[clang] [coroutine] Suppress unreachable-code warning on coroutine statements. (PR #77454)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Haojian Wu (hokein)

<details>
<summary>Changes</summary>

This fixes #<!-- -->69219.

Consider an example:

```
CoTask my_coroutine() {
    std::abort();
    co_return 1; // unreachable code warning.
}
```

Clang emits a CFG-based unreachable warning on the `co_return` statement (precisely the `1` subexpr). If we remove this statement, the program semantic is changed (my_coroutine is not a coroutine anymore).

The fix to suppress the warning for this particular case.

---
Full diff: https://github.com/llvm/llvm-project/pull/77454.diff


2 Files Affected:

- (modified) clang/lib/Analysis/ReachableCode.cpp (+41-1) 
- (added) clang/test/SemaCXX/coroutine-unreachable-warning.cpp (+50) 


``````````diff
diff --git a/clang/lib/Analysis/ReachableCode.cpp b/clang/lib/Analysis/ReachableCode.cpp
index 1bf0d9aec8620e..d839d2f999609d 100644
--- a/clang/lib/Analysis/ReachableCode.cpp
+++ b/clang/lib/Analysis/ReachableCode.cpp
@@ -17,6 +17,7 @@
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ParentMap.h"
+#include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/StmtCXX.h"
 #include "clang/Analysis/AnalysisDeclContext.h"
 #include "clang/Analysis/CFG.h"
@@ -60,6 +61,45 @@ static bool isTrivialDoWhile(const CFGBlock *B, const Stmt *S) {
   return false;
 }
 
+// Check if the block starts with a coroutine statement and see if the given
+// unreachable 'S' is the substmt of the coroutine statement.
+//
+// We suppress the unreachable warning for cases where an unreachable code is
+// a substmt of the coroutine statement, becase removing it will change the
+// function semantic if this is the only coroutine statement of the coroutine.
+static bool isInCoroutineStmt(const CFGBlock *Block, const Stmt* S) {
+  // The coroutine statement, co_return, co_await, or co_yield.
+  const Stmt* CoroStmt = nullptr;
+  // Find the first coroutine statement in the block.
+  for (CFGBlock::const_iterator I = Block->begin(), E = Block->end(); I != E;
+       ++I)
+    if (std::optional<CFGStmt> CS = I->getAs<CFGStmt>()) {
+      const Stmt *S = CS->getStmt();
+      if (llvm::isa<CoreturnStmt>(S) || llvm::isa<CoroutineSuspendExpr>(S)) {
+        CoroStmt = S ;
+        break;
+      }
+    }
+  if (!CoroStmt)
+    return false;
+
+  struct Checker : RecursiveASTVisitor<Checker> {
+    const Stmt *StmtToCheck;
+    bool CoroutineSubStmt = false;
+    Checker(const Stmt *S) : StmtToCheck(S) {}
+    bool VisitStmt(const Stmt *S) {
+      if (S == StmtToCheck)
+        CoroutineSubStmt = true;
+      return true;
+    }
+    // The 'S' stmt captured in the CFG can be implicit.
+    bool shouldVisitImplicitCode() const { return true; }
+  };
+  Checker checker(S);
+  checker.TraverseStmt(const_cast<Stmt *>(CoroStmt));
+  return checker.CoroutineSubStmt;
+}
+
 static bool isBuiltinUnreachable(const Stmt *S) {
   if (const auto *DRE = dyn_cast<DeclRefExpr>(S))
     if (const auto *FDecl = dyn_cast<FunctionDecl>(DRE->getDecl()))
@@ -623,7 +663,7 @@ void DeadCodeScan::reportDeadCode(const CFGBlock *B,
   if (isa<BreakStmt>(S)) {
     UK = reachable_code::UK_Break;
   } else if (isTrivialDoWhile(B, S) || isBuiltinUnreachable(S) ||
-             isBuiltinAssumeFalse(B, S, C)) {
+             isBuiltinAssumeFalse(B, S, C) || isInCoroutineStmt(B, S)) {
     return;
   }
   else if (isDeadReturn(B, S)) {
diff --git a/clang/test/SemaCXX/coroutine-unreachable-warning.cpp b/clang/test/SemaCXX/coroutine-unreachable-warning.cpp
new file mode 100644
index 00000000000000..6ac5c34262b7e0
--- /dev/null
+++ b/clang/test/SemaCXX/coroutine-unreachable-warning.cpp
@@ -0,0 +1,50 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -std=c++20 -fsyntax-only -verify -Wunreachable-code
+
+#include "Inputs/std-coroutine.h"
+
+extern void abort (void) __attribute__ ((__noreturn__));
+
+struct task {
+  struct promise_type {
+    std::suspend_always initial_suspend();
+    std::suspend_always final_suspend() noexcept;
+    void return_void();
+    std::suspend_always yield_value(int) { return {}; }
+      task get_return_object();
+    void unhandled_exception();
+  };
+};
+
+task test1() {
+  abort();
+  co_yield 1;
+}
+
+task test2() {
+  abort();
+  1; // expected-warning {{code will never be executed}}
+  co_yield 1;
+}
+
+task test3() {
+  abort();
+  co_return;
+}
+
+task test4() {
+  abort();
+  1; // expected-warning {{code will never be executed}}
+  co_return;
+}
+
+
+task test5() {
+  abort();
+  co_await std::suspend_never{};
+}
+
+task test6() {
+  abort();
+  1; // expected-warning {{code will never be executed}}
+  co_await std::suspend_never{};
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/77454