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Re: Crashes in "C-h h"

From: Paul Eggert
Subject: Re: Crashes in "C-h h"
Date: Thu, 4 Jul 2019 00:15:12 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2

Eli Zaretskii wrote:
Is this with USE_LSB or without?  I think we need to time both

That benchmark was with USE_LSB. I see no reason to think the !USE_LSB case would cause FIXNUMP+XFIXNUM to beat EQ+make_fixnum, but just to double-check here are the timings for that benchmark compiled with the additional GCC flags -m32 -DWIDE_EMACS_INT (the only significant !USE_LSB platform these days) and with 'long' replaced by 'long long' to get the loop to work on a 32-bit platform:

1030-lnxsrv09 $ time ./a.out 0 0

real    0m51.933s
user    0m51.927s
sys     0m0.001s
1031-lnxsrv09 $ time ./a.out 0 0 0

real    2m16.960s
user    2m16.960s
sys     0m0.000s
1032-lnxsrv09 $ time ./a.out 0 0 0 0

real    0m59.678s
user    0m59.677s
sys     0m0.001s

This indicates that on this !USE_LSB platform, the EQ+make_fixnum code is about ten times faster than the FIXNUMP+XFIXNUM code on this benchmark. I'm not surprised by the performance disparity, as branch-prediction failures can be pretty expensive.

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