Re: [pushed][PATCH v3] LoongArch: Avoid non-returning indirect jumps through $ra [PR110136]

[WANG Xuerui <i@xen0n.name>]

Hi,

On 6/15/23 17:03, Xi Ruoyao wrote:
> Xuerui: I guess this makes it sensible to show "ret" instead of "jirl
> $zero, $ra, 0" in objdump -d output, but I don't know how to implement
> it.  Do you have some idea?

Thanks for the suggestion! Actually I have previously made this patch 
series [1] which included just that. But the Loongson maintainers said 
they're working on linker relaxation at that time so they would have to 
postpone processing it, and I've never had a review since then; it's 
expected to conflict with the relaxation patches so some rebasing would 
be needed, but IIRC all review comments should be addressed. You can 
take the series if you'd like to ;-)

[1]: https://sourceware.org/pipermail/binutils/2023-February/126088.html

>
> On Thu, 2023-06-15 at 16:27 +0800, Lulu Cheng wrote:
>> Pushed to trunk and gcc-12 gcc-13.
>> r14-1866
>> r13-7448
>> r12-9698
>>
>> 在 2023/6/15 上午9:30, Lulu Cheng 写道:
>>> Micro-architecture unconditionally treats a "jr $ra" as "return from
>>> subroutine",
>>> hence doing "jr $ra" would interfere with both subroutine return
>>> prediction and
>>> the more general indirect branch prediction.
>>>
>>> Therefore, a problem like PR110136 can cause a significant increase
>>> in branch error
>>> prediction rate and affect performance. The same problem exists with
>>> "indirect_jump".
>>>
>>> gcc/ChangeLog:
>>>
>>>          * config/loongarch/loongarch.md: Modify the register
>>> constraints for template
>>>          "jumptable" and "indirect_jump" from "r" to "e".
>>>
>>> Co-authored-by: Andrew Pinski <apinski@marvell.com>
>>> ---
>>> v1 -> v2:
>>>     1. Modify the description.
>>>     2. Modify the register constraints of the template
>>> "indirect_jump".
>>> v2 -> v3:
>>>     1. Modify the description.
>>> ---
>>>    gcc/config/loongarch/loongarch.md | 8 ++++++--
>>>    1 file changed, 6 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/gcc/config/loongarch/loongarch.md
>>> b/gcc/config/loongarch/loongarch.md
>>> index 816a943d155..b37e070660f 100644
>>> --- a/gcc/config/loongarch/loongarch.md
>>> +++ b/gcc/config/loongarch/loongarch.md
>>> @@ -2895,6 +2895,10 @@ (define_insn "*jump_pic"
>>>    }
>>>      [(set_attr "type" "branch")])
>>>    
>>> +;; Micro-architecture unconditionally treats a "jr $ra" as "return
>>> from subroutine",
>>> +;; non-returning indirect jumps through $ra would interfere with
>>> both subroutine
>>> +;; return prediction and the more general indirect branch
>>> prediction.
>>> +
>>>    (define_expand "indirect_jump"
>>>      [(set (pc) (match_operand 0 "register_operand"))]
>>>      ""
>>> @@ -2905,7 +2909,7 @@ (define_expand "indirect_jump"
>>>    })
>>>    
>>>    (define_insn "@indirect_jump<mode>"
>>> -  [(set (pc) (match_operand:P 0 "register_operand" "r"))]
>>> +  [(set (pc) (match_operand:P 0 "register_operand" "e"))]
>>>      ""
>>>      "jr\t%0"
>>>      [(set_attr "type" "jump")
>>> @@ -2928,7 +2932,7 @@ (define_expand "tablejump"
>>>    
>>>    (define_insn "@tablejump<mode>"
>>>      [(set (pc)
>>> -       (match_operand:P 0 "register_operand" "r"))
>>> +       (match_operand:P 0 "register_operand" "e"))
>>>       (use (label_ref (match_operand 1 "" "")))]
>>>      ""
>>>      "jr\t%0"