* g++ V3.3.1 not faster with __restrict
@ 2003-11-21 10:26 Kaspar Thommen
2003-11-21 10:42 ` Falk Hueffner
0 siblings, 1 reply; 2+ messages in thread
From: Kaspar Thommen @ 2003-11-21 10:26 UTC (permalink / raw)
To: gcc-help
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Hi,
I ran some benchmarks using g++ V3.3.1. Two functions are called from a main
program. While both doing exactly the same array manipulation (dot product
on doubles), one of the functions uses the __restrict keyword to tell the
compiler that pointer aliasing is "turned off" in that function.
To my surprise, the __restrict version is not faster than the conventional
one. In fact, they show the same speed. Do I have to specify any compilation
option to tell g++ to really take advantage from the __restrict information?
Didn't find anything in the docs however. I tried using -o, -o2, -o3, giving
always equal speed for both functions.
Regards
Kaspar
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^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: g++ V3.3.1 not faster with __restrict
2003-11-21 10:26 g++ V3.3.1 not faster with __restrict Kaspar Thommen
@ 2003-11-21 10:42 ` Falk Hueffner
0 siblings, 0 replies; 2+ messages in thread
From: Falk Hueffner @ 2003-11-21 10:42 UTC (permalink / raw)
To: Kaspar Thommen; +Cc: gcc-help
"Kaspar Thommen" <kaspar@gmx.ch> writes:
> To my surprise, the __restrict version is not faster than the conventional
> one. In fact, they show the same speed. Do I have to specify any compilation
> option to tell g++ to really take advantage from the __restrict information?
No, although probably -O2 is required to show any difference.
> Didn't find anything in the docs however. I tried using -o, -o2,
> -o3, giving always equal speed for both functions.
Try -funroll-loops or -funroll-all-loops, this produces more
possibilities to take advantage of the non-aliasing.
--
Falk
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