[llvm] [llvm][ARM] Correct the properties of trap instructions (PR #113287)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-arm

Author: David Spickett (DavidSpickett)

<details>
<summary>Changes</summary>

Fixes #<!-- -->113154

The encodings used for llvm.trap() on ARM were all marked as barriers and terminators. This lead to stack frame destroy code being inserted before the trap if the trap was the last thing in the function and it had no return statement.
```
void fn() {
  volatile int i = 0;
  __builtin_trap();
}
```
Produced:
```
fn:
        push    {r11, lr}   << stack frame create
<...>
        mov     sp, r11
        pop     {r11, lr}   << stack frame destroy
        .inst   0xe7ffdefe  << trap
        bx      lr
```
All the other targets don't mark them this way, instead they mark them with isTrap. I've changed ARM to do this, which fixes the code generation:
```
fn:
        push    {r11, lr}   << stack frame create
<...>
        .inst   0xe7ffdefe  << trap
        mov     sp, r11
        pop     {r11, lr}   << stack frame destroy
        bx      lr
```
I've updated the existing trap test to force the need for a stack frame, then check that the instruction immediately after the trap is resetting the stack pointer.

---
Full diff: https://github.com/llvm/llvm-project/pull/113287.diff


3 Files Affected:

- (modified) llvm/lib/Target/ARM/ARMInstrInfo.td (+2-2) 
- (modified) llvm/lib/Target/ARM/ARMInstrThumb.td (+1-1) 
- (modified) llvm/test/CodeGen/ARM/trap.ll (+35-14) 


``````````diff
diff --git a/llvm/lib/Target/ARM/ARMInstrInfo.td b/llvm/lib/Target/ARM/ARMInstrInfo.td
index ed68c6ff20cde5..d24d4af36f0d86 100644
--- a/llvm/lib/Target/ARM/ARMInstrInfo.td
+++ b/llvm/lib/Target/ARM/ARMInstrInfo.td
@@ -2378,13 +2378,13 @@ def UDF : AInoP<(outs), (ins imm0_65535:$imm16), MiscFrm, NoItinerary,
  *  - In ARM: UDF #60896;
  *  - In Thumb: UDF #254 followed by a branch-to-self.
  */
-let isBarrier = 1, isTerminator = 1 in
+let isTrap = 1 in
 def TRAPNaCl : AXI<(outs), (ins), MiscFrm, NoItinerary,
                "trap", [(trap)]>,
            Requires<[IsARM,UseNaClTrap]> {
   let Inst = 0xe7fedef0;
 }
-let isBarrier = 1, isTerminator = 1 in
+let isTrap = 1 in
 def TRAP : AXI<(outs), (ins), MiscFrm, NoItinerary,
                "trap", [(trap)]>,
            Requires<[IsARM,DontUseNaClTrap]> {
diff --git a/llvm/lib/Target/ARM/ARMInstrThumb.td b/llvm/lib/Target/ARM/ARMInstrThumb.td
index 2ad78f8cd8d4c7..b92f42874bbddb 100644
--- a/llvm/lib/Target/ARM/ARMInstrThumb.td
+++ b/llvm/lib/Target/ARM/ARMInstrThumb.td
@@ -675,7 +675,7 @@ def tSVC : T1pI<(outs), (ins imm0_255:$imm), IIC_Br,
 }
 
 // The assembler uses 0xDEFE for a trap instruction.
-let isBarrier = 1, isTerminator = 1 in
+let isTrap = 1 in
 def tTRAP : TI<(outs), (ins), IIC_Br,
                "trap", [(trap)]>, Encoding16, Sched<[WriteBr]> {
   let Inst = 0xdefe;
diff --git a/llvm/test/CodeGen/ARM/trap.ll b/llvm/test/CodeGen/ARM/trap.ll
index fe046e70166553..a5d2b2081f28f3 100644
--- a/llvm/test/CodeGen/ARM/trap.ll
+++ b/llvm/test/CodeGen/ARM/trap.ll
@@ -1,3 +1,6 @@
+;; Check encodings of trap instructions and that their properties are set
+;; correctly so that they are not placed after the stack frame is destroyed.
+
 ; RUN: llc < %s -mtriple=arm-apple-darwin | FileCheck %s -check-prefix=DARWIN
 ; RUN: llc < %s -mtriple=arm-apple-darwin -trap-func=_trap | FileCheck %s -check-prefix=FUNC
 ; RUN: llc < %s -mtriple=arm-apple-darwin -trap-func=_trap -O0 | FileCheck %s -check-prefix=FUNC
@@ -29,22 +32,31 @@
 ; rdar://7961298
 ; rdar://9249183
 
-define void @t() nounwind {
+define void @t() noinline optnone {
 entry:
+  ;; So that we have a stack frame.
+  %1 = alloca i32, align 4
+  store volatile i32 0, ptr %1, align 4
+
 ; DARWIN-LABEL: t:
-; DARWIN: trap
+; DARWIN:      trap
+; DARWIN-NEXT: add sp, sp, #4
 
 ; FUNC-LABEL: t:
-; FUNC: bl __trap
+; FUNC:      bl __trap
+; FUNC-NEXT: add sp, sp, #4
 
 ; NACL-LABEL: t:
-; NACL: .inst 0xe7fedef0
+; NACL:      .inst 0xe7fedef0
+; NACL-NEXT: add sp, sp, #4
 
 ; ARM-LABEL: t:
-; ARM: .inst 0xe7ffdefe
+; ARM:      .inst 0xe7ffdefe
+; ARM-NEXT: add sp, sp, #4
 
 ; THUMB-LABEL: t:
-; THUMB: .inst.n 0xdefe
+; THUMB:      .inst.n 0xdefe
+; THUMB-NEXT: add sp, #4
 
 ; ENCODING-NACL: e7fedef0    trap
 
@@ -53,25 +65,34 @@ entry:
 ; ENCODING-THUMB: defe  trap
 
   call void @llvm.trap()
-  unreachable
+  ret void
 }
 
-define void @t2() nounwind {
+define void @t2() {
 entry:
+  ;; So that we have a stack frame.
+  %1 = alloca i32, align 4
+  store volatile i32 0, ptr %1, align 4
+
 ; DARWIN-LABEL: t2:
-; DARWIN: udf #254
+; DARWIN:      udf #254
+; DARWIN-NEXT: add sp, sp, #4
 
 ; FUNC-LABEL: t2:
-; FUNC: bl __trap
+; FUNC:      bl __trap
+; FUNC-NEXT: add sp, sp, #4
 
 ; NACL-LABEL: t2:
-; NACL: bkpt #0
+; NACL:      bkpt #0
+; NACL-NEXT: add sp, sp, #4
 
 ; ARM-LABEL: t2:
-; ARM: bkpt #0
+; ARM:      bkpt #0
+; ARM-NEXT: add sp, sp, #4
 
 ; THUMB-LABEL: t2:
-; THUMB: bkpt #0
+; THUMB:      bkpt #0
+; THUMB-NEXT: add sp, #4
 
 ; ENCODING-NACL: e1200070    bkpt #0
 
@@ -80,7 +101,7 @@ entry:
 ; ENCODING-THUMB: be00  bkpt #0
 
   call void @llvm.debugtrap()
-  unreachable
+  ret void
 }
 
 declare void @llvm.trap() nounwind

``````````

</details>


https://github.com/llvm/llvm-project/pull/113287