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Groups > comp.lang.c++ > #70083
| From | David Brown <david.brown@hesbynett.no> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: A question about if constexpr |
| Date | 2019-11-28 12:37 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <qrobhp$nj1$1@dont-email.me> (permalink) |
| References | <9d930f21-6639-49a3-8ef2-5de724f18255@googlegroups.com> <cone.1574898034.956666.129095.1004@monster.email-scan.com> <qro3p4$gd7$1@dont-email.me> <h49nj1Fa201U1@mid.individual.net> |
On 28/11/2019 11:58, Bo Persson wrote:
> On 2019-11-28 at 10:24, David Brown wrote:
>> On 28/11/2019 00:40, Sam wrote:
>>> Daniel writes:
>>>
>>>> Consider
>>>>
>>>> #include <string>
>>>> #include <iostream>
>>>>
>>>> int main()
>>>> {
>>>> constexpr int n = 1;
>>>> constexpr int m = 10;
>>>> std::string s = "abcd";
>>>>
>>>> if (n < m || s.empty())
>>>> {
>>>> std::cout << "ok\n";
>>>> }
>>>> }
>>>>
>>>> Is it reasonable that vs2019 produces a warning
>>>>
>>>> warning C4127: conditional expression is constant
>>>> consider using 'if constexpr' statement instead
>>>
>>> The compiler does not reject this code as ill-formed; and only issues a
>>> non-fatal diagnostic. This is entirely the compiler's prerogative.
>>>
>>> Whether this or something else is "reasonable" is often a somewhat
>>> subjective matter. Someone might find it reasonable, this is a suggested
>>> optimization.
>>
>> It should not be a "suggested optimisation". If the compiler is not
>> generating identical code for an "if constexpr" and an "if" where the
>> expression happens to be known at compile time (whether it is
>> technically a C++ constant or not), then the compiler is doing a poor
>> job at optimising.
>>
>> I had a check at <https://godbolt.org> with the simpler test variation:
>>
>>
>> #include <string>
>>
>> int foo()
>> {
>> constexpr int n = 1;
>> constexpr int m = 10;
>> std::string s = "abcd";
>>
>> if (n < m || s.empty())
>> {
>> return 1;
>> }
>> return 0;
>> }
>>
>> It turns out that MSVC (/O2 /Wall /std:c++17) generates a whole pile of
>> muck for that code. It takes about 40 lines of assembly (excluding
>> string library functions) while it creates and deletes the string, adds
>> calls to "__security_check_cookie", whatever that is. However, it
>> generates the same pile when "constexpr" is added. (It also generates
>> an extraordinary quantity of warnings from its own headers here - is
>> that normal for MSVC, or is it an artefact of the godbolt.org setup or
>> my incomplete knowledge of MSVC command line parameters?)
>>
>> gcc and clang, on the other hand generates a single "return 1;" result
>> whether you have "constexpr" or not. And no warnings with "-Wall
>> -Wextra".
>>
>>
>
> You should probably blaim this on the settings for Godbolt.org
>
> Running locally, and adding a main function containing return foo(), it
> results in
>
> --- C:\Users\BoP\source\reference\quick_test.cpp
> -------------------------------
> return foo();
> 00007FF6BB091000 mov eax,1
> }
> 00007FF6BB091005 ret
>
>
> On the other hand, Visual Studio reports that it is using the compiler
> options
>
> //permissive- /GS- /Zc:rvalueCast /GL /W4 /Gy /Zc:wchar_t /Zi /Gm- /O2
> /Ob2 /Fd"x64\Release\vc142.pdb" /Zc:inline /fp:precise /D "_UNICODE" /D
> "UNICODE" /errorReport:prompt /GF /WX /Zc:forScope /GR /arch:AVX2 /Gd
> /Oy /Oi /MD /std:c++latest /FC /Fa"x64\Release\" /EHsc /nologo
> /Fo"x64\Release\" /Ot /Fp"x64\Release\quick_test_ms.pch"
> /diagnostics:column
>
>
> Don't know exactly which one makes the difference. :-)
>
OK. As I said, I don't know the command line switches for MSVC - I just
used the ones I knew for optimisation, warnings and standard version.
