[AArch64] Allow PAC mnemonics in the HINT space with PAC disabled

[pbarrio]

Summary

It is important to emit HINT instructions instead of PAC ones when
PAC is disabled. This allows compatibility with other assemblers
(e.g. GAS). This was implemented in commit da33762de853.

Still, developers of assembly code will want to write code that is
compatible with both pre- and post-PAC CPUs. They could use HINT
mnemonics, but the new mnemonics are a lot more readable (e.g.
paciaz instead of hint #24), and they will result in the same
encodings. So, while LLVM should not *emit* the new mnemonics when
PAC is disabled, this patch will at least make LLVM *accept*
assembly code that uses them.

[ostannard]

It would be nice to also print the mnemonic as a comment on the hint instruction, as these will commonly show up in compiled code. It looks like AArch64InstPrinter has access to a CommentStream which could be used for this.

That said, this patch LGTM on its own, the above is just a nice-to-have which could be done later.

[pbarrio]

Rebase to trigger C.I.

[pbarrio]

Another rebase to pass C.I. - looks like it was broken outside this commit

llvm/lib/Target/AArch64/AArch64InstrInfo.td

let isAuthenticated = 1 in {
     def AUTIB1716  : SystemNoOperands<0b110, "hint\t#14">;
   }
 }
 
 let Uses = [LR], Defs = [LR], CRm = 0b0000 in {
   def XPACLRI   : SystemNoOperands<0b111, "hint\t#7">;
 }
 
+// In order to be able to write readable assembly, LLVM should accept assembly
+// inputs that use pointer authentication mnemonics, even with PA disabled.
+// However, in order to be compatible with other assemblers (e.g. GAS), LLVM
+// should not emit these mnemonics unless PA is enabled.
+def : InstAlias<"paciaz", (PACIAZ), 0>;
+def : InstAlias<"pacibz", (PACIBZ), 0>;
+def : InstAlias<"autiaz", (AUTIAZ), 0>;
+def : InstAlias<"autibz", (AUTIBZ), 0>;
+def : InstAlias<"paciasp", (PACIASP), 0>;
+def : InstAlias<"pacibsp", (PACIBSP), 0>;
+def : InstAlias<"autiasp", (AUTIASP), 0>;
+def : InstAlias<"autibsp", (AUTIBSP), 0>;
+def : InstAlias<"pacia1716", (PACIA1716), 0>;
+def : InstAlias<"pacib1716", (PACIB1716), 0>;
+def : InstAlias<"autia1716", (AUTIA1716), 0>;
+def : InstAlias<"autib1716", (AUTIB1716), 0>;
+def : InstAlias<"xpaclri", (XPACLRI), 0>;
+
 // These pointer authentication instructions require armv8.3a
 let Predicates = [HasPA] in {
 
-  // When compiling with PA, there is a better mnemonic for these instructions.
+  // When PA is enabled, a better mnemonic should be emitted.
   def : InstAlias<"paciaz", (PACIAZ), 1>;
   def : InstAlias<"pacibz", (PACIBZ), 1>;
   def : InstAlias<"autiaz", (AUTIAZ), 1>;
   def : InstAlias<"autibz", (AUTIBZ), 1>;
   def : InstAlias<"paciasp", (PACIASP), 1>;
   def : InstAlias<"pacibsp", (PACIBSP), 1>;
   def : InstAlias<"autiasp", (AUTIASP), 1>;
   def : InstAlias<"autibsp", (AUTIBSP), 1>;

llvm/test/MC/AArch64/armv8.3a-signed-pointer.s

 // RUN: llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=+v8.3a -o - %s 2>&1 | \
 // RUN: FileCheck --check-prefixes=CHECK,ALL %s
 
 // RUN: not llvm-mc -triple aarch64-none-linux-gnu -show-encoding %s -o - > %t.1 2>%t.2
-// RUN: FileCheck --check-prefixes=ALL %s < %t.1
+// RUN: FileCheck --check-prefixes=NO83,ALL %s < %t.1
 // RUN: FileCheck --check-prefix=CHECK-REQ %s < %t.2
 
