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 <apin...@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"
