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

Summary:
It is important to emit HINT instructions instead of BTI  ones when
BTI is disabled. This allows compatibility with other assemblers
(e.g. GAS).

Still, developers of assembly code will want to write code that is
compatible with both pre- and post-BTI CPUs. They could use HINT
mnemonics, but the new mnemonics are a lot more readable (e.g.
bti c instead of hint rust-lang#34), and they will result in the same
encodings. So, while LLVM should not *emit* the new mnemonics when
BTI is disabled, this patch will at least make LLVM *accept*
assembly code that uses them.

Reviewers: pbarrio, tamas.petz, ostannard

Reviewed By: pbarrio, ostannard

Subscribers: ostannard, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81257

llvm/lib/Target/AArch64/AArch64InstrInfo.td

@@ -726,6 +726,12 @@ def : InstAlias<"sevl", (HINT 0b101)>;
 def : InstAlias<"dgh",  (HINT 0b110)>;
 def : InstAlias<"esb",  (HINT 0b10000)>, Requires<[HasRAS]>;
 def : InstAlias<"csdb", (HINT 20)>;
+// In order to be able to write readable assembly, LLVM should accept assembly
+// inputs that use Branch Target Indentification mnemonics, even with BTI disabled.
+// However, in order to be compatible with other assemblers (e.g. GAS), LLVM
+// should not emit these mnemonics unless BTI is enabled.
+def : InstAlias<"bti",  (HINT 32), 0>;
+def : InstAlias<"bti $op", (HINT btihint_op:$op), 0>;
 def : InstAlias<"bti",  (HINT 32)>, Requires<[HasBTI]>;
 def : InstAlias<"bti $op", (HINT btihint_op:$op)>, Requires<[HasBTI]>;
 

llvm/test/MC/AArch64/armv8.5a-bti.s

@@ -1,6 +1,6 @@
-// RUN:     llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=+bti   < %s      | FileCheck %s
-// RUN:     llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=+v8.5a < %s      | FileCheck %s
-// RUN: not llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=-bti   < %s 2>&1 | FileCheck %s --check-prefix=NOBTI
+// RUN: llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=+bti   < %s | FileCheck %s
+// RUN: llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=+v8.5a < %s | FileCheck %s
+// RUN: llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=-bti   < %s | FileCheck %s --check-prefix=NOBTI
 
 bti
 bti c
@@ -12,14 +12,10 @@ bti jc
 // CHECK: bti j    // encoding: [0x9f,0x24,0x03,0xd5]
 // CHECK: bti jc   // encoding: [0xdf,0x24,0x03,0xd5]
 
-// NOBTI:      instruction requires: bti
-// NOBTI-NEXT: bti
-// NOBTI:      instruction requires: bti
-// NOBTI-NEXT: bti
-// NOBTI:      instruction requires: bti
-// NOBTI-NEXT: bti
-// NOBTI:      instruction requires: bti
-// NOBTI-NEXT: bti
+// NOBTI: hint #32 // encoding: [0x1f,0x24,0x03,0xd5]
+// NOBTI: hint #34 // encoding: [0x5f,0x24,0x03,0xd5]
+// NOBTI: hint #36 // encoding: [0x9f,0x24,0x03,0xd5]
+// NOBTI: hint #38 // encoding: [0xdf,0x24,0x03,0xd5]
 
 hint #32
 hint #34