[AArch64][GlobalISel] Make LR livein to entry in llvm.returnaddress selection

[paquette]

Summary

This fixes a couple verifier failures on this bot:

http://green.lab.llvm.org/green/job/test-suite-verify-machineinstrs-aarch64-globalisel-O0-g/

The failures show up in eeprof-1.c and pr17377.c in the GCC C Torture Suite.

Specifically:

*** Bad machine code: MBB has allocatable live-in, but isn't entry or landing-pad. ***
- function:    foo
- basic block: %bb.3 if.end (0x7fac7106dfc8)
- p. register: $lr

and

*** Bad machine code: Using an undefined physical register ***
- function:    f
- basic block: %bb.1 entry (0x7f8941092588)
- instruction: %18:gpr64 = COPY $lr
- operand 1:   $lr

Unlike SDAG, we were setting LR as a live in to the block containing the returnaddress.

Also, this ensures that we don't add LR as a livein to the entry block twice.

In MachineBasicBlock.h there's a comment saying

Note that it is an error to add the same register to the same set more than once unless the intention is to call sortUniqueLiveIns after all registers are added.

so it's probably good to avoid adding LR twice.

Surprisingly the verifier doesn't complain about that. Maybe it should.

[aemerson]

LGTM.

llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp

case Intrinsic::returnaddress: {
     if (Depth == 0 && IntrinID == Intrinsic::returnaddress) {
       if (MFReturnAddr) {
         MIRBuilder.buildCopy({DstReg}, MFReturnAddr);
         I.eraseFromParent();
         return true;
       }
       MFI.setReturnAddressIsTaken(true);
       MF.addLiveIn(AArch64::LR, &AArch64::GPR64spRegClass);
-      I.getParent()->addLiveIn(AArch64::LR);
       // Insert the copy from LR/X30 into the entry block, before it can be
       // clobbered by anything.
+      MachineBasicBlock &EntryBlock = *MF.begin();
+      if (!EntryBlock.isLiveIn(AArch64::LR))
+        EntryBlock.addLiveIn(AArch64::LR);
       MachineIRBuilder EntryBuilder(MF);
-      EntryBuilder.setInstr(*MF.begin()->begin());
+      EntryBuilder.setInstr(*EntryBlock.begin());
       EntryBuilder.buildCopy({DstReg}, {Register(AArch64::LR)});
       MFReturnAddr = DstReg;
       I.eraseFromParent();
       return true;
     }
 
     MFI.setFrameAddressIsTaken(true);
     Register FrameAddr(AArch64::FP);

llvm/test/CodeGen/AArch64/GlobalISel/select-returnaddress-liveins.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64 -global-isel -run-pass=instruction-select -verify-machineinstrs %s -o - | FileCheck %s
+
+--- |
+  define void @lr_other_block() { ret void }
+  define void @already_live_in() { ret void }
+  declare i8* @llvm.returnaddress(i32 immarg)
+...
+---
+name:            lr_other_block
+alignment:       4
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: lr_other_block
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK:   liveins: $w0, $x0, $lr
+  ; CHECK:   [[COPY:%[0-9]+]]:gpr64 = COPY $lr
+  ; CHECK:   B %bb.1
+  ; CHECK: bb.1:
+  ; CHECK:   $x0 = COPY [[COPY]]
+  ; CHECK:   RET_ReallyLR implicit $x0
+  ; LR should be added as a livein to the entry block.
+
+  bb.0:
+    ; We should have lr as a livein to bb.0, and a copy from LR.
+    liveins: $w0, $x0
+    G_BR %bb.1
+  bb.1:
+    %4:gpr(p0) = G_INTRINSIC intrinsic(@llvm.returnaddress), 0
+    $x0 = COPY %4
+    RET_ReallyLR implicit $x0
+...
+---
+name:            already_live_in
+alignment:       4
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: already_live_in
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK:   liveins: $w0, $x0, $lr
+  ; CHECK:   [[COPY:%[0-9]+]]:gpr64 = COPY $lr
+  ; CHECK:   B %bb.1
+  ; CHECK: bb.1:
+  ; CHECK:   $x0 = COPY [[COPY]]
+  ; CHECK:   RET_ReallyLR implicit $x0
+  ; We should not have LR listed as a livein twice.
+
+  bb.0:
+    liveins: $w0, $x0, $lr
+    G_BR %bb.1
+  bb.1:
+    %4:gpr(p0) = G_INTRINSIC intrinsic(@llvm.returnaddress), 0
+    $x0 = COPY %4
+    RET_ReallyLR implicit $x0
+...