[FastISel] Support unreachable with NoTrapAfterNoReturn

[nikic]

Currently FastISel triggers a fallback if there is an unreachable terminator and the TrapUnreachable option is enabled (the ISD::TRAP selection does not actually work).

Add handling for NoTrapAfterNoReturn, in which case we don't actually need to emit a trap. The test is just there to make sure there is no FastISel fallback (which is why I'm not testing the case without noreturn). We have other tests that check the actual unreachable codegen variations.

[llvmbot]

@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-llvm-selectiondag

Author: Nikita Popov (nikic)

Changes

Currently FastISel triggers a fallback if there is an unreachable terminator and the TrapUnreachable option is enabled (the ISD::TRAP selection does not actually work).

Add handling for NoTrapAfterNoReturn, in which case we don't actually need to emit a trap. The test is just there to make sure there is no FastISel fallback (which is why I'm not testing the case without noreturn). We have other tests that check the actual unreachable codegen variations.

---

Full diff: https://github.com/llvm/llvm-project/pull/118296.diff

2 Files Affected:

• (modified) llvm/lib/CodeGen/SelectionDAG/FastISel.cpp (+12-4)
• (added) llvm/test/CodeGen/X86/no-trap-after-noreturn-fastisel.ll (+13)

diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index eede879e7e80d1..d5551758c073e9 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1851,11 +1851,19 @@ bool FastISel::selectOperator(const User *I, unsigned Opcode) {
     return false;
   }
 
-  case Instruction::Unreachable:
-    if (TM.Options.TrapUnreachable)
+  case Instruction::Unreachable: {
+    if (TM.Options.TrapUnreachable) {
+      if (TM.Options.NoTrapAfterNoreturn) {
+        const auto *Call =
+            dyn_cast_or_null<CallInst>(cast<Instruction>(I)->getPrevNode());
+        if (Call && Call->doesNotReturn())
+          return true;
+      }
+
       return fastEmit_(MVT::Other, MVT::Other, ISD::TRAP) != 0;
-    else
-      return true;
+    }
+    return true;
+  }
 
   case Instruction::Alloca:
     // FunctionLowering has the static-sized case covered.
diff --git a/llvm/test/CodeGen/X86/no-trap-after-noreturn-fastisel.ll b/llvm/test/CodeGen/X86/no-trap-after-noreturn-fastisel.ll
new file mode 100644
index 00000000000000..5149209f79d156
--- /dev/null
+++ b/llvm/test/CodeGen/X86/no-trap-after-noreturn-fastisel.ll
@@ -0,0 +1,13 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=x86_64-unknown-linux-gnu -O0 -trap-unreachable -no-trap-after-noreturn -fast-isel-abort=3 < %s | FileCheck %s
+
+declare void @foo()
+
+define void @noreturn_unreachable() nounwind {
+; CHECK-LABEL: noreturn_unreachable:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    pushq %rax
+; CHECK-NEXT:    callq foo@PLT
+  call void @foo() noreturn
+  unreachable
+}

[arsenm]

CallBase?

[arsenm]

Check invoke case?

[nikic]

invoke is a terminator, so it can't be followed by unreachable. (Same is true for callbr.)

[aengelke]

LGTM. Was there any concrete use case where this caused a noticeable amount of fallbacks?

[nikic]

LGTM. Was there any concrete use case where this caused a noticeable amount of fallbacks?

Yes, this is one of the more common cause of fallbacks for Rust code. For the test case I'm looking at the distribution looks like this:

array(7) {
  ["invoke"]=>
  int(7870)
  ["unreachable"]=>
  int(3160)
  ["landingpad"]=>
  int(630)
  ["args"]=>
  int(566)
  ["ret"]=>
  int(397)
  ["other"]=>
  int(324)
  ["switch"]=>
  int(237)
}