Topic: [[likely]] attribute for switch case
Author: david.bolvansky@gmail.com
Date: Tue, 8 May 2018 08:48:53 -0700 (PDT)
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Hello,
Since in C++20 we can write:
char * b(int e) {
if ([[likely]] e == 0)
return "0";
else if (e == 1)
return "1";
else return "undef";
}
Can likely attribute be added even for switch case? Like for example:
switch(e) {
case 0 [[likely]]:
return "0";
case 1:
return "1";
default:
return "undef";
}
I read the "likely" attribute
proposal http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0479r0.html
and they do not mention this use case for switches.
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<div dir=3D"ltr">Hello,<div><br></div><div>Since in C++20 we can write:</di=
v><div><div style=3D"color: rgb(0, 0, 0);" liberation=3D"" mono",=3D"" cour=
ier,=3D"" monospace;font-weight:=3D"" normal;font-size:=3D"" 14px;line-heig=
ht:=3D"" 19px;white-space:=3D"" pre;"=3D""><br><div style=3D"background-col=
or: rgb(255, 255, 254);"><span style=3D"color: #0000ff;">char</span> * b(<s=
pan style=3D"color: #0000ff;">int</span> e) {</div><div> <span style=3D"=
background-color: rgb(255, 255, 254); color: rgb(0, 0, 255);">=C2=A0 =C2=A0=
</span><span style=3D"background-color: rgb(255, 255, 254); color: rgb(0, 0=
, 255);">if</span><span style=3D"background-color: rgb(255, 255, 254);">=C2=
=A0 (</span><span style=3D"background-color: rgb(255, 255, 254);">[[likely]=
]=C2=A0</span><span style=3D"background-color: rgb(255, 255, 254);">e =3D=
=3D </span><span style=3D"background-color: rgb(255, 255, 254); color: rgb(=
9, 136, 90);">0</span><span style=3D"background-color: rgb(255, 255, 254);"=
>)</span></div><div style=3D"background-color: rgb(255, 255, 254);"> =
<span style=3D"color: rgb(0, 0, 255);">=C2=A0 =C2=A0</span><span style=3D"=
color: rgb(0, 0, 255);">=C2=A0 =C2=A0</span><span style=3D"color: rgb(0, 0,=
255);">return</span> <span style=3D"color: rgb(163, 21, 21);">"0"=
;</span>;</div><div style=3D"background-color: rgb(255, 255, 254);"> <sp=
an style=3D"color: rgb(0, 0, 255);">=C2=A0 =C2=A0</span><span style=3D"colo=
r: #0000ff;">else</span> <span style=3D"color: #0000ff;">if</span> (e =3D=
=3D <span style=3D"color: #09885a;">1</span>)</div><div style=3D"background=
-color: rgb(255, 255, 254);"> <span style=3D"color: rgb(0, 0, 255);"=
>=C2=A0 =C2=A0</span><span style=3D"color: rgb(0, 0, 255);">=C2=A0 =C2=A0</=
span><span style=3D"color: rgb(0, 0, 255);">return</span> <span style=3D"co=
lor: rgb(163, 21, 21);">"1"</span>;</div><div style=3D"background=
-color: rgb(255, 255, 254);"> <span style=3D"color: #0000ff;">=C2=A0 =C2=
=A0else</span> <span style=3D"color: #0000ff;">return</span> <span style=3D=
"color: #a31515;">"undef"</span>;</div><div style=3D"background-c=
olor: rgb(255, 255, 254);">}</div><div style=3D"background-color: rgb(255, =
255, 254);"><br></div><div style=3D"background-color: rgb(255, 255, 254);">=
Can likely attribute be added even for switch case? Like for example:</div>=
<div style=3D"background-color: rgb(255, 255, 254);"><br></div><div style=
=3D"background-color: rgb(255, 255, 254);">switch(e) {</div><div style=3D"b=
ackground-color: rgb(255, 255, 254);">=C2=A0 =C2=A0 case 0 [[likely]]:</div=
><div style=3D"background-color: rgb(255, 255, 254);">=C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 return "0";</div><div style=3D"background-color: rg=
b(255, 255, 254);">=C2=A0 =C2=A0 case 1:</div><div style=3D"background-colo=
r: rgb(255, 255, 254);">=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return "1&qu=
ot;;</div><div style=3D"background-color: rgb(255, 255, 254);">=C2=A0 =C2=
