[8u] RFR: 8151284: aarch32: on-stack replacement does not work properly when invoked from compiled

[Edward Nevill]

On Fri, 2016-03-04 at 18:30 +0300, Andrey Petushkov wrote:
> Hi All!
> 
> Below is a fix for the On Stack Replacement functionality. The bug is caused by borrowing code from aarch64 but using sp for both sp and esp purposes. As a result the correct value of sp gets lost after removing of the frame. The trivial fix is to save the value across the frame pop routine.
> 

Hi Andrey,

Thanks for this. One thing confuses me about this code. Here is the code from templateTable_aarch32.cpp post your patch

      // We have the address of an on stack replacement routine in r0
      // We need to prepare to execute the OSR method. First we must
      // migrate the locals and monitors off of the stack.

<<<< LOOK HERE
      __ str(r0, Address( __ pre(sp, -wordSize)));               // save the nmethod

      call_VM(noreg, CAST_FROM_FN_PTR(address, SharedRuntime::OSR_migration_begin));

      // r0 is OSR buffer, move it to expected parameter location
      __ mov(j_rarg0, r0);

      // remove activation
      // get sender sp
      __ ldr(rscratch1,
          Address(rfp, frame::interpreter_frame_sender_sp_offset * wordSize));
      // remove frame anchor
      __ leave();
      __ mov(sp, rscratch1);
      // Ensure compiled code always sees stack at proper alignment
      __ align_stack();

      // and begin the OSR nmethod
<<<<< LOOK HERE
      __ ldr(r14, Address( __ post(sp, wordSize)));
      __ ldr(rscratch1, Address(r14, nmethod::osr_entry_point_offset()));
      __ b(rscratch1);

At the first point marked "LOOK HERE" it does

      __ str(r0, Address( __ pre(sp, -wordSize)));               // save the nmethod

to save the nmethod on the stack. It then modifies the stack pointer and the, at the second "LOOK HERE" it does

      __ ldr(r14, Address( __ post(sp, wordSize)));

This would seem to be broken? The aarch64 code uses r19 which is preserved around the call.

Is there a callee save register we can use to preserve the nmethod around the call?

Alternatively I think the ldr r14, ... should go before the leave().

But I have no way of testing this.

All the best,
Ed.