 // ALL: .text
   mrs x0, apiakeylo_el1
   mrs x0, apiakeyhi_el1
   mrs x0, apibkeylo_el1
   mrs x0, apibkeyhi_el1
   mrs x0, apdakeylo_el1

 // CHECK-REQ-NEXT:  msr apgakeylo_el1, x0
 // CHECK-REQ: error: expected writable system register or pstate
 // CHECK-REQ-NEXT:  msr apgakeyhi_el1, x0
 
 // ALL-EMPTY:
 // ALL-EMPTY:
   paciasp
 // CHECK-NEXT: paciasp        // encoding: [0x3f,0x23,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: paciasp
+// NO83-NEXT: hint #25        // encoding: [0x3f,0x23,0x03,0xd5]
   autiasp
 // CHECK-NEXT: autiasp        // encoding: [0xbf,0x23,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: autiasp
+// NO83-NEXT: hint #29        // encoding: [0xbf,0x23,0x03,0xd5]
   paciaz
 // CHECK-NEXT: paciaz         // encoding: [0x1f,0x23,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: paciaz
+// NO83-NEXT: hint #24        // encoding: [0x1f,0x23,0x03,0xd5]
   autiaz
 // CHECK-NEXT: autiaz         // encoding: [0x9f,0x23,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: autiaz
+// NO83-NEXT: hint #28        // encoding: [0x9f,0x23,0x03,0xd5]
   pacia1716
 // CHECK-NEXT: pacia1716      // encoding: [0x1f,0x21,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: pacia1716
+// NO83-NEXT: hint #8         // encoding: [0x1f,0x21,0x03,0xd5]
   autia1716
 // CHECK-NEXT: autia1716      // encoding: [0x9f,0x21,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: autia1716
+// NO83-NEXT: hint #12        // encoding: [0x9f,0x21,0x03,0xd5]
   pacibsp
 // CHECK-NEXT: pacibsp        // encoding: [0x7f,0x23,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: pacibsp
+// NO83-NEXT: hint #27        // encoding: [0x7f,0x23,0x03,0xd5]
   autibsp
 // CHECK-NEXT: autibsp        // encoding: [0xff,0x23,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: autibsp
+// NO83-NEXT: hint #31        // encoding: [0xff,0x23,0x03,0xd5]
   pacibz
 // CHECK-NEXT: pacibz         // encoding: [0x5f,0x23,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: pacibz
+// NO83-NEXT: hint #26        // encoding: [0x5f,0x23,0x03,0xd5]
   autibz
 // CHECK-NEXT: autibz         // encoding: [0xdf,0x23,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: autibz
+// NO83-NEXT: hint #30        // encoding: [0xdf,0x23,0x03,0xd5]
   pacib1716
 // CHECK-NEXT: pacib1716      // encoding: [0x5f,0x21,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: pacib1716
+// NO83-NEXT: hint #10        // encoding: [0x5f,0x21,0x03,0xd5]
   autib1716
 // CHECK-NEXT: autib1716      // encoding: [0xdf,0x21,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: autib1716
+// NO83-NEXT: hint #14        // encoding: [0xdf,0x21,0x03,0xd5]
   xpaclri
 // CHECK-NEXT: xpaclri        // encoding: [0xff,0x20,0x03,0xd5]
-// CHECK-REQ: error: instruction requires: pa
-// CHECK-REQ-NEXT: xpaclri
+// NO83-NEXT: hint #7         // encoding: [0xff,0x20,0x03,0xd5]
 
 // ALL-EMPTY:
   pacia x0, x1
 // CHECK-NEXT: pacia x0, x1     // encoding: [0x20,0x00,0xc1,0xda]
 // CHECK-REQ-NEXT:      ^
 // CHECK-REQ-NEXT: error: instruction requires: pa
 // CHECK-REQ-NEXT: pacia x0, x1
   autia x0, x1