[InstCombine] Avoid use after free in DenseMap, when built with GCC

Previously, this used a statement like this:
    Map[A] = Map[B];

This is equivalent to the following:
    const auto &Src = Map[B];
    auto &Dest = Map[A];
    Dest = Src;

The second statement, "auto &Dest = Map[A];" can insert a new
element into the DenseMap, which can potentially grow and reallocate
the DenseMap's internal storage, which will invalidate the existing
reference to the source. When doing the actual assignment,
the Src reference is dereferenced, accessing memory that was
freed when the DenseMap grew.

This issue hasn't shown up when LLVM was built with Clang, because
the right hand side ended up dereferenced before evaulating the
left hand side. (If the value type is a larger data type, Clang doesn't
do this but behaves like GCC.)

With GCC, a cast to Value* isn't enough to make it dereference the
right hand side reference before invoking operator[] (while that is
enough to make Clang/LLVM do the right thing for larger types), but
storing it in an intermediate variable in a separate statement works.

This fixes PR42065.

Differential Revision: https://reviews.llvm.org/D62624

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362150 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index b3eb75ea8a8b72034909f295ec5ac4c3a8e78d06..a04e56916f8ef2b17c3d349f3209abc62e7a2354 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -703,7 +703,10 @@ static Value *rewriteGEPAsOffset(Value *Start, Value *Base,
       continue;
 
     if (auto *CI = dyn_cast<CastInst>(Val)) {
-      NewInsts[CI] = NewInsts[CI->getOperand(0)];
+      // Don't get rid of the intermediate variable here; the store can grow
+      // the map which will invalidate the reference to the input value.
+      Value *V = NewInsts[CI->getOperand(0)];
+      NewInsts[CI] = V;
       continue;
     }
     if (auto *GEP = dyn_cast<GEPOperator>(Val)) {