commit 68945a8686708c6e4c74b9aeb231b8c4e1731181
Author: Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed Feb 10 10:45:12 2021 -0800

    [Thumb2] support `movs pc, lr` alias for `subs pc, lr, #0`/`eret`
    
    This is used by the Linux kernel built with CONFIG_THUMB2_KERNEL.
    
    Because different operands are not permitted to `movs`, the diagnostics now provide multiple suggestions along the lines of using a non-pc destination operand or lr source operand.
    
    Forked from D95586.
    
    Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
    
    Reviewed By: DavidSpickett
    
    Differential Revision: https://reviews.llvm.org/D96304

diff --git a/llvm/lib/Target/ARM/ARMInstrThumb2.td b/llvm/lib/Target/ARM/ARMInstrThumb2.td
index 5642cab32e7c..b70d66869b06 100644
--- a/llvm/lib/Target/ARM/ARMInstrThumb2.td
+++ b/llvm/lib/Target/ARM/ARMInstrThumb2.td
@@ -4145,6 +4145,18 @@ def t2SUBS_PC_LR : T2I <(outs), (ins imm0_255:$imm), NoItinerary,
   let Inst{7-0} = imm;
 }
 
+// B9.3.19 SUBS PC, LR (Thumb)
+// In the Thumb instruction set, MOVS{<c>}{<q>} PC, LR is a pseudo-instruction
+// for SUBS{<c>}{<q>} PC, LR, #0.
+def : t2InstAlias<"movs${p}\tpc, lr", (t2SUBS_PC_LR 0, pred:$p)>;
+def : t2InstAlias<"movs${p}.w\tpc, lr", (t2SUBS_PC_LR 0, pred:$p)>;
+
+// ERET - Return from exception in Hypervisor mode.
+// B9.3.3, B9.3.20: ERET is an alias for "SUBS PC, LR, #0" in an implementation that
+// includes virtualization extensions.
+def t2ERET : InstAlias<"eret${p}", (t2SUBS_PC_LR 0, pred:$p), 1>,
+             Requires<[IsThumb2, HasVirtualization]>;
+
 // Hypervisor Call is a system instruction.
 let isCall = 1 in {
 def t2HVC : T2XI <(outs), (ins imm0_65535:$imm16), IIC_Br, "hvc.w\t$imm16", []>,
@@ -4160,12 +4172,6 @@ def t2HVC : T2XI <(outs), (ins imm0_65535:$imm16), IIC_Br, "hvc.w\t$imm16", []>,
 // Alias for HVC without the ".w" optional width specifier
 def : t2InstAlias<"hvc\t$imm16", (t2HVC imm0_65535:$imm16)>;
 
-// ERET - Return from exception in Hypervisor mode.
-// B9.3.3, B9.3.20: ERET is an alias for "SUBS PC, LR, #0" in an implementation that
-// includes virtualization extensions.
-def t2ERET : InstAlias<"eret${p}", (t2SUBS_PC_LR 0, pred:$p), 1>,
-             Requires<[IsThumb2, HasVirtualization]>;
-
 //===----------------------------------------------------------------------===//
 // Non-Instruction Patterns
 //
diff --git a/llvm/test/MC/ARM/lsl-zero-errors.s b/llvm/test/MC/ARM/lsl-zero-errors.s
index ad39470a4a50..e021aa9eb986 100644
--- a/llvm/test/MC/ARM/lsl-zero-errors.s
+++ b/llvm/test/MC/ARM/lsl-zero-errors.s
@@ -72,9 +72,11 @@
 // CHECK-THUMBV7: note: operand must be a register in range [r0, r12] or r14
 // CHECK-THUMBV8: note: operand must be a register in range [r0, r14]
 
-// CHECK-THUMBV7: error: operand must be a register in range [r0, r12] or r14
-// CHECK-THUMBV8: error: operand must be a register in range [r0, r14]
+// CHECK-NONARM: error: invalid instruction, any one of the following would fix this:
 // CHECK-NONARM-NEXT: movs pc, r0, lsl #0
+// CHECK-THUMBV7: note: operand must be a register in range [r0, r12] or r14
+// CHECK-THUMBV8: note: operand must be a register in range [r0, r14]
+// CHECK-NONARM: note: invalid operand for instruction
 
