Re: [PATCH 2/2] Reduce qsort stack consumption

[Corinna Vinschen <vinschen at redhat dot com>]

Hi Håkan,

On Mar 13 09:57, Håkan Lindqvist wrote:
> From 761f70c0603a94e032bc061885250c4e4cd5a19d Mon Sep 17 00:00:00 2001
> From: Hakan Lindqvist <hakan.lindqvist@enea.com>
> Date: Mon, 12 Mar 2018 14:55:01 +0100
> Subject: [PATCH 2/2] Reduce qsort stack consumption
> 
> Classical function call recursion wastes a lot of stack space.
> Each recursion level requires a full stack frame comprising all
> local variables and additional space as dictated by the
> processor calling convention.
> 
> This implementation instead stores the variables that are unique
> for each recursion level in a parameter stack array, and uses
> iteration to emulate recursion. Function call recursion is not
> used until the array is full.
> 
> To ensure the stack consumption isn't worsened by this design, the
> size of the parameter stack array is chosen to be similar to the
> stack frame excluding the array. Each function call recursion level
> can handle 8 iterative recursion levels.
> 
> Stack consumption will worsen when sorting tiny arrays that do not
> need recursion (of 6 elements or less). It will be about equal for
> up to 15 elements, and be an improvement for larger arrays. The best
> case improvement is a stack size reduction down to about one quarter
> of the stack consumption before the change.
> 
> A design where the parameter stack array is large enough for the
> worst case recursion level was rejected because it would worsen
> the stack consumption when sorting arrays smaller than about 1500
> elements. The worst case is 31 levels on a 32-bit system.
> 
> A design with a dynamic parameter array size was rejected because
> of limitations in some compilers.
> ---
> newlib/libc/search/qsort.c | 60 +++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 54 insertions(+), 6 deletions(-)

can you please resend this patch?  It's indentation is hopelessly broken
for some reason and so doesn't apply cleanly.  Maybe it works better as
attachment.

Your other patch is fine, though.


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
Red Hat

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