[v3] LoongArch: Avoid non-returning indirect jumps through $ra [PR110136]
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Commit Message
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(-)
Comments
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"
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"
>
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"
@@ -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"