Date: Thu, 30 Jun 2005 17:35:17 +0300 From: "Andriy Tkachuk" <andrit@ukr.net> To: "Max Laier" <max@love2party.net>, <freebsd-hackers@freebsd.org> Subject: Re: hot path optimizations in uma_zalloc() & uma_zfree() Message-ID: <003901c57d82$37891b80$de921bd9@ertpc> References: <000d01c57cf7$b9b6f9f0$29931bd9@ertpc> <200506301415.38106.max@love2party.net>
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I just checked the object code - you right, it almost the same:
- bucket->ub_bucket[bucket->ub_cnt] = item;
- 22b9: 0f bf 43 08 movswl 0x8(%ebx),%eax
- 22bd: 8b 4d 0c mov 0xc(%ebp),%ecx
- 22c0: 89 4c 83 0c mov %ecx,0xc(%ebx,%eax,4)
- bucket->ub_cnt++;
- 22c4: 8d 42 01 lea 0x1(%edx),%eax
- 22c7: 66 89 43 08 mov %ax,0x8(%ebx)
+ bucket->ub_bucket[bucket->ub_cnt++] = item;
+ 22b9: 0f bf c2 movswl %dx,%eax
+ 22bc: 8b 4d 0c mov 0xc(%ebp),%ecx
+ 22bf: 89 4c 83 0c mov %ecx,0xc(%ebx,%eax,4)
+ 22c3: 8d 42 01 lea 0x1(%edx),%eax
+ 22c6: 66 89 43 08 mov %ax,0x8(%ebx)
but still there is some minor difference in first line.
I'm not familiar with assembler, can somebody explain
whether this difference is assential or not?
in decrementation there is no difference at all:
- bucket->ub_cnt--;
+ item = bucket->ub_bucket[--bucket->ub_cnt];
1bbe: 66 ff 49 08 decw 0x8(%ecx)
- item = bucket->ub_bucket[bucket->ub_cnt];
1bc2: 0f bf 41 08 movswl 0x8(%ecx),%eax
1bc6: 8b 44 81 0c mov 0xc(%ecx,%eax,4),%eax
1bca: 89 45 f0 mov %eax,0xfffffff0(%ebp)
----- Original Message -----
From: "Max Laier" <max@love2party.net>
To: <freebsd-hackers@freebsd.org>
Cc: "ant" <andrit@ukr.net>
Sent: Thursday, June 30, 2005 3:15 PM
Subject: Re: hot path optimizations in uma_zalloc() & uma_zfree()
> Another optimization is very trivial, for example:
> - bucket->ub_cnt--;
> - item = bucket->ub_bucket[bucket->ub_cnt];
> + item = bucket->ub_bucket[--bucket->ub_cnt];
> (see the patch)
Might be me, but this doesn't change the generated object code at all (modulo
the changed __line__ in debugging).
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