[llvm-dev] Maintaining musttail call contract

[Johannes Doerfert via llvm-dev]

Hi Xun,

On 10/30/20 9:47 PM, Xun Li via llvm-dev wrote:
> Hi,
>
> A musttail call must be preceded by an optional bitcast instruction
> and a return instruction. It seems that there isn't a mechanism to
> make sure the musttail call contract isn't broken by any instruction
> insertion in the IR.
> As far as I can tell, all IR transformations don't really look at
> what's before a ReturnInst when inserting instructions. For example,
> all Instrumentation passes ignored them, and would insert
> instrumentation instructions in between musttail call and ReturnInst,
> which would break the contract.
> I fixed it for ASAN in https://reviews.llvm.org/D87777 and fixed it
> for TSAN in https://reviews.llvm.org/D87620.
> But lately I also found that SafeStack pass has the same problem. For
> instance, running "opt --safe-stack" on the following IR would crash
> Clang:
>
> target triple = "x86_64-unknown-linux-gnu"
>
> declare i32 @foo(i32* %p)
> declare void @alloca_test_use([10 x i8]*)
>
> define i32 @call_foo(i32* %a) safestack {
>    %x = alloca [10 x i8], align 1
>    call void @alloca_test_use([10 x i8]* %x)
>    %r = musttail call i32 @foo(i32* %a)
>    ret i32 %r
> }
>
> I am pretty sure that SafeStack isn't the last pass that has this
> problem and there will be more like this in the future.
> I am trying to see if there are good systematic ways to solve this
> problem without having to require every pass author to be careful when
> inserting instructions before ReturnInst.
>
> One solution I can think of is to update
> IRBuilderBase::SetInsertPoint(Instruction *I) to always check if "I"
> is a ReturnInst followed by a musttail call contract, and if so, move
> the insertion point to before the call. Since this approach adds
> overhead to every insertion point creation, I am not sure if it's the
> best solution.
>
> Advice and feedback much appreciated!

I am not a fan of the IRBuilder solution, it doesn't cover all the
cases as not everything uses the builder and instructions can be moved,
and it will be some behind the scenes magic that could cause other
things to break silently.

My suggestion:
   1) Make the verifier aware of the restriction, it will flag broken
      IR and catch (almost) all cases (I would assume).
   2) If, for some reason I can't imagine right now, we build this broken
      IR and want to keep it "for a while", write a late pass that tries
      to "repair" the IR but only if it can keep the semantics otherwise
      intact, so it cannot unconditionally move instructions but only if
      it is otherwise correct anyway. If it can't, abort.

~ Johannes