From cc3953e504f54e3b20f6d4689809c335f43419cd Mon Sep 17 00:00:00 2001
From: Avi Kivity <avi@scylladb.com>
Date: Mon, 21 Oct 2024 19:12:22 +0300
Subject: [PATCH] build: disable Seastar exception hack

In [1], Seastar started to bypass a lock in libgcc's exception throwing
mechanism to allow scalability on large machines. The problem is documented
in [2] and reported as fixed.

In [3], testing results on a 2s96c192t machine are reported. The problem
appears indeed fixed with gcc 14's runtime (which we use, even though we
build with clang).

Given the new results, we can safely drop the exception scalability hack.
As [1] states that the hack causes the loss of a translation cache, we
may gain some performance this way.

With that, we disable the cache by defining some random macro.

[1] https://github.com/scylladb/seastar/464f5e3ae43b366b05573018fc46321863bf2fae
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71744
[3] https://github.com/scylladb/seastar/issues/2479#issuecomment-2427098413

Closes scylladb/scylladb#21217
---
 configure.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/configure.py b/configure.py
index 0a4393261a..5d0f8e6724 100755
--- a/configure.py
+++ b/configure.py
@@ -1662,6 +1662,9 @@ user_ldflags = forced_ldflags + ' ' + args.user_ldflags
 curdir = os.getcwd()
 user_cflags = args.user_cflags + f" -ffile-prefix-map={curdir}=."
 
+# Since gcc 13, libgcc doesn't need the exception workaround
+user_cflags += ' -DSEASTAR_NO_EXCEPTION_HACK'
+
 if args.target != '':
     user_cflags += ' -march=' + args.target