[llvm-dev] clang emits calls to consexpr function.

[Michiel Derhaeg via llvm-dev]

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
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