[IR] CallBrInst: scan+update arg list when indirect dest list changes

Summary:
There's an unspoken invariant of callbr that the list of BlockAddress
Constants in the "function args" list match the BasicBlocks in the
"other labels" list. (This invariant is being added to the LangRef in
https://reviews.llvm.org/D67196).

When modifying the any of the indirect destinations of a callbr
instruction (possible jump targets), we need to update the function
arguments if the argument is a BlockAddress whose BasicBlock refers to
the indirect destination BasicBlock being replaced.  Otherwise, many
transforms that modify successors will end up violating that invariant.
A recent change to the arm64 Linux kernel exposed this bug, which
prevents the kernel from booting.

I considered maintaining a mapping from indirect destination BasicBlock
to argument operand BlockAddress, but this ends up being a one to
potentially many (though usually one) mapping.  Also, the list of
arguments to a function (or more typically inline assembly) ends up
being less than 10.  The implementation is significantly simpler to just
rescan the full list of arguments. Because of the one to potentially
many relationship, the full arg list must be scanned (we can't stop at
the first instance).

Thanks to the following folks that reported the issue and helped debug
it:
* Nathan Chancellor
* Will Deacon
* Andrew Murray
* Craig Topper

Link: https://bugs.llvm.org/show_bug.cgi?id=43222
Link: ClangBuiltLinux/linux#649
Link: https://lists.infradead.org/pipermail/linux-arm-kernel/2019-September/678330.html

Reviewers: craig.topper, chandlerc

Reviewed By: craig.topper

Subscribers: void, javed.absar, kristof.beyls, hiraditya, llvm-commits, nathanchance, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67252

llvm-svn: 371262

Author: nickdesaulniers

llvm/include/llvm/IR/Instructions.h

@@ -3942,6 +3942,9 @@ class CallBrInst : public CallBase {
             ArrayRef<BasicBlock *> IndirectDests, ArrayRef<Value *> Args,
             ArrayRef<OperandBundleDef> Bundles, const Twine &NameStr);
 
+  /// Should the Indirect Destinations change, scan + update the Arg list.
+  void updateArgBlockAddresses(unsigned i, BasicBlock *B);
+
   /// Compute the number of operands to allocate.
   static int ComputeNumOperands(int NumArgs, int NumIndirectDests,
                                 int NumBundleInputs = 0) {
@@ -4079,7 +4082,7 @@ class CallBrInst : public CallBase {
     return cast<BasicBlock>(*(&Op<-1>() - getNumIndirectDests() - 1));
   }
   BasicBlock *getIndirectDest(unsigned i) const {
-    return cast<BasicBlock>(*(&Op<-1>() - getNumIndirectDests() + i));
+    return cast_or_null<BasicBlock>(*(&Op<-1>() - getNumIndirectDests() + i));
   }
   SmallVector<BasicBlock *, 16> getIndirectDests() const {
     SmallVector<BasicBlock *, 16> IndirectDests;
@@ -4091,6 +4094,7 @@ class CallBrInst : public CallBase {
     *(&Op<-1>() - getNumIndirectDests() - 1) = reinterpret_cast<Value *>(B);
   }
   void setIndirectDest(unsigned i, BasicBlock *B) {
+    updateArgBlockAddresses(i, B);
     *(&Op<-1>() - getNumIndirectDests() + i) = reinterpret_cast<Value *>(B);
   }
 
@@ -4100,11 +4104,10 @@ class CallBrInst : public CallBase {
     return i == 0 ? getDefaultDest() : getIndirectDest(i - 1);
   }
 
