreSP=*/IsSEH);
   }
 }
+
+static int computeSPAdjust4SpillFPBP(MachineFunction &MF,
+                                     const TargetRegisterClass *RC,
+                                     unsigned SpillRegNum) {
+  const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
+  unsigned AllocSize = TRI->getSpillSize(*RC) * SpillRegNum;
+  Align StackAlign = MF.getFrameInfo().getStackAlignment();
+  unsigned AlignedSize = alignTo(AllocSize, StackAlign);
+  return AlignedSize - AllocSize;
+}
+
+void X86FrameLowering::spillFPBPUsingSP(MachineFunction &MF,
+                                        MachineBasicBlock::iterator BeforeMI,
+                                        bool SpillFP, bool SpillBP) const {
+  const TargetRegisterClass *RC;
+  unsigned RegNum = 0;
+  MachineBasicBlock *MBB = BeforeMI->getParent();
+  DebugLoc DL = BeforeMI->getDebugLoc();
+
+  // Spill FP.
+  if (SpillFP) {
+    Register FP = TRI->getFrameRegister(MF);
+    if (STI.isTarget64BitILP32())
+      FP = Register(getX86SubSuperRegister(FP, 64));
+    RC = TRI->getMinimalPhysRegClass(FP);
+    ++RegNum;
+
+    BuildMI(*MBB, BeforeMI, DL,
+            TII.get(getPUSHOpcode(MF.getSubtarget<X86Subtarget>())))
+        .addReg(FP);
+  }
+
+  // Spill BP.
+  if (SpillBP) {
+    Register BP = TRI->getBaseRegister(MF);
+    if (STI.isTarget64BitILP32())
+      BP = Register(getX86SubSuperRegister(BP, 64));
+    RC = TRI->getMinimalPhysRegClass(BP);
+    ++RegNum;
+
+    BuildMI(*MBB, BeforeMI, DL,
+            TII.get(getPUSHOpcode(MF.getSubtarget<X86Subtarget>())))
+        .addReg(BP);
+  }
+
+  // Make sure SP is aligned.
+  int SPAdjust = computeSPAdjust4SpillFPBP(MF, RC, RegNum);
+  if (SPAdjust)
+    emitSPUpdate(*MBB, BeforeMI, DL, -SPAdjust, false);
+
+  // Emit unwinding information.
+  if (SpillFP && needsDwarfCFI(MF)) {
+    // Emit .cfi_remember_state to remember old frame.
+    unsigned CFIIndex =
+        MF.addFrameInst(MCCFIInstruction::createRememberState(nullptr));
+    BuildMI(*MBB, BeforeMI, DL, TII.get(TargetOpcode::CFI_INSTRUCTION))
+        .addCFIIndex(CFIIndex);
+
+    // Setup new CFA value with DW_CFA_def_cfa_expression:
----------------
efriedma-quic wrote:

If you're spilling the base pointer, you shouldn't need to touch the unwind: we normally restore the stack pointer from fp, anyway.

If you're spilling the frame pointer, you'd have to do something complicated to get correct unwind; probably represent it as a separate function (.seh_endproc/.seh_startproc/etc.).  You can't specify unwind opcodes outside the prolog.

https://github.com/llvm/llvm-project/pull/81048