On Sat, 2023-09-30 at 07:48 -0700, Alexei Starovoitov wrote:
> On Fri, Sep 29, 2023 at 2:03 PM Dave Thaler
> <dthaler=40microsoft.com@xxxxxxxxxxxxxx> wrote:
> >
> > Perhaps text like the proposed snippet quoted in the exchange above should be
> > added around the new text that now appears in the doc, i.e. the ambiguous text
> > is currently:
> > > For signed operations (``BPF_SDIV`` and ``BPF_SMOD``), for ``BPF_ALU``,
> > > 'imm' is interpreted as a 32-bit signed value. For ``BPF_ALU64``, 'imm'
> > > is first :term:`sign extended<Sign Extend>` from 32 to 64 bits, and then
> > > interpreted as a 64-bit signed value.
>
> That's what we have in the doc and it's a correct description.
> Which part is ambiguous?
>
As far as I understand Dave suggests to add exact specification for
the SMOD instruction as "signed modulo" might have different definitions [1].
I double checked and current clang implementation generates SMOD for
LLVM's 'srem' operation [2], which follows C semantics and defines
remainder as:
remainder = a - n * trunc(divident / divisor)
> This instruction returns the remainder of a division (where the
> result is either zero or has the same sign as the dividend, op1)
And this is consistent with interpreter logic in core.c, e.g.:
ALU64_MOD_K:
switch (OFF) {
case 0: ... break;
case 1:
AX = div64_s64(DST, IMM); /* implemented as '/' */
DST = DST - AX * IMM;
break;
}
CONT;
[1] https://en.wikipedia.org/wiki/Modulo
[2] https://llvm.org/docs/LangRef.html#srem-instruction
