Re: Use static inline functions for Float <-> Datum conversions

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Use static inline functions for Float <-> Datum conversions
Date: 2016-08-31 12:22:30
Message-ID: 752262dc-0a51-22bd-9cfa-88532d2d8dc2@iki.fi
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On 08/31/2016 02:38 PM, Tom Lane wrote:
> Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
>> Now that we are OK with static inline functions, we can save some cycles
>> from floating-point functions, by turning Float4GetDatum,
>> Float8GetDatum, and DatumGetFloat8 into static inlines.
>
> Looks good to me.

Ok, will push.

> I wonder whether there is a compiler-dependent way of avoiding the union
> trick ... or maybe gcc is already smart enough that it doesn't matter?

It seems to compile into a single instruction, so it can't get any
better from a performance point of view.

float8pl:
.LFB79:
.loc 1 871 0
.cfi_startproc
.LVL297:
.LBB959:
.LBB960:
.loc 2 733 0
movsd 40(%rdi), %xmm2
.LBE960:
.LBE959:
.LBB961:
.LBB962:
movsd 32(%rdi), %xmm1
...

A union is probably what language pedantics would prefer anyway, and
anything else would be more of a trick.

- Heikki

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