llvm dev - Feb 2019 - clang emits calls to consexpr function.

Hi Devs,
consider below testcase

$cat test.cpp

constexpr int product()
{

    return 10*20;
}
int main()
{
const   int x = product();
    return 0;
}


$./clang test.cpp -std=c++11 -S
$./clang -v
clang version 9.0.0
Target: x86_64-unknown-linux-gnu

$cat test.s

main:
        .cfi_startproc
# %bb.0:
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset %rbp, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register %rbp
        subq    $16, %rsp
        movl    $0, -4(%rbp)
        callq   _Z7productv    //here you can see the calls to product
function
        xorl    %ecx, %ecx
        movl    %eax, -8(%rbp)
        movl    %ecx, %eax
        addq    $16, %rsp
        popq    %rbp
        .cfi_def_cfa %rsp, 8
        retq


while g++ do not emits calls to constexpr function
$g++ test.cpp -std=c++11
$cat test.s
main:
.LFB1:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        movl    $200, -4(%rbp)
        movl    $0, %eax
        popq    %rbp
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc

is this bug in clang compiler?
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190205/ea40cfba/attachment.html>

On Tue, Feb 5, 2019 at 1:41 PM kamlesh kumar via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi Devs,
> consider below testcase
>
> $cat test.cpp
>
> constexpr int product()
> {
>
>     return 10*20;
> }
> int main()
> {
> const   int x = product();
>     return 0;
> }
>
>
> $./clang test.cpp -std=c++11 -S
> $./clang -v
> clang version 9.0.0
> Target: x86_64-unknown-linux-gnu
>
> $cat test.s
>
> main:
>         .cfi_startproc
> # %bb.0:
>         pushq   %rbp
>         .cfi_def_cfa_offset 16
>         .cfi_offset %rbp, -16
>         movq    %rsp, %rbp
>         .cfi_def_cfa_register %rbp
>         subq    $16, %rsp
>         movl    $0, -4(%rbp)
>         callq   _Z7productv    //here you can see the calls to product
function
>         xorl    %ecx, %ecx
>         movl    %eax, -8(%rbp)
>         movl    %ecx, %eax
>         addq    $16, %rsp
>         popq    %rbp
>         .cfi_def_cfa %rsp, 8
>         retq
>
>
> while g++ do not emits calls to constexpr function
> $g++ test.cpp -std=c++11
> $cat test.s
> main:
> .LFB1:
>         .cfi_startproc
>         pushq   %rbp
>         .cfi_def_cfa_offset 16
>         .cfi_offset 6, -16
>         movq    %rsp, %rbp
>         .cfi_def_cfa_register 6
>         movl    $200, -4(%rbp)
>         movl    $0, %eax
>         popq    %rbp
>         .cfi_def_cfa 7, 8
>         ret
>         .cfi_endproc
>
> is this bug in clang compiler?
It's not a bug. constexpr just means the value can be used as a
constant, it doesn't mean the compiler *has to* compute it as a
constant. If you turn on some optimization, the call will get inlined
and constant folded: https://gcc.godbolt.org/z/M806yO

And if you define "x" as constexpr instead of const, you will force
the
compiler to compute product at compile-time without opimizations
enabled.

On 05/02 14:08, Hans Wennborg via llvm-dev wrote:
> On Tue, Feb 5, 2019 at 1:41 PM kamlesh kumar via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi Devs,
> > consider below testcase
> >
> > $cat test.cpp
> >
> > constexpr int product()
> > {
> >
> >     return 10*20;
> > }
> > int main()
> > {
> > const   int x = product();
> >     return 0;
> > }
> >
> >
> > $./clang test.cpp -std=c++11 -S
> > $./clang -v
> > clang version 9.0.0
> > Target: x86_64-unknown-linux-gnu
> >
> > $cat test.s
> >
> > main:
> >         .cfi_startproc
> > # %bb.0:
> >         pushq   %rbp
> >         .cfi_def_cfa_offset 16
> >         .cfi_offset %rbp, -16
> >         movq    %rsp, %rbp
> >         .cfi_def_cfa_register %rbp
> >         subq    $16, %rsp
> >         movl    $0, -4(%rbp)
> >         callq   _Z7productv    //here you can see the calls to product
function
> >         xorl    %ecx, %ecx
> >         movl    %eax, -8(%rbp)
> >         movl    %ecx, %eax
> >         addq    $16, %rsp
> >         popq    %rbp
> >         .cfi_def_cfa %rsp, 8
> >         retq
> >
> >
> > while g++ do not emits calls to constexpr function
> > $g++ test.cpp -std=c++11
> > $cat test.s
> > main:
> > .LFB1:
> >         .cfi_startproc
> >         pushq   %rbp
> >         .cfi_def_cfa_offset 16
> >         .cfi_offset 6, -16
> >         movq    %rsp, %rbp
> >         .cfi_def_cfa_register 6
> >         movl    $200, -4(%rbp)
> >         movl    $0, %eax
> >         popq    %rbp
> >         .cfi_def_cfa 7, 8
> >         ret
> >         .cfi_endproc
> >
> > is this bug in clang compiler?
> 
> It's not a bug. constexpr just means the value can be used as a
> constant, it doesn't mean the compiler *has to* compute it as a
> constant. If you turn on some optimization, the call will get inlined
> and constant folded: https://gcc.godbolt.org/z/M806yO
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev