[PATCH v2] LoongArch: Modify the register constraints for template "jumptable" and "indirect_jump" from "r" to "e" [PR110136]

[Lulu Cheng]

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
+;; 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"
-- 
2.31.1