Smallest saturating arithmetic functions in x86 assembly: Is 45 bytes the limit?

http://blog.regehr.org/archives/278/comment-page-1

by SlyShy • 16 years ago

23 3 16 years ago

blog.regehr.org

ssp• 16 years ago

You can shave a byte off the unsigned addition by using the sbb instruction

    00000000 <original>:
    0:	8b 44 24 04          	mov    0x4(%esp),%eax
    4:	31 d2                	xor    %edx,%edx
    6:	f7 d2                	not    %edx
    8:	03 44 24 08          	add    0x8(%esp),%eax
    c:	0f 42 c2             	cmovb  %edx,%eax
    f:	c3                   	ret    

    00000010 <one_shorter>:
     0:	8b 44 24 04          	mov    0x4(%esp),%eax
    14:	03 44 24 08          	add    0x8(%esp),%eax
    18:	89 c3                	mov    %eax,%ebx
    1a:	19 c3                	sbb    %eax,%ebx
    1c:	09 d8                	or     %ebx,%eax
    1e:	c3                   	ret

By (ab)using the short pop instructions we can save another byte:

   0000001f <another>:
   1f:	58                   	pop    %eax
   20:	5b                   	pop    %ebx
   21:	01 d8                	add    %ebx,%eax
   23:	89 c3                	mov    %eax,%ebx
   25:	19 c3                	sbb    %eax,%ebx
   27:	09 d8                	or     %ebx,%eax
   29:	83 ec 08             	sub    $0x8,%esp
   2c:	c3                   	ret

vilda• 16 years ago

Not sure about the goals, but instructions like sar, btc, and loop have significant performance penalty.

daeken• 16 years ago

When you're aiming for small code, performance is the last of your worries. A couple years back I decided to implement a complete bootloader that could read a kernel from NTFS in the MBR -- 510 bytes usable. While performance would've been nice, just getting it into that space was a massive(ly fun) chore.

Smallest saturating arithmetic functions in x86 assembly: Is 45 bytes the limit?

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