[v2] LoongArch: Modify the register constraints for template "jumptable" and "indirect_jump" from "r" to "e" [PR110136]
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| linaro-tcwg-bot/tcwg_gcc_build--master-aarch64 |
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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".
---
gcc/config/loongarch/loongarch.md | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
Comments
On 2023/6/8 10:27, Lulu Cheng wrote:
> 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".
> ---
> 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..43a2ecc8957 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",
> +;; hence doing "jr $ra" would interfere with both subroutine return prediction and
Not all cases of "doing 'jr $ra'" are harmful, obviously. Paraphrasing
it like e.g. "non-returning indirect jumps through $ra" would be better.
You could simplify the patch title a lot with this too: "Avoid
non-returning indirect jumps through $ra" is shorter and does not
duplicate the ChangeLog message.
> +;; 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",
+;; hence doing "jr $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"