[llvm-commits] [llvm] r155466 - in /llvm/trunk: lib/Analysis/ConstantFolding.cpp test/Transforms/GlobalOpt/constantfold-initializers.ll

[Chandler Carruth]

Author: chandlerc
Date: Tue Apr 24 13:42:47 2012
New Revision: 155466

URL: http://llvm.org/viewvc/llvm-project?rev=155466&view=rev
Log:
Fix a crash on valid (if UB) bitcode that is produced for some global
constants in C++11 mode. I have no idea why it required such particular
circumstances to get here, the code seems clearly to rely upon unchecked
assumptions.

Specifically, when we decide to form an index into a struct type, we may
have gone through (at least one) zero-length array indexing round, which
would have left the offset un-adjusted, and thus not necessarily valid
for use when indexing the struct type.

This is just an canonicalization step, so the correct thing is to refuse
to canonicalize nonsensical GEPs of this form. Implemented, and test
case added.

Fixes PR12642. Pair debugged and coded with Richard Smith. =] I credit
him with most of the debugging, and preventing me from writing the wrong
code.

Modified:
    llvm/trunk/lib/Analysis/ConstantFolding.cpp
    llvm/trunk/test/Transforms/GlobalOpt/constantfold-initializers.ll

Modified: llvm/trunk/lib/Analysis/ConstantFolding.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ConstantFolding.cpp?rev=155466&r1=155465&r2=155466&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ConstantFolding.cpp (original)
+++ llvm/trunk/lib/Analysis/ConstantFolding.cpp Tue Apr 24 13:42:47 2012
@@ -681,6 +681,7 @@
   // This makes it easy to determine if the getelementptr is "inbounds".
   // Also, this helps GlobalOpt do SROA on GlobalVariables.
   Type *Ty = Ptr->getType();
+  assert(Ty->isPointerTy() && "Forming regular GEP of non-pointer type");
   SmallVector<Constant*, 32> NewIdxs;
   do {
     if (SequentialType *ATy = dyn_cast<SequentialType>(Ty)) {
@@ -711,10 +712,17 @@
       }
       Ty = ATy->getElementType();
     } else if (StructType *STy = dyn_cast<StructType>(Ty)) {
-      // Determine which field of the struct the offset points into. The
-      // getZExtValue is at least as safe as the StructLayout API because we
-      // know the offset is within the struct at this point.
+      // If we end up with an offset that isn't valid for this struct type, we
+      // can't re-form this GEP in a regular form, so bail out. The pointer
+      // operand likely went through casts that are necessary to make the GEP
+      // sensible.
       const StructLayout &SL = *TD->getStructLayout(STy);
+      if (Offset.uge(SL.getSizeInBytes()))
+        break;
+
+      // Determine which field of the struct the offset points into. The
+      // getZExtValue is fine as we've already ensured that the offset is
+      // within the range representable by the StructLayout API.
       unsigned ElIdx = SL.getElementContainingOffset(Offset.getZExtValue());
       NewIdxs.push_back(ConstantInt::get(Type::getInt32Ty(Ty->getContext()),
                                          ElIdx));

Modified: llvm/trunk/test/Transforms/GlobalOpt/constantfold-initializers.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/GlobalOpt/constantfold-initializers.ll?rev=155466&r1=155465&r2=155466&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/GlobalOpt/constantfold-initializers.ll (original)
+++ llvm/trunk/test/Transforms/GlobalOpt/constantfold-initializers.ll Tue Apr 24 13:42:47 2012
@@ -12,6 +12,11 @@
 @xs = global [2 x i32] zeroinitializer, align 4
 ; CHECK: @xs = global [2 x i32] [i32 1, i32 1]
 
+; PR12642
+%PR12642.struct = type { i8 }
+ at PR12642.s = global <{}> zeroinitializer, align 1
+ at PR12642.p = constant %PR12642.struct* bitcast (i8* getelementptr (i8* bitcast (<{}>* @PR12642.s to i8*), i64 1) to %PR12642.struct*), align 8
+
 define internal void @test1() {
 entry:
   store i32 1, i32* getelementptr inbounds ([2 x i32]* @xs, i64 0, i64 0)