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A question about if constexpr Daniel <danielaparker@gmail.com> - 2019-11-27 05:45 -0800
Re: A question about if constexpr "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-11-27 15:03 +0100
Re: A question about if constexpr Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2019-11-29 16:33 +0000
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-29 16:52 -0800
Re: A question about if constexpr David Brown <david.brown@hesbynett.no> - 2019-11-30 12:03 +0100
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-30 03:47 -0800
Re: A question about if constexpr David Brown <david.brown@hesbynett.no> - 2019-11-30 15:08 +0100
Re: A question about if constexpr Sam <sam@email-scan.com> - 2019-11-27 18:40 -0500
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-27 23:16 -0800
Re: A question about if constexpr Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2019-11-29 16:35 +0000
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-29 16:41 -0800
Re: A question about if constexpr David Brown <david.brown@hesbynett.no> - 2019-11-28 10:24 +0100
Re: A question about if constexpr Bo Persson <bo@bo-persson.se> - 2019-11-28 11:58 +0100
Re: A question about if constexpr David Brown <david.brown@hesbynett.no> - 2019-11-28 12:37 +0100
Re: A question about if constexpr Robert Wessel <robertwessel2@yahoo.com> - 2019-11-28 14:04 -0600
Re: A question about if constexpr David Brown <david.brown@hesbynett.no> - 2019-11-29 09:23 +0100
Re: A question about if constexpr Robert Wessel <robertwessel2@yahoo.com> - 2019-11-29 14:42 -0600
Re: A question about if constexpr Daniel <danielaparker@gmail.com> - 2019-11-28 21:09 -0800
Re: A question about if constexpr "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-11-29 07:41 +0100
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-28 23:50 -0800
Re: A question about if constexpr David Brown <david.brown@hesbynett.no> - 2019-11-29 09:38 +0100
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-29 17:15 -0800
Re: A question about if constexpr Paavo Helde <myfirstname@osa.pri.ee> - 2019-11-30 08:19 +0200
Re: A question about if constexpr Ned Latham <nedlatham@woden.valhalla.oz> - 2019-11-30 00:24 -0600
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-30 02:13 -0800
Re: A question about if constexpr Ned Latham <nedlatham@woden.valhalla.oz> - 2019-11-30 04:56 -0600
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-30 03:20 -0800
Re: A question about if constexpr Ned Latham <nedlatham@woden.valhalla.oz> - 2019-11-30 05:38 -0600
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-30 04:57 -0800
Re: A question about if constexpr Ned Latham <nedlatham@woden.valhalla.oz> - 2019-11-30 09:46 -0600
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-30 11:09 -0800
Re: A question about if constexpr Ned Latham <nedlatham@woden.valhalla.oz> - 2019-11-30 20:03 -0600
Re: A question about if constexpr Öö Tiib <ootiib@hot.ee> - 2019-11-30 23:43 -0800
Re: A question about if constexpr Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-01 04:56 -0600
Re: A question about if constexpr Christian Gollwitzer <auriocus@gmx.de> - 2019-12-01 18:33 +0100
Re: A question about if constexpr Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-01 19:27 -0600
Re: A question about if constexpr boltar@nowhere.co.uk - 2019-12-02 08:34 +0000
Re: A question about if constexpr Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-02 04:21 -0600
Re: A question about if constexpr boltar@nowhere.co.uk - 2019-12-02 08:33 +0000
Re: A question about if constexpr Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2019-12-03 17:05 +0000
Re: A question about if constexpr boltar@nowhere.co.uk - 2019-12-03 17:09 +0000
Re: A question about if constexpr Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2019-12-03 18:03 +0000
Re: A question about if constexpr boltar@nowhere.co.uk - 2019-12-04 16:49 +0000
Re: A question about if constexpr Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2019-12-04 18:36 +0000
Re: A question about if constexpr boltar@nowhere.co.uk - 2019-12-05 12:51 +0000
Re: A question about if constexpr David Brown <david.brown@hesbynett.no> - 2019-11-29 09:30 +0100
Re: A question about if constexpr Paavo Helde <myfirstname@osa.pri.ee> - 2019-11-29 10:51 +0200
Re: A question about if constexpr Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 13:52 -0800
Re: A question about if constexpr Bonita Montero <Bonita.Montero@gmail.com> - 2019-12-01 14:42 +0100
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