[AArch64] PAUTH_PROLOGUE should not be duplicated with PAuthLR (llvm#124775)

When using PAuthLR, the PAUTH_PROLOGUE expands into a sequence of
instructions which takes the address of one of those instructions, and
uses that address to compute the return address signature. If this is
duplicated, there will be two different addresses used in calculating
the signature, so the epilogue will only be correct for (at most) one of
them.

This change also restricts code generation when using v8.3-A return
address signing, without PAuthLR. This isn't strictly needed, as
duplicating the prologue there would be valid. We could fix this by
having two copies of PAUTH_PROLOGUE, with and without isNotDuplicable,
but I don't think it's worth adding the extra complexity to a security
feature for that.

Author: ostannard

llvm/lib/Target/AArch64/AArch64InstrInfo.td

@@ -1773,7 +1773,12 @@ def : InstAlias<"xpaclri", (XPACLRI), 0>;
 
 let Uses = [LR, SP], Defs = [LR] in {
 // Insertion point of LR signing code.
-def PAUTH_PROLOGUE : Pseudo<(outs), (ins), []>, Sched<[]>;
+def PAUTH_PROLOGUE : Pseudo<(outs), (ins), []>, Sched<[]> {
+  // When using PAuthLR, the address of one of the instructions this expands
+  // into is used as an input to the signature calculation, so this must not be
+  // duplicated.
+  let isNotDuplicable = 1;
+}
 // Insertion point of LR authentication code.
 // The RET terminator of the containing machine basic block may be replaced
 // with a combined RETA(A|B) instruction when rewriting this Pseudo.

llvm/test/CodeGen/AArch64/pauthlr-prologue-duplication.mir

@@ -0,0 +1,88 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple aarch64-none-elf -run-pass=block-placement -O3 -o - %s | FileCheck %s
+
+## Check that block-placement does not perform tail duplication on the
+## PAUTH_EPILOGUE instruction. If that happened, the two prologues would use
+## different addresses while calculating the return address signature, so the
+## epilogue could only be correct for (at most) one of them.
+
+--- |
+  define void @test() "frame-pointer"="non-leaf" {
+  entry:
+    ret void
+  }
+
+  declare void @f()
+...
+---
+name:            test
+body:             |
+  ; CHECK-LABEL: name: test
+  ; CHECK: bb.0.entry:
+  ; CHECK-NEXT:   successors: %bb.1(0x30000000), %bb.2(0x50000000)
+  ; CHECK-NEXT:   liveins: $w0, $w1, $lr
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   CBZW renamable $w0, %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT:   liveins: $w0, $w1, $lr
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   B %bb.3
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT:   liveins: $w1, $lr
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   renamable $w8 = MOVZWi 1, 0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   successors: %bb.5(0x30000000), %bb.4(0x50000000)
+  ; CHECK-NEXT:   liveins: $w1, $w8, $lr
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   frame-setup PAUTH_PROLOGUE implicit-def $lr, implicit killed $lr, implicit $sp
+  ; CHECK-NEXT:   CBZW killed renamable $w1, %bb.5
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.4:
+  ; CHECK-NEXT:   successors: %bb.5(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   BL @f, csr_aarch64_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.5:
+  ; CHECK-NEXT:   BL @f, csr_aarch64_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp
+  ; CHECK-NEXT:   frame-destroy PAUTH_EPILOGUE implicit-def $lr, implicit killed $lr, implicit $sp
+  ; CHECK-NEXT:   TCRETURNdi @f, 0, csr_aarch64_aapcs, implicit $sp
+  bb.0.entry:
+    successors: %bb.1(0x30000000), %bb.2(0x50000000)
+    liveins: $w0, $w1, $lr
+
+    CBNZW renamable $w0, %bb.2
+
+  bb.1:
+    successors: %bb.3(0x80000000)
+    liveins: $w1, $lr
+
+    renamable $w8 = MOVZWi 1, 0
+    B %bb.3
+
+  bb.2:
+    successors: %bb.3(0x80000000)
+    liveins: $w0, $w1, $lr
+
+  bb.3:
+    successors: %bb.5(0x30000000), %bb.4(0x50000000)
+    liveins: $w1, $w8, $lr
+
+    frame-setup PAUTH_PROLOGUE implicit-def $lr, implicit killed $lr, implicit $sp
+    CBZW killed renamable $w1, %bb.5
+
+  bb.4:
+    successors: %bb.5(0x80000000)
+
+    BL @f, csr_aarch64_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp
+
+  bb.5:
+    BL @f, csr_aarch64_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp
+    frame-destroy PAUTH_EPILOGUE implicit-def $lr, implicit killed $lr, implicit $sp
+    TCRETURNdi @f, 0, csr_aarch64_aapcs, implicit $sp
+...