Re: [boost] [uuids] On boost::uuids::uuid operator == performance.

Subject: Re: [boost] [uuids] On boost::uuids::uuid operator == performance.
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2012-04-16 05:59:26

On 16/04/12 01:10, Michael Kochetkov wrote:

> boost uses the following comparison operator:
> inline bool operator==(uuid const&  lhs, uuid const&  rhs) /* throw() */
> {
>      return std::equal(lhs.begin(), lhs.end(), rhs.begin());
> }
>
> which end up with the following code:
> 	lea	edx, DWORD PTR _id2$[esp+92]
> 	lea	ecx, DWORD PTR _id1$[esp+108]
> 	lea	eax, DWORD PTR _id1$[esp+92]
> 	call	?_Equal_at_std@@YA_NPBE00_at_Z		; std::_Equal

A good implementation of std::equal would call memcmp here, which would 
already be optimized.

That doesn't seem to be the case.

Can you try replacing this by memcmp(lhs.begin(), rhs.begin(), lhs.size()) ?

> inline
> bool comp(const boost::uuids::uuid&  l, const boost::uuids::uuid&  r) {
> 	return *reinterpret_cast<const uint32_t*>(l.data) ==
> *reinterpret_cast<const uint32_t*>(r.data)
> 		&&  *reinterpret_cast<const uint32_t*>(l.data+4) ==
> *reinterpret_cast<const uint32_t*>(r.data+4)
> 		&&  *reinterpret_cast<const uint32_t*>(l.data+8) ==
> *reinterpret_cast<const uint32_t*>(r.data+8)
> 		&&  *reinterpret_cast<const uint32_t*>(l.data+12) ==
> *reinterpret_cast<const uint32_t*>(r.data+12);
> }

This code is specific to 32-bit architectures and also breaks the 
strict-aliasing rule.