Re: [Seqan-dev] Fwd: Fwd: cmake default without -O3 and order of args/options in cmake call

Marcel Ehrhardt <marehr2@zedat.fu-berlin.de> · Mon, 10 Aug 2020 13:39:46 +0200

Dear Stefan,

thank you for writing us!

(I'll answer below)

On 8/10/20 1:01 PM, Hannes Hauswedell wrote:

-------- Forwarded Message --------
Subject: Fwd: cmake default without -O3 and order of args/options in
cmake call
Date: Sun, 9 Aug 2020 18:03:47 +0200
From: Stefan Kurtz <kurtz@zbh.uni-hamburg.de>
Reply-To: kurtz@zbh.uni-hamburg.de
Organization: Universitaet Hamburg
To: eqan-dev-owner@lists.fu-berlin.de
CC: Hannes Hauswedell <hannes.hauswedell@fu-berlin.de>

As suggested on the seqan web pages I had send an E-mail to
eqan-dev@lists.fu-berlin.de, but
I received the following message:
---
Your message has been rejected, probably because you are not
subscribed to the mailing list and the list's policy is to prohibit
non-members from posting to it.  If you think that your messages are
being rejected in error, contact the mailing list owner at
seqan-dev-owner@lists.fu-berlin.de.
--

Please forward the message to the Seqan developers. Kind regards, Stefan
Kurtz

-------- Forwarded Message --------
Reply-To: kurtz@zbh.uni-hamburg.de
To: seqan-dev@lists.fu-berlin.de
From: Stefan Kurtz <kurtz@zbh.uni-hamburg.de>
Subject: cmake default without -O3 and order of args/options in cmake call
Organization: Universitaet Hamburg
Message-ID: <cb4e36e9-814a-db1e-461a-c853efcc4300@zbh.uni-hamburg.de>
Date: Sun, 9 Aug 2020 17:56:49 +0200
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Dear Seqan3-developers,

this email refers to the 'Quick Setup (Cmake)' section of SeqAn3  3.0.2.

1) I followed the description for the Cmake-setup and noticed in the
example CMakeList.txt
shown at the end of https://docs.seqan.de/seqan/3-master-user/setup.html
that compiler optimization options such as -O3 is missing, which will
lead to poor performance for users who do not recognize it.
I know use
add_compile_options(-DNDEBUG -Wall -Wextra -Wunused-parameter
-Wunused-variable -pedantic -Werror -O3)

in CmakeLists.txt.

Thank you for pointing this out. It is a known fact / best practice in 

the CMake community that `-DCMAKE_BUILD_TYPE=Release` must be specified 

to enable optimized builds, e.g. `cmake -DCMAKE_BUILD_TYPE=Release 

<seqan3-source-path> `, but since our documentations should be 

accessible for "new"  C++ programmers or "new" CMake users, we should 

add a section that teaches how to enable Release builds.

The best-practice in CMake for header-only libraries, like SeqAn2 or 

SeqAn3, is to specify only the minimal required flags that are needed to 

compile any given source that uses SeqAn. Additional flags, like 

hardening options or performance options is up to the application 

developer and/or package maintainer. The rational is simple, if we would 

add the `-O3` flag to our cmake export, no-one could do a debug-build 

any more, because we then would dictate you to always have the `-O3` 

flag in your Makefile.

2) The cmake-setup specifies that one has to call
      cmake ../source -DCMAKE_CXX_COMPILER=/path/to/executable/g++-7

      The help pages of cmake version 3.16.3 on macOS and
      cmake version 3.10.2 on Linux state that options come before
arguments, but only the macOS version enforces this. So on my
      macOS system only
       cmake  -DCMAKE_CXX_COMPILER=/path/to/executable/g++-7 ../source
      works. To simplify the use for macOS, it is probably better
      use this order in the cmake-setup section (and elsewhere) which
works on both platforms.

Thank you for this, too :)

I created this issue: https://github.com/seqan/seqan3/issues/2017

So if you have a github account, we could move any further discussion there.

Kind regards, Stefan Kurtz