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Date: Wed, 12 Aug 2020 14:34:30 +0300
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: [PATCH 4/3] setjmp: optimize longjmp prologues
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
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