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

[Xi Ruoyao <xry111@xry111.site>]

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?

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"
> 

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University