 // CHECK-NONARM: error: invalid instruction, any one of the following would fix this:
 // CHECK-NONARM-NEXT: movs r0, pc, lsl #0
@@ -82,9 +84,11 @@
 // CHECK-NONARM: note: invalid operand for instruction
 // CHECK-NONARM: note: invalid operand for instruction
 
-// CHECK-THUMBV7: error: operand must be a register in range [r0, r12] or r14
-// CHECK-THUMBV8: error: operand must be a register in range [r0, r14]
+// CHECK-NONARM: error: invalid instruction, any one of the following would fix this:
 // CHECK-NONARM-NEXT: movs pc, pc, lsl #0
+// CHECK-THUMBV7: note: operand must be a register in range [r0, r12] or r14
+// CHECK-THUMBV8: note: operand must be a register in range [r0, r14]
+// CHECK-NONARM: note: invalid operand for instruction
 
 // CHECK-ARM: mov pc, r0                @ encoding: [0x00,0xf0,0xa0,0xe1]
 // CHECK-ARM: mov r0, pc                @ encoding: [0x0f,0x00,0xa0,0xe1]
diff --git a/llvm/test/MC/ARM/thumb-mov.s b/llvm/test/MC/ARM/thumb-mov.s
index 5ceb0082dddc..6f662f3ee2c7 100644
--- a/llvm/test/MC/ARM/thumb-mov.s
+++ b/llvm/test/MC/ARM/thumb-mov.s
@@ -13,13 +13,15 @@
         movs pc, r0
         movs r0, pc
         movs pc, pc
-// CHECK: error: operand must be a register in range [r0, r14]
+// CHECK: error: invalid instruction, any one of the following would fix this:
 // CHECK-NEXT: movs pc, r0
 // CHECK: note: operand must be a register in range [r0, r14]
-// CHECK-NEXT: movs r0, pc
 // CHECK: note: invalid operand for instruction
+// CHECK: error: invalid instruction, any one of the following would fix this:
 // CHECK-NEXT: movs r0, pc
-// CHECK: error: invalid instruction
+// CHECK: note: operand must be a register in range [r0, r14]
+// CHECK: note: invalid operand for instruction
+// CHECK: error: invalid operand for instruction
 // CHECK-NEXT: movs pc, pc
 
         // mov.w selects t2MOVr
@@ -39,13 +41,15 @@
         movs.w pc, r0
         movs.w r0, pc
         movs.w pc, pc
-// CHECK: error: operand must be a register in range [r0, r14]
+// CHECK: error: invalid instruction, any one of the following would fix this:
 // CHECK-NEXT: movs.w pc, r0
 // CHECK: note: operand must be a register in range [r0, r14]
-// CHECK-NEXT: movs.w r0, pc
 // CHECK: note: invalid operand for instruction
+// CHECK: error: invalid instruction, any one of the following would fix this:
 // CHECK-NEXT: movs.w r0, pc
-// CHECK: error: invalid instruction
+// CHECK: note: operand must be a register in range [r0, r14]
+// CHECK: note: invalid operand for instruction
+// CHECK: error: invalid operand for instruction
 // CHECK-NEXT: movs.w pc, pc
 
 
@@ -104,3 +108,11 @@
         mov.w r0, sp
 // CHECK: mov.w sp, r0                @ encoding: [0x4f,0xea,0x00,0x0d]
 // CHECK: mov.w r0, sp                @ encoding: [0x4f,0xea,0x0d,0x00]
+
+        // `movs pc, lr` is an alias for `subs pc, lr, #0`/`eret`.
+        movs   pc, lr
+        movs.w pc, lr
+// CHECK-V7: subs pc, lr, #0             @ encoding: [0xde,0xf3,0x00,0x8f]
+// CHECK-V7: subs pc, lr, #0             @ encoding: [0xde,0xf3,0x00,0x8f]
+// CHECK-V8: eret                        @ encoding: [0xde,0xf3,0x00,0x8f]
+// CHECK-V8: eret                        @ encoding: [0xde,0xf3,0x00,0x8f]