[libc-dev] __builtin_* vs llvm-libc provided ones?

Siva Chandra via libc-dev libc-dev at lists.llvm.org
Tue Feb 2 11:07:56 PST 2021

On Mon, Feb 1, 2021 at 2:58 AM Ebrahim Byagowi <ebraminio at gmail.com> wrote:

> To describe in maybe some better way, for example this is all I need to
> get ceilf floorf etc in a .wasm module being built by -nostdlib -nostdinc
> #define ceilf __builtin_ceilf
> #define floorf __builtin_floorf
> #define abs __builtin_abs
> #define fabs __builtin_fabs
> so I wondered if I could get more of libc this way (parts make sense ofc)
> or at least to know what will be the relation between those builtin
> implementations and the upcoming libc.

IIUC, there are two parts to your question:
1. Can we implement a libc function as a macro resolving to a builtin: Not
if the standard requires the function to be a real addressable function.
One can choose to also provide a macro, but an addressable function
declaration should be available. See section 7.1.4 of the C11 standard for
more information.
2. What is the difference between builtins and the libc flavors of the
functions: Typically, builtins resolve to the hardware instruction
implementing the operation. If a hardware implementation is not available,
the compiler builtin calls into the libc itself. With respect to math
functions, you will notice this wilh the `long double` flavors. That said,
we have implemented the math functions from first principles (as in, the
implementations do not assume any special hardware support) in LLVM libc.
However, we are just about starting to add machine specific implementations
(https://reviews.llvm.org/D95850). This should make the libc functions
equivalent to the compiler builtins.
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