reverse-stepi stepping more than one instruction

If I run reverse-stepi after running to a certain event with rr replay -g, then I've got a situation where reverse-stepi steps more than one instruction:

(rr) disas
Dump of assembler code for function core::slice::from_raw_parts<u8>:
   0x000056197ad5aab0 <+0>:	push   %rbp
   0x000056197ad5aab1 <+1>:	mov    %rsp,%rbp
   0x000056197ad5aab4 <+4>:	mov    %rdi,-0x40(%rbp)
   0x000056197ad5aab8 <+8>:	mov    %rsi,-0x38(%rbp)
   0x000056197ad5aabc <+12>:	mov    -0x40(%rbp),%rax
   0x000056197ad5aac0 <+16>:	mov    %rax,-0x30(%rbp)
   0x000056197ad5aac4 <+20>:	mov    -0x38(%rbp),%rax
   0x000056197ad5aac8 <+24>:	mov    %rax,-0x28(%rbp)
   0x000056197ad5aacc <+28>:	mov    -0x30(%rbp),%rax
   0x000056197ad5aad0 <+32>:	mov    -0x28(%rbp),%rcx
   0x000056197ad5aad4 <+36>:	mov    %rax,-0x20(%rbp)
   0x000056197ad5aad8 <+40>:	mov    %rcx,-0x18(%rbp)
   0x000056197ad5aadc <+44>:	mov    -0x20(%rbp),%rax
=> 0x000056197ad5aae0 <+48>:	mov    -0x18(%rbp),%rcx
   0x000056197ad5aae4 <+52>:	mov    %rax,-0x10(%rbp)
   0x000056197ad5aae8 <+56>:	mov    %rcx,-0x8(%rbp)
   0x000056197ad5aaec <+60>:	mov    -0x10(%rbp),%rax
   0x000056197ad5aaf0 <+64>:	mov    -0x8(%rbp),%rdx
   0x000056197ad5aaf4 <+68>:	mov    %rax,-0x48(%rbp)
   0x000056197ad5aaf8 <+72>:	mov    %rdx,-0x50(%rbp)
   0x000056197ad5aafc <+76>:	mov    -0x48(%rbp),%rax
   0x000056197ad5ab00 <+80>:	mov    -0x50(%rbp),%rdx
   0x000056197ad5ab04 <+84>:	pop    %rbp
   0x000056197ad5ab05 <+85>:	retq   
End of assembler dump.
(rr) reverse-stepi

Thread 1 received signal SIGINT, Interrupt.
core::slice::{{impl}}::index<u8> (self=..., slice=...) at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcore/slice.rs:708
708	in /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcore/slice.rs
(rr) disas
Dump of assembler code for function core::slice::{{impl}}::index<u8>:
   0x000056197ad5b040 <+0>:	push   %rbp
   0x000056197ad5b041 <+1>:	mov    %rsp,%rbp
   0x000056197ad5b044 <+4>:	sub    $0x90,%rsp
   0x000056197ad5b04b <+11>:	mov    (%rdi),%rax
   0x000056197ad5b04e <+14>:	mov    %rax,-0x58(%rbp)
   0x000056197ad5b052 <+18>:	mov    0x8(%rdi),%rax
   0x000056197ad5b056 <+22>:	mov    %rax,-0x50(%rbp)
   0x000056197ad5b05a <+26>:	mov    %rsi,-0x48(%rbp)
   0x000056197ad5b05e <+30>:	mov    %rdx,-0x40(%rbp)
   0x000056197ad5b062 <+34>:	mov    -0x58(%rbp),%rax
   0x000056197ad5b066 <+38>:	mov    -0x50(%rbp),%rcx
   0x000056197ad5b06a <+42>:	mov    %rax,-0x38(%rbp)
   0x000056197ad5b06e <+46>:	mov    %rcx,-0x30(%rbp)
   0x000056197ad5b072 <+50>:	mov    -0x48(%rbp),%rax
   0x000056197ad5b076 <+54>:	mov    -0x40(%rbp),%rcx
   0x000056197ad5b07a <+58>:	mov    %rax,-0x28(%rbp)
   0x000056197ad5b07e <+62>:	mov    %rcx,-0x20(%rbp)
   0x000056197ad5b082 <+66>:	mov    -0x38(%rbp),%rax
   0x000056197ad5b086 <+70>:	cmp    -0x30(%rbp),%rax
   0x000056197ad5b08a <+74>:	jbe    0x56197ad5b099 <core::slice::{{impl}}::index<u8>+89>
   0x000056197ad5b08c <+76>:	mov    -0x38(%rbp),%rdi
   0x000056197ad5b090 <+80>:	mov    -0x30(%rbp),%rsi
   0x000056197ad5b094 <+84>:	callq  0x56197af5c380 <core::slice::slice_index_order_fail>
=> 0x000056197ad5b099 <+89>:	mov    -0x30(%rbp),%rax
   0x000056197ad5b09d <+93>:	mov    -0x28(%rbp),%rdi
   0x000056197ad5b0a1 <+97>:	mov    -0x20(%rbp),%rsi
   0x000056197ad5b0a5 <+101>:	mov    %rax,-0x60(%rbp)
---Type <return> to continue, or q <return> to quit---q
Quit
(rr) stepi

Thread 1 received signal SIGINT, Interrupt.
0x000056197ad5aae0 in core::slice::from_raw_parts<u8> (p=0x56197af69140 <str.V> "raw_capacity overflowraw_cap overflow\000", len=0) at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcore/slice.rs:2161
2161	in /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcore/slice.rs
(rr) 

Thread 1 received signal SIGKILL, Killed.
0x000056197ad5aae0 in core::slice::from_raw_parts<u8> (p=0x56197af69140 <str.V> "raw_capacity overflowraw_cap overflow\000", len=0) at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcore/slice.rs:2161
2161	in /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcore/slice.rs
(rr) 
[Inferior 1 (process 26877) exited normally]
(rr) 

If you need more information, please tell me what is necessary.

Start by reproducing the reported sequence: run rr replay -g, use reverse-stepi, and compare the disassembly before and after stepping. Determine why the command moves from core::slice::from_raw_parts<u8> to core::slice::{{impl}}::index<u8> instead of one instruction; done means reverse-stepi reliably advances to the immediately preceding instruction.