Tail calls: look through AssertZExt to find register copy.

[t.p.northover]

Summary

arm64_32 guarantees the high 32 bits of pointer parameters are passed as 0, and this is modelled in the IR by inserting an AssertZExt after the copyFromReg. The function deciding whether registers that need to be preserved actually are wasn't expecting this so it banned perfectly legitimate tail calls.

[Gerolf]

This is a straightforward fix. LGTM.

[t.p.northover]

Thanks. Looks like I pushed it a while back (6c85668d28), which is weird.

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

for (unsigned I = 0, E = ArgLocs.size(); I != E; ++I) {
     MCRegister Reg = ArgLoc.getLocReg();
     // Only look at callee saved registers.
     if (MachineOperand::clobbersPhysReg(CallerPreservedMask, Reg))
       continue;
     // Check that we pass the value used for the caller.
     // (We look for a CopyFromReg reading a virtual register that is used
     //  for the function live-in value of register Reg)
     SDValue Value = OutVals[I];
+    if (Value->getOpcode() == ISD::AssertZext)
+      Value = Value.getOperand(0);
     if (Value->getOpcode() != ISD::CopyFromReg)
       return false;
     Register ArgReg = cast<RegisterSDNode>(Value->getOperand(1))->getReg();
     if (MRI.getLiveInPhysReg(ArgReg) != Reg)
       return false;
   }
   return true;
 }

llvm/test/CodeGen/AArch64/swiftself.ll

define void @swiftself_passthrough(i8* swiftself %addr0) {
   ret void
 }
 
 ; We can use a tail call if the callee swiftself is the same as the caller one.
 ; This should also work with fast-isel.
 ; CHECK-LABEL: swiftself_tail:
 ; OPTAARCH64: b {{_?}}swiftself_param
 ; OPTAARCH64-NOT: ret
-; OPTARM64_32: bl {{_?}}swiftself_param
+; OPTARM64_32: b {{_?}}swiftself_param
 define i8* @swiftself_tail(i8* swiftself %addr0) {
   call void asm sideeffect "", "~{x20}"()
-  %res = tail call i8* @swiftself_param(i8* swiftself %addr0)
+  %res = musttail call i8* @swiftself_param(i8* swiftself %addr0)
   ret i8* %res
 }
 
 ; We can not use a tail call if the callee swiftself is not the same as the
 ; caller one.
 ; CHECK-LABEL: swiftself_notail:
 ; CHECK: mov x20, x0
 ; CHECK: bl {{_?}}swiftself_param