=A0 default:</div><div style=3D"background-color: rgb(255, 255, 254);">=C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return "undef";</div><div style=3D=
"background-color: rgb(255, 255, 254);"><br></div><div style=3D"background-=
color: rgb(255, 255, 254);">}</div><div style=3D"background-color: rgb(255,=
255, 254);"><br></div><div style=3D"background-color: rgb(255, 255, 254);"=
><br></div><div style=3D"background-color: rgb(255, 255, 254);">I read the =
"likely" attribute proposal=C2=A0http://www.open-std.org/jtc1/sc2=
2/wg21/docs/papers/2016/p0479r0.html and they do not mention this use case =
for switches.</div></div></div></div>
<p></p>
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Author: Andrey Semashev <andrey.semashev@gmail.com>
Date: Tue, 8 May 2018 19:08:44 +0300
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On 05/08/18 18:48, david.bolvansky@gmail.com wrote:
> Hello,
>=20
> Since in C++20 we can write:
>=20
> char * b(int e) {
> if=C2=A0 ([[likely]] e =3D=3D 0)
> return "0";
> else if (e =3D=3D 1)
> return "1";
> =C2=A0 =C2=A0else return "undef";
> }
>=20
> Can likely attribute be added even for switch case? Like for example:
>=20
> switch(e) {
> =C2=A0 =C2=A0 case 0 [[likely]]:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return "0";
> =C2=A0 =C2=A0 case 1:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return "1";
> =C2=A0 =C2=A0 default:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return "undef";
>=20
> }
>=20
>=20
> I read the "likely" attribute=20
> proposal=C2=A0http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p04=
79r0.html=20
> and they do not mention this use case for switches.
N4741 includes and example with `switch`/`case` in [dcl.attr.likelihood]/3.
I suspect your example with an `if` statement is not doing what you=20
think it does. The [[likely]] applies to the condition expression, which=20
basically says "this condition is likely to be executed" and not "this=20
condition is likely to be true". The correct example is also given in=20
the aforementioned section.
This is the most unfortunate aspect of this proposal as it makes it=20
incompatible with __builtin_expect and tons of code that wraps it in macros=
..
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..
.
Author: david.bolvansky@gmail.com
Date: Tue, 8 May 2018 09:29:26 -0700 (PDT)
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Thanks, N4741 really has it already.
D=C5=88a utorok, 8. m=C3=A1ja 2018 18:08:48 UTC+2 Andrey Semashev nap=C3=AD=
sal(-a):
>
> On 05/08/18 18:48, david.b...@gmail.com <javascript:> wrote:=20
> > Hello,=20
> >=20
> > Since in C++20 we can write:=20
> >=20
> > char * b(int e) {=20
> > if ([[likely]] e =3D=3D 0)=20
> > return "0";=20
> > else if (e =3D=3D 1)=20
> > return "1";=20
> > else return "undef";=20
> > }=20
> >=20
> > Can likely attribute be added even for switch case? Like for example:=
=20
> >=20
> > switch(e) {=20
> > case 0 [[likely]]:=20
> > return "0";=20
> > case 1:=20
> > return "1";=20
> > default:=20
> > return "undef";=20
> >=20
> > }=20
> >=20
> >=20
> > I read the "likely" attribute=20
> > proposal=20
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0479r0.html=20
> > and they do not mention this use case for switches.=20
>
> N4741 includes and example with `switch`/`case` in=20
> [dcl.attr.likelihood]/3.=20
>
> I suspect your example with an `if` statement is not doing what you=20
> think it does. The [[likely]] applies to the condition expression, which=
=20
> basically says "this condition is likely to be executed" and not "this=20
> condition is likely to be true". The correct example is also given in=20
> the aforementioned section.=20
>
