[llvm] [GVN] Fix use-after-free in load PRE with select available value (PR #69314)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

replaceValuesPerBlockEntry() only handled simple and coerced load values, however the load may also be referenced by a select value.

Additionally, I suspect that the previous code might have been incorrect if a load had an offset, as it always constructed the AvailableValue from scratch.

Fixes https://github.com/llvm/llvm-project/issues/69301.

---
Full diff: https://github.com/llvm/llvm-project/pull/69314.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/GVN.cpp (+8-3) 
- (added) llvm/test/Transforms/GVN/pr69301.ll (+59) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 6619ed780e77557..5e58af0edc1556f 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -945,9 +945,14 @@ static void replaceValuesPerBlockEntry(
     SmallVectorImpl<AvailableValueInBlock> &ValuesPerBlock, Value *OldValue,
     Value *NewValue) {
   for (AvailableValueInBlock &V : ValuesPerBlock) {
-    if ((V.AV.isSimpleValue() && V.AV.getSimpleValue() == OldValue) ||
-       (V.AV.isCoercedLoadValue() && V.AV.getCoercedLoadValue() == OldValue))
-      V = AvailableValueInBlock::get(V.BB, NewValue);
+    if (V.AV.Val == OldValue)
+      V.AV.Val = NewValue;
+    if (V.AV.isSelectValue()) {
+      if (V.AV.V1 == OldValue)
+        V.AV.V1 = NewValue;
+      if (V.AV.V2 == OldValue)
+        V.AV.V2 = NewValue;
+    }
   }
 }
 
diff --git a/llvm/test/Transforms/GVN/pr69301.ll b/llvm/test/Transforms/GVN/pr69301.ll
new file mode 100644
index 000000000000000..dc54ef559dcffd7
--- /dev/null
+++ b/llvm/test/Transforms/GVN/pr69301.ll
@@ -0,0 +1,59 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
+; RUN: opt -S -passes=gvn < %s | FileCheck %s
+
+; Make sure we don't have use-after-free due to dangling values in
+; select available value.
+
+define i64 @test(i1 %c, ptr %p) {
+; CHECK-LABEL: define i64 @test(
+; CHECK-SAME: i1 [[C:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[ADD:%.*]], [[LOOP_LATCH:%.*]] ]
+; CHECK-NEXT:    [[PTR_IV:%.*]] = phi ptr [ [[P]], [[ENTRY]] ], [ [[SELECT:%.*]], [[LOOP_LATCH]] ]
+; CHECK-NEXT:    [[ICMP:%.*]] = icmp eq i64 [[IV]], 0
+; CHECK-NEXT:    br i1 [[ICMP]], label [[LOOP_EXIT_CRIT_EDGE:%.*]], label [[LOOP_CONT:%.*]]
+; CHECK:       loop.exit_crit_edge:
+; CHECK-NEXT:    [[RES_PRE:%.*]] = load i64, ptr [[PTR_IV]], align 8
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       loop.cont:
+; CHECK-NEXT:    [[ADD]] = add i64 [[IV]], -1
+; CHECK-NEXT:    [[RES_PRE1:%.*]] = load i64, ptr [[PTR_IV]], align 8
+; CHECK-NEXT:    br i1 [[C]], label [[EXITSPLIT:%.*]], label [[LOOP_LATCH]]
+; CHECK:       loop.latch:
+; CHECK-NEXT:    [[LOAD6:%.*]] = load i64, ptr [[P]], align 8
+; CHECK-NEXT:    [[ICMP7:%.*]] = icmp ugt i64 [[RES_PRE1]], [[LOAD6]]
+; CHECK-NEXT:    [[TMP0:%.*]] = select i1 [[ICMP7]], i64 [[RES_PRE1]], i64 [[LOAD6]]
+; CHECK-NEXT:    [[SELECT]] = select i1 [[ICMP7]], ptr [[PTR_IV]], ptr [[P]]
+; CHECK-NEXT:    br label [[LOOP]]
+; CHECK:       exitsplit:
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[RES:%.*]] = phi i64 [ [[RES_PRE1]], [[EXITSPLIT]] ], [ [[RES_PRE]], [[LOOP_EXIT_CRIT_EDGE]] ]
+; CHECK-NEXT:    ret i64 [[RES]]
+;
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %entry ], [ %add, %loop.latch ]
+  %ptr.iv = phi ptr [ %p, %entry ], [ %select, %loop.latch ]
+  %icmp = icmp eq i64 %iv, 0
+  br i1 %icmp, label %exit, label %loop.cont
+
+loop.cont:
+  %add = add i64 %iv, -1
+  br i1 %c, label %exit, label %loop.latch
+
+loop.latch:
+  %load = load i64, ptr %ptr.iv, align 8
+  %load6 = load i64, ptr %p, align 8
+  %icmp7 = icmp ugt i64 %load, %load6
+  %select = select i1 %icmp7, ptr %ptr.iv, ptr %p
+  br label %loop
+
+exit:
+  %res = load i64, ptr %ptr.iv, align 8
+  ret i64 %res
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/69314