Re: [PATCH for-9.1 09/19] target/i386: move 60-BF opcodes to new decoder

[Paolo Bonzini <pbonzini@redhat.com>]

On Thu, Apr 11, 2024 at 9:47 AM Richard Henderson
<richard.henderson@linaro.org> wrote:
> > +    case MO_32:
> > +#ifdef TARGET_X86_64
> > +        /*
> > +         * This could also use the same algorithm as MO_16.  It produces fewer
> > +         * TCG ops and better code if flags are needed, but it requires a 64-bit
> > +         * multiply even if they are not (and thus the high part of the multiply
> > +         * is dead).
> > +         */
>
> Is 64-bit multiply ever slower these days?
> My intuition says "slow" multiply is at least a decade out of date.

I was thinking more about TCG_TARGET_REG_BITS == 32.

> > +        tcg_gen_negsetcondi_i32(TCG_COND_LT, s->tmp2_i32, s->tmp2_i32, 0);
>
> This seems like something the optimizer should handle, but doesn't.

I wanted to avoid using TARGET_LONG_BITS - 1, but it's not a problem
to use extract. I've changed it.

At least the ppc and x86 backends do support it and convert it to SAR,
so I didn't notice in my test that it was the backend doing it and not
the optimizer!

Paolo