[IRGen] Handle infinite cycles in findDominatingStoreToReturnValue.

[fhahn]

Summary

If there is an infinite cycle in the IR, the loop will never exit. Keep
track of visited basic blocks in a set and return nullptr if a block is
visited again.

Fixes #62830.

[rjmccall]

Nice catch.

clang/lib/CodeGen/CGCall.cpp

static llvm::StoreInst *findDominatingStoreToReturnValue(CodeGenFunction &CGF) {
   llvm::StoreInst *store =
       GetStoreIfValid(CGF.ReturnValue.getPointer()->user_back());
   if (!store) return nullptr;
 
   // Now do a first-and-dirty dominance check: just walk up the
   // single-predecessors chain from the current insertion point.
   llvm::BasicBlock *StoreBB = store->getParent();
   llvm::BasicBlock *IP = CGF.Builder.GetInsertBlock();
+  llvm::SmallPtrSet<llvm::BasicBlock *, 4> SeenBBs;
   while (IP != StoreBB) {
-    if (!(IP = IP->getSinglePredecessor()))
+    if (!SeenBBs.insert(IP).second || !(IP = IP->getSinglePredecessor()))
       return nullptr;
   }
 
   // Okay, the store's basic block dominates the insertion point; we
   // can do our thing.
   return store;
 }
 

clang/test/CodeGen/dominating-store-infinite-cycle.c

+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 2
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
+
+// Test for PR62830 where there are 2 infinite cycles using goto. Make sure
+// clang codegen doesn't hang.
+int a;
+
+// CHECK-LABEL: define dso_local i32 @main
+// CHECK-SAME: () #[[ATTR0:[0-9]+]] {
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[RETVAL:%.*]] = alloca i32, align 4
+// CHECK-NEXT:    store i32 0, ptr [[RETVAL]], align 4
+// CHECK-NEXT:    br label [[L1:%.*]]
+// CHECK:       L1:
+// CHECK-NEXT:    br label [[L1]]
+// CHECK:       L2:
+// CHECK-NEXT:    [[TMP0:%.*]] = load i32, ptr @a, align 4
+// CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i32 [[TMP0]], 0
+// CHECK-NEXT:    br i1 [[TOBOOL]], label [[IF_END:%.*]], label [[IF_THEN:%.*]]
+// CHECK:       if.then:
+// CHECK-NEXT:    br label [[L2:%.*]]
+// CHECK:       if.end:
+// CHECK-NEXT:    [[TMP1:%.*]] = load i32, ptr [[RETVAL]], align 4
+// CHECK-NEXT:    ret i32 [[TMP1]]
+//
+int main() {
+L1:
+  goto L1;
+
+L2:
+  if (!a)
+    goto L2;
+}