From: Alexander Monakov <amonakov@ispras.ru>
To: musl@lists.openwall.com
Subject: [musl] [PATCH] setjmp: optimize x86-64 longjmp prologues
Date: Wed, 12 Aug 2020 02:10:08 +0300 [thread overview]
Message-ID: <20200811231008.8896-1-amonakov@ispras.ru> (raw)
In-Reply-To: <20200811181116.8433-1-amonakov@ispras.ru>
Use a branchless sequence that is one byte shorter as well.
---
src/setjmp/x32/longjmp.s | 9 ++++-----
src/setjmp/x86_64/longjmp.s | 9 ++++-----
2 files changed, 8 insertions(+), 10 deletions(-)
diff --git a/src/setjmp/x32/longjmp.s b/src/setjmp/x32/longjmp.s
index bb88afa1..c8ad86fa 100644
--- a/src/setjmp/x32/longjmp.s
+++ b/src/setjmp/x32/longjmp.s
@@ -5,11 +5,10 @@
.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 %esi,%eax /* CF==0 iff val==0 */
+ cmc /* CF = !val */
+ 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..c8ad86fa 100644
--- a/src/setjmp/x86_64/longjmp.s
+++ b/src/setjmp/x86_64/longjmp.s
@@ -5,11 +5,10 @@
.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 %esi,%eax /* CF==0 iff val==0 */
+ cmc /* CF = !val */
+ 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
next prev parent reply other threads:[~2020-08-11 23:10 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 ` Alexander Monakov [this message]
2020-08-12 11:34 ` [musl] [PATCH 4/3] setjmp: optimize longjmp prologues Alexander Monakov
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