[libre-riscv-dev] spike-sv non-default element widths

[lkcl]

On Wed, Oct 10, 2018 at 10:02 PM lkcl <lkcl at libre-riscv.org> wrote:
>
> On Wed, Oct 10, 2018 at 9:34 PM Jacob Lifshay <programmerjake at gmail.com> wrote:
> >
> > On Wed, Oct 10, 2018 at 7:45 AM Luke Kenneth Casson Leighton <lkcl at lkcl.net>
> > wrote:
> >
> > > .... actually.. ah.... for signed and unsigned integers, is it
> > > _actually_ the case that the size of the type will affect the answer,
> > > if typecast down to e.g. int16_t/uint16_t or int8_t/uint8_t?
> > >
> > >  by that i mean:
> > >
> > >  uint64_t x = 0..255;
> > >  uint64_t y = 0..255;
> > >  uint8_t x1 = (uint8_t)x;
> > >  uint8_t y1 = (uint8_t)y;
> > >
> > >  assert (x / y) == (x1 / y1)
> > >
> > > or any other operator.  will that hold for all possible values of x
> > > and y?  i mean, i _could_ write a for-loop to check... :)
> > >
> > Assuming the compare only operates on the lower 8 bits, it holds for
> > mul/add/sub/bitwise-logic/left-shift. It doesn't hold for div/mod or
> > right-shift. That's why there are only 1 of add/sub/and/or/xor/mul in RV
> > instead of a signed/unsigned version of each of them.
> >
> > It works similarly for any wider bitwidth simulating a narrower bitwidth as
> > long as the upper bits of the results are ignored.

 https://git.libre-riscv.org/?p=riscv-isa-sim.git;a=blob;f=operators/operators.t.cc;h=b5ac0185adba82ad8424401a50807f63ecb383ea;hb=c3d43aa1e221c74936adac7673f1923cbec5fdd0

 ok so for all possible int8_t values 0-255 on +-*/% >> << when first
typecast to uint8_t into a uint16_t, a uint16_t answer is correct

            int8_t x=0, y=0;
            int16_t x1, y1;
            int8_t z = x op y;
            x1 = (uint8_t)x;
            y1 = (uint8_t)y;
            uint16_t z1 = x1 op y1;

all answers match.

so i believe it would be safe to do the following algorithm (for integers)

* work out the maximum bitwidth of RS1 and RS2, called MB
* take RS1 of width N (8/16/32)
* typecast it to unsigned integer of width N
* sign-extend from N bitwidth to MB bitwidth
* return it in reg_t (which is a uint64_t)
* likewise with RS2 (for its own version of N)
* carrry out the answer
* let sign-extending take place (sext32)
* on storing in RD, truncate answer to bitwidth M

all three bitwidths *may* be different.

l.