-  void setSuccessor(unsigned idx, BasicBlock *NewSucc) {
-    assert(idx < getNumIndirectDests() + 1 &&
+  void setSuccessor(unsigned i, BasicBlock *NewSucc) {
+    assert(i < getNumIndirectDests() + 1 &&
            "Successor # out of range for callbr!");
-    *(&Op<-1>() - getNumIndirectDests() -1 + idx) =
-        reinterpret_cast<Value *>(NewSucc);
+    return i == 0 ? setDefaultDest(NewSucc) : setIndirectDest(i - 1, NewSucc);
   }
 
   unsigned getNumSuccessors() const { return getNumIndirectDests() + 1; }

llvm/lib/IR/Instructions.cpp

@@ -823,6 +823,17 @@ void CallBrInst::init(FunctionType *FTy, Value *Fn, BasicBlock *Fallthrough,
   setName(NameStr);
 }
 
+void CallBrInst::updateArgBlockAddresses(unsigned i, BasicBlock *B) {
+  assert(getNumIndirectDests() > i && "IndirectDest # out of range for callbr");
+  if (BasicBlock *OldBB = getIndirectDest(i)) {
+    BlockAddress *Old = BlockAddress::get(OldBB);
+    BlockAddress *New = BlockAddress::get(B);
+    for (unsigned ArgNo = 0, e = getNumArgOperands(); ArgNo != e; ++ArgNo)
+      if (dyn_cast<BlockAddress>(getArgOperand(ArgNo)) == Old)
+        setArgOperand(ArgNo, New);
+  }
+}
+
 CallBrInst::CallBrInst(const CallBrInst &CBI)
     : CallBase(CBI.Attrs, CBI.FTy, CBI.getType(), Instruction::CallBr,
                OperandTraits<CallBase>::op_end(this) - CBI.getNumOperands(),

llvm/unittests/IR/InstructionsTest.cpp

@@ -1061,5 +1061,56 @@ TEST(InstructionsTest, FNegInstruction) {
   FNeg->deleteValue();
 }
 
+TEST(InstructionsTest, CallBrInstruction) {
+  LLVMContext Context;
+  std::unique_ptr<Module> M = parseIR(Context, R"(
+define void @foo() {
+entry:
+  callbr void asm sideeffect "// XXX: ${0:l}", "X"(i8* blockaddress(@foo, %branch_test.exit))
+          to label %land.rhs.i [label %branch_test.exit]
+
+land.rhs.i:
+  br label %branch_test.exit
+
+branch_test.exit:
+  %0 = phi i1 [ true, %entry ], [ false, %land.rhs.i ]
+  br i1 %0, label %if.end, label %if.then
+
+if.then:
+  ret void
+
+if.end:
+  ret void
+}
+)");
+  Function *Foo = M->getFunction("foo");
+  auto BBs = Foo->getBasicBlockList().begin();
+  CallBrInst &CBI = cast<CallBrInst>(BBs->front());
+  ++BBs;
+  ++BBs;
+  BasicBlock &BranchTestExit = *BBs;
+  ++BBs;
+  BasicBlock &IfThen = *BBs;
+
+  // Test that setting the first indirect destination of callbr updates the dest
+  EXPECT_EQ(&BranchTestExit, CBI.getIndirectDest(0));
+  CBI.setIndirectDest(0, &IfThen);
+  EXPECT_EQ(&IfThen, CBI.getIndirectDest(0));
+
+  // Further, test that changing the indirect destination updates the arg
+  // operand to use the block address of the new indirect destination basic
+  // block. This is a critical invariant of CallBrInst.
+  BlockAddress *IndirectBA = BlockAddress::get(CBI.getIndirectDest(0));
+  BlockAddress *ArgBA = cast<BlockAddress>(CBI.getArgOperand(0));
+  EXPECT_EQ(IndirectBA, ArgBA)
+      << "After setting the indirect destination, callbr had an indirect "
+         "destination of '"
+      << CBI.getIndirectDest(0)->getName() << "', but a argument of '"
+      << ArgBA->getBasicBlock()->getName() << "'. These should always match:\n"
+      << CBI;
+  EXPECT_EQ(IndirectBA->getBasicBlock(), &IfThen);
+  EXPECT_EQ(ArgBA->getBasicBlock(), &IfThen);
+}
+
 } // end anonymous namespace
 } // end namespace llvm