[llvm] [FastISel] Support unreachable with NoTrapAfterNoReturn (PR #118296)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-llvm-selectiondag

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

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
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 at PLT
+  call void @foo() noreturn
+  unreachable
+}

``````````

</details>


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