From 5430c86e71927492399129f3df80824c6c334ddf Mon Sep 17 00:00:00 2001
From: Lulu Cheng <chenglulu@loongson.cn>
Date: Wed, 7 Jun 2023 10:21:58 +0800
Subject: [PATCH] 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>
---
 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 @@
 }
   [(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_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_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"