Most of the time I know the number of bits and I'd just use the proper hex
mask:

uint32_t filled = (test_var & 0x80000000) ? 0xFFFFFFFF : 0;

Or if you want to avoid branches and are okay with a few extra cycles:
uint32_t filled = ((test_var >> 31) & 0x01) * 0xFFFFFFFF;
(That's just from the top of my head. Maybe there's something better than
multiplication?)

On Fri, Jun 27, 2025 at 9:11 AM Tiago Freire via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Isn't sign_fill just:
>
> //preparation
> using u_t = std::make_unsigned_t<T>;
> constexpr u_t mask = u_t{1} << (std::numeric_limits<u_t>::digits - 1);
>
> //actual conversion
> T filled = ( std::bit_cast<u_t>(test_var) & mask ) ? T{-1} : T{0};
>
>
> -----Original Message-----
> From: Jan Schultke <janschultke_at_[hidden]>
> Sent: Friday, June 27, 2025 08:50
> To: Tiago Freire <tmiguelf_at_[hidden]>
> Cc: std-proposals_at_[hidden]
> Subject: Re: [std-proposals] More C++ bit manipulation utilities
>
> > I’ve used a version of all of these, semi-regularly.
> > However, they are so trivial to implement that it has never occurred to
> me to write a function to do those.
>
> How do you write sign_fill usually then? Do you just use ">>
> (std::numeric_limits<T>::digits - 1)" in every place where you sign-fill?
>
> > It might be easier to sell these by packaging them with a set of bit
> > operations, like a shift out (i.e. left or right shift overflow bits
> > goes into a separate integer), mask multiple bits, bit order swaps,
> > etc…
>
> I'm not sure what you mean by "mask multiple bits"; that sounds much like
> bit_mask, which is on the list.
>
> bit_reverse (bit order swaps) is design-approved for C++29 as part of Bit
> permutations already.
>
> obtaining the shifted-out bits sounds interesting. That's useful for
> bit-shifting numbers in multi-precision arithmetic.
> --
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