> This is the most unfortunate aspect of this proposal as it makes it=20
> incompatible with __builtin_expect and tons of code that wraps it in=20
> macros.=20
>
--=20
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<div dir=3D"ltr">Thanks, N4741 really has it already.<br><br>D=C5=88a utoro=
k, 8. m=C3=A1ja 2018 18:08:48 UTC+2 Andrey Semashev nap=C3=ADsal(-a):<block=
quote class=3D"gmail_quote" style=3D"margin: 0;margin-left: 0.8ex;border-le=
ft: 1px #ccc solid;padding-left: 1ex;">On 05/08/18 18:48, <a href=3D"javasc=
ript:" target=3D"_blank" gdf-obfuscated-mailto=3D"KTwhuBEiBgAJ" rel=3D"nofo=
llow" onmousedown=3D"this.href=3D'javascript:';return true;" onclic=
k=3D"this.href=3D'javascript:';return true;">david.b...@gmail.com</=
a> wrote:
<br>> Hello,
<br>>=20
<br>> Since in C++20 we can write:
<br>>=20
<br>> char * b(int e) {
<br>> if=C2=A0 ([[likely]] e =3D=3D 0)
<br>> return "0";
<br>> else if (e =3D=3D 1)
<br>> return "1";
<br>> =C2=A0=C2=A0 =C2=A0else return "undef";
<br>> }
<br>>=20
<br>> Can likely attribute be added even for switch case? Like for examp=
le:
<br>>=20
<br>> switch(e) {
<br>> =C2=A0=C2=A0 =C2=A0 case 0 [[likely]]:
<br>> =C2=A0=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return "0";
<br>> =C2=A0=C2=A0 =C2=A0 case 1:
<br>> =C2=A0=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return "1";
<br>> =C2=A0=C2=A0 =C2=A0 default:
<br>> =C2=A0=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return "undef";
<br>>=20
<br>> }
<br>>=20
<br>>=20
<br>> I read the "likely" attribute=20
<br>> proposal=C2=A0<a href=3D"http://www.open-std.org/jtc1/sc22/wg21/do=
cs/papers/2016/p0479r0.html" target=3D"_blank" rel=3D"nofollow" onmousedown=
=3D"this.href=3D'http://www.google.com/url?q\x3dhttp%3A%2F%2Fwww.open-s=
td.org%2Fjtc1%2Fsc22%2Fwg21%2Fdocs%2Fpapers%2F2016%2Fp0479r0.html\x26sa\x3d=
D\x26sntz\x3d1\x26usg\x3dAFQjCNFxGsWJ-xIPfVshaBevbFktekHjfQ';return tru=
e;" onclick=3D"this.href=3D'http://www.google.com/url?q\x3dhttp%3A%2F%2=
Fwww.open-std.org%2Fjtc1%2Fsc22%2Fwg21%2Fdocs%2Fpapers%2F2016%2Fp0479r0.htm=
l\x26sa\x3dD\x26sntz\x3d1\x26usg\x3dAFQjCNFxGsWJ-xIPfVshaBevbFktekHjfQ'=
;return true;">http://www.open-std.<wbr>org/jtc1/sc22/wg21/docs/<wbr>papers=
/2016/p0479r0.html</a>=20
<br>> and they do not mention this use case for switches.
<br>
<br>N4741 includes and example with `switch`/`case` in [dcl.attr.likelihood=
]/3.
<br>
<br>I suspect your example with an `if` statement is not doing what you=20
<br>think it does. The [[likely]] applies to the condition expression, whic=
h=20
<br>basically says "this condition is likely to be executed" and =
not "this=20
<br>condition is likely to be true". The correct example is also given=
in=20
<br>the aforementioned section.
<br>
<br>This is the most unfortunate aspect of this proposal as it makes it=20
<br>incompatible with __builtin_expect and tons of code that wraps it in ma=
cros.
<br></blockquote></div>
<p></p>
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.
Author: david.bolvansky@gmail.com
Date: Tue, 8 May 2018 09:33:50 -0700 (PDT)
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>
> Clang 6.0/trunk ignores it yet, GCC takes this hint.
>
> https://godbolt.org/g/dpSDqd
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<blockquote class=3D"gmail_quote" style=3D"margin: 0;margin-left: 0.8ex;bor=
der-left: 1px #ccc solid;padding-left: 1ex;"><div dir=3D"ltr"><blockquote c=
lass=3D"gmail_quote" style=3D"margin:0;margin-left:0.8ex;border-left:1px #c=
cc solid;padding-left:1ex">Clang 6.0/trunk ignores it yet, GCC takes this h=
int.</blockquote></div></blockquote><div>=C2=A0https://godbolt.org/g/dpSDqd=
</div>
<p></p>
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