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Message-Id: <20200811231008.8896-1-amonakov@ispras.ru>
Date: Wed, 12 Aug 2020 02:10:08 +0300
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: [PATCH] setjmp: optimize x86-64 longjmp prologues
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
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