[llvm] [CFGMST] Remove special case for entry block with no successors (PR #140804)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Arthur Eubanks (aeubanks)

<details>
<summary>Changes</summary>

This breaks in the case where there are unreachable blocks after an entry block with no successors, which don't get visited in CFGMST, causing crashes.

Fixes #<!-- -->135828.

---
Full diff: https://github.com/llvm/llvm-project/pull/140804.diff


3 Files Affected:

- (modified) llvm/include/llvm/Transforms/Instrumentation/CFGMST.h (-6) 
- (added) llvm/test/Transforms/PGOProfile/Inputs/unreachable-block.proftext (+9) 
- (added) llvm/test/Transforms/PGOProfile/unreachable-block.ll (+12) 


``````````diff
diff --git a/llvm/include/llvm/Transforms/Instrumentation/CFGMST.h b/llvm/include/llvm/Transforms/Instrumentation/CFGMST.h
index f6bf045f7de2c..6f0c63b888353 100644
--- a/llvm/include/llvm/Transforms/Instrumentation/CFGMST.h
+++ b/llvm/include/llvm/Transforms/Instrumentation/CFGMST.h
@@ -133,12 +133,6 @@ template <class Edge, class BBInfo> class CFGMST {
     LLVM_DEBUG(dbgs() << "  Edge: from fake node to " << Entry->getName()
                       << " w = " << EntryWeight << "\n");
 
-    // Special handling for single BB functions.
-    if (succ_empty(Entry)) {
-      addEdge(Entry, nullptr, EntryWeight);
-      return;
-    }
-
     static const uint32_t CriticalEdgeMultiplier = 1000;
 
     for (BasicBlock &BB : F) {
diff --git a/llvm/test/Transforms/PGOProfile/Inputs/unreachable-block.proftext b/llvm/test/Transforms/PGOProfile/Inputs/unreachable-block.proftext
new file mode 100644
index 0000000000000..9be174fb32bca
--- /dev/null
+++ b/llvm/test/Transforms/PGOProfile/Inputs/unreachable-block.proftext
@@ -0,0 +1,9 @@
+# IR level Instrumentation Flag
+:ir
+foo
+# Func Hash:
+742261418966908927
+# Num Counters:
+1
+# Counter Values:
+1
diff --git a/llvm/test/Transforms/PGOProfile/unreachable-block.ll b/llvm/test/Transforms/PGOProfile/unreachable-block.ll
new file mode 100644
index 0000000000000..bd55185649989
--- /dev/null
+++ b/llvm/test/Transforms/PGOProfile/unreachable-block.ll
@@ -0,0 +1,12 @@
+; RUN: llvm-profdata merge %S/Inputs/unreachable-block.proftext -o %t.profdata
+; RUN: opt < %s -passes=pgo-instr-use -pgo-test-profile-file=%t.profdata -S
+
+declare ptr @bar()
+
+define ptr @foo() {
+entry:
+  ret ptr null
+
+2:
+  ret ptr null
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/140804