[clang] [Clang][CodeGen] Don't emit assumptions if current block is unreachable. (PR #106936)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-codegen

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Fixes https://github.com/llvm/llvm-project/issues/106898.

When emitting an infinite loop, clang codegen will delete the whole block and leave builder's current block as nullptr:
https://github.com/llvm/llvm-project/blob/837ee5b46a5f7f898f0de7e46a19600b896a0a1f/clang/lib/CodeGen/CGStmt.cpp#L597-L600

Then clang will create `zext (icmp slt %a, %b)` without parent block for `a < b`. It will crash here:
https://github.com/llvm/llvm-project/blob/837ee5b46a5f7f898f0de7e46a19600b896a0a1f/clang/lib/CodeGen/CGExprScalar.cpp#L416-L420

Even if we disabled this optimization, it still crashes in `Builder.CreateAssumption`:
https://github.com/llvm/llvm-project/blob/837ee5b46a5f7f898f0de7e46a19600b896a0a1f/llvm/lib/IR/IRBuilder.cpp#L551-L561

This patch disables assumptions emission if current block is null. As an alternative, we can fix the optimization in `EmitIntToBoolConversion` and use `CGM.getIntrinsic` as we do for `__builtin_assume`:
https://github.com/llvm/llvm-project/blob/837ee5b46a5f7f898f0de7e46a19600b896a0a1f/clang/lib/CodeGen/CGBuiltin.cpp#L3422-L3431


---
Full diff: https://github.com/llvm/llvm-project/pull/106936.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGStmt.cpp (+1-1) 
- (modified) clang/test/SemaCXX/cxx23-assume.cpp (+9) 


``````````diff
diff --git a/clang/lib/CodeGen/CGStmt.cpp b/clang/lib/CodeGen/CGStmt.cpp
index 7158a06e6bc3b3..b138c87a853495 100644
--- a/clang/lib/CodeGen/CGStmt.cpp
+++ b/clang/lib/CodeGen/CGStmt.cpp
@@ -752,7 +752,7 @@ void CodeGenFunction::EmitAttributedStmt(const AttributedStmt &S) {
     } break;
     case attr::CXXAssume: {
       const Expr *Assumption = cast<CXXAssumeAttr>(A)->getAssumption();
-      if (getLangOpts().CXXAssumptions &&
+      if (getLangOpts().CXXAssumptions && Builder.GetInsertBlock() &&
           !Assumption->HasSideEffects(getContext())) {
         llvm::Value *AssumptionVal = EvaluateExprAsBool(Assumption);
         Builder.CreateAssumption(AssumptionVal);
diff --git a/clang/test/SemaCXX/cxx23-assume.cpp b/clang/test/SemaCXX/cxx23-assume.cpp
index 9138501d726dd6..eeae59daea3f70 100644
--- a/clang/test/SemaCXX/cxx23-assume.cpp
+++ b/clang/test/SemaCXX/cxx23-assume.cpp
@@ -158,3 +158,12 @@ foo (int x, int y)
   return x + y;
 }
 }
+
+// Do not crash when assumptions are unreachable.
+namespace gh106898 {
+int foo () { 
+    while(1);
+    int a = 0, b = 1;
+    __attribute__((assume (a < b)));
+}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/106936