Re: [PATCH 03/72] qemu/host-utils: Add wrappers for carry builtins

[Richard Henderson]

On 5/10/21 7:57 AM, Alex Bennée wrote:
> Richard Henderson <richard.henderson@linaro.org> writes:
>
> > These builtins came in clang 3.8, but are not present in gcc through
> > version 11.  Even in clang the optimization is not ideal except for
> > x86_64, but no worse than the hand-coding that we currently do.
>
> Given this statement....

I think you mis-read the "except for x86_64" part?

Anyway, these are simply bugs to be filed against clang, so that hopefully 
clang-12 will do a good job with the builtin.  And as I said, while the 
generated code is not ideal, it's no worse.

> > +static inline uint64_t uadd64_carry(uint64_t x, uint64_t y, bool *pcarry)
> > +{
> > +#if __has_builtin(__builtin_addcll)
> > +    unsigned long long c = *pcarry;
> > +    x = __builtin_addcll(x, y, c, &c);
>
> what happens when unsigned long long isn't the same as uint64_t? Doesn't
> C99 only specify a minimum?

If you only look at C99, sure.  But looking at the set of supported hosts, 
unsigned long long is always a 64-bit type.

> > +    *pcarry = c & 1;
>
> Why do we need to clamp it here? Shouldn't the compiler automatically do
> that due to the bool?

This produces a single AND insn, instead of CMP + SETcc.


r~