Re: [PATCH v7 04/17] tracing: add static function tracer support for MIPS

Ralf Baechle
2009-11-11T03:14:33Z

On Wed, Nov 11, 2009 at 10:42:31AM +0800, Wu Zhangjin wrote:

> > -mlong-calls really degrades performance. I have seen things like 6%
> > drop in network packet forwarding rates with -mlong-calls.
> >
>
> so much drop? seems only two instructions added for it: lui, addi. from
> this view point, I think the -fno-omit-frame-pointer(add, sd, move...)
> will also bring with much drop.

The calling sequence is quite badly bloated. Example:

Normal 32/64-bit subroutine call:

jal symbol

32-bit with -mlong-call:

lui $25, %hi(foo)
addiu $25, %lo(foo)
jalr $25

64-bit with -mlong-call:

lui $25, %highest(foo)
lui $2, %hi(foo)
daddiu $25, %higher(foo)
daddiu $2, %lo(foo)
dsll $25, 32
daddu $25, $2
jalr $25

So not considering the possible cost of the delay slot that's 1 vs. 3 vs. 7
instructions. Last I checked ages ago gcc didn't apropriately consider this
cost when generating -mlong-calls code and Linux these days also is
optimized under the assumption that subroutine calls are cheap.

It's time that we get a -G optimization that works for the kernel; it would
allow to cut down the -mlong-calls calling sequence to just:

lw/ld $25, offset($gp)
jalr $25

> It's time to remove them? -mlong-calls, -fno-omit-frame-pointer.
>
> > It would be better to fix all the tools so that they could handle both
> > -mlong-calls and -mno-long-calls code.
> >
>
> It's totally possible, will try to make it work later. I just wanted the
> stuff simple, but if it really brings us with much drop, it's time to
> fix it.

Ralf