[gcc r12-9698] LoongArch: Avoid non-returning indirect jumps through $ra [PR110136]

[gcc r12-9698] LoongArch: Avoid non-returning indirect jumps through $ra [PR110136]

[LuluCheng]

https://gcc.gnu.org/g:ddec24e5abe99033c8d6bbe544b4c2b35a0232f2

commit r12-9698-gddec24e5abe99033c8d6bbe544b4c2b35a0232f2
Author: Lulu Cheng <chenglulu@loongson.cn>
Date:   Wed Jun 7 10:21:58 2023 +0800

    LoongArch: Avoid non-returning indirect jumps through $ra [PR110136]
    
    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:
    
            PR target/110136
            * 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>
    
    (cherry picked from commit 5430c86e71927492399129f3df80824c6c334ddf)

Diff:
---
 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 8f8412fba84..a64142c7a78 100644
--- a/gcc/config/loongarch/loongarch.md
+++ b/gcc/config/loongarch/loongarch.md
@@ -2622,6 +2622,10 @@
 }
   [(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"))]
   ""
@@ -2632,7 +2636,7 @@
 })
 
 (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")
@@ -2655,7 +2659,7 @@
 
 (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"