From: Alexander Monakov <amonakov@ispras.ru>
To: musl@lists.openwall.com
Subject: [musl] [PATCH 4/3] setjmp: optimize longjmp prologues
Date: Wed, 12 Aug 2020 14:34:30 +0300 [thread overview]
Message-ID: <20200812113430.9254-1-amonakov@ispras.ru> (raw)
In-Reply-To: <20200811231008.8896-1-amonakov@ispras.ru>
Use a branchless sequence that is one byte shorter on 64-bit, same size
on 32-bit. Thanks to Pete Cawley for suggesting this variant.
---
I'm sending a revised variant after Pete Cawley (@corsix) suggested a
preferable variant on Twitter. A similar cmp-adc combo can be used to
replace the branchy sequence in i386 longjmp code.
src/setjmp/i386/longjmp.s | 6 ++----
src/setjmp/x32/longjmp.s | 8 +++-----
src/setjmp/x86_64/longjmp.s | 8 +++-----
3 files changed, 8 insertions(+), 14 deletions(-)
diff --git a/src/setjmp/i386/longjmp.s b/src/setjmp/i386/longjmp.s
index b429f135..8188f06b 100644
--- a/src/setjmp/i386/longjmp.s
+++ b/src/setjmp/i386/longjmp.s
@@ -6,10 +6,8 @@ _longjmp:
longjmp:
mov 4(%esp),%edx
mov 8(%esp),%eax
- test %eax,%eax
- jnz 1f
- inc %eax
-1:
+ cmp $1,%eax
+ adc $0, %al
mov (%edx),%ebx
mov 4(%edx),%esi
mov 8(%edx),%edi
diff --git a/src/setjmp/x32/longjmp.s b/src/setjmp/x32/longjmp.s
index bb88afa1..1b2661c3 100644
--- a/src/setjmp/x32/longjmp.s
+++ b/src/setjmp/x32/longjmp.s
@@ -5,11 +5,9 @@
.type longjmp,@function
_longjmp:
longjmp:
- mov %esi,%eax /* val will be longjmp return */
- test %esi,%esi
- jnz 1f
- inc %eax /* if val==0, val=1 per longjmp semantics */
-1:
+ xor %eax,%eax
+ cmp $1,%esi /* CF = val ? 0 : 1 */
+ adc %esi,%eax /* eax = val + !val */
mov (%rdi),%rbx /* rdi is the jmp_buf, restore regs from it */
mov 8(%rdi),%rbp
mov 16(%rdi),%r12
diff --git a/src/setjmp/x86_64/longjmp.s b/src/setjmp/x86_64/longjmp.s
index bb88afa1..1b2661c3 100644
--- a/src/setjmp/x86_64/longjmp.s
+++ b/src/setjmp/x86_64/longjmp.s
@@ -5,11 +5,9 @@
.type longjmp,@function
_longjmp:
longjmp:
- mov %esi,%eax /* val will be longjmp return */
- test %esi,%esi
- jnz 1f
- inc %eax /* if val==0, val=1 per longjmp semantics */
-1:
+ xor %eax,%eax
+ cmp $1,%esi /* CF = val ? 0 : 1 */
+ adc %esi,%eax /* eax = val + !val */
mov (%rdi),%rbx /* rdi is the jmp_buf, restore regs from it */
mov 8(%rdi),%rbp
mov 16(%rdi),%r12
--
2.11.0
prev parent reply other threads:[~2020-08-12 11:34 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-11 18:11 [musl] [PATCH 1/3] setjmp: fix x86-64 longjmp argument adjustment Alexander Monakov
2020-08-11 18:11 ` [musl] [PATCH 2/3] setjmp: avoid useless REX-prefix on xor %eax, %eax Alexander Monakov
2020-08-11 18:11 ` [musl] [PATCH 3/3] setjmp: optimize x86 longjmp epilogues Alexander Monakov
2020-08-11 18:45 ` [musl] [PATCH 1/3] setjmp: fix x86-64 longjmp argument adjustment Rich Felker
2020-08-12 13:29 ` Alexander Monakov
2020-08-12 21:44 ` Szabolcs Nagy
2020-08-12 22:13 ` Szabolcs Nagy
2020-08-13 8:01 ` Jens Gustedt
2020-08-13 8:53 ` Alexander Monakov
2020-08-11 23:10 ` [musl] [PATCH] setjmp: optimize x86-64 longjmp prologues Alexander Monakov
2020-08-12 11:34 ` Alexander Monakov [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200812113430.9254-1-amonakov@ispras.ru \
--to=amonakov@ispras.ru \
--cc=musl@lists.openwall.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://git.vuxu.org/mirror/musl/
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).