Strange combinational circuit

[Martin Schoeberl]

Hi all,

I have following Chisel code:

val limit = r1 === CNT_MAX
val tickb = when(limit) { Bits(1) } .otherwise { Bits(0) }

The intention is that this resembles the C/Java expression: val = cond ? trueVal : falseVal.
I know that one can use a Mux or in this case directly tick = limit. However, this code
compiles and it does something.

More interesting is looking at two versions:

val limit = r1 === CNT_MAX

val ticka = when(limit) { Bits(0) } .otherwise { Bits(1) }
val tickb = when(limit) { Bits(1) } .otherwise { Bits(0) }

Both variables (ticka and tickb) get the same value: 0 when limit is true, 1 when false.

So either:
(1) this is code that should not be legal Chisel code, then it should not be accepted by the compiler; or
(2) this is an error in Chisel; or
(3) I'm completely off and don't know what I do and don't understand all about Chisel/Scala ;-)

Anyway, if one would like to look into this, here is a small example to reproduce the issue
and look it up with gtkwave:

git clone git://github.com/schoeberl/comphdl.git
cd comphdl/chisel
make test MAIN=error.CombErrMain
gtkwave ErrorContainer.vcd

Cheers,
Martin

[Jonathan Bachrach]

On Feb 15, 2013, at 5:56 PM, Martin Schoeberl <mar...@jopdesign.com> wrote:

> Hi all,
>
> I have following Chisel code:
>
> val limit = r1 === CNT_MAX
> val tickb = when(limit) { Bits(1) } .otherwise { Bits(0) }
>
> The intention is that this resembles the C/Java expression: val = cond ? trueVal : falseVal.
> I know that one can use a Mux or in this case directly tick = limit. However, this code
> compiles and it does something.

"when" is not an expression but a statement and is not supposed to be used to return a value.
instead it is meant for doing conditional updates (e.g., a := b).
you need to use Mux for expressions.

you can use when to do a counter by doing:

val limit = r1 === CNT_MAX

when (limit) { c := c + Bits(1) }

or

val limit = r1 === CNT_MAX

val tickb = Mux(limit, Bits(1), Bit(0))
c := c + tickb

> More interesting is looking at two versions:
>
> val limit = r1 === CNT_MAX
>
> val ticka = when(limit) { Bits(0) } .otherwise { Bits(1) }
> val tickb = when(limit) { Bits(1) } .otherwise { Bits(0) }
>
> Both variables (ticka and tickb) get the same value: 0 when limit is true, 1 when false.
>
> So either:
> (1) this is code that should not be legal Chisel code, then it should not be accepted by the compiler; or
> (2) this is an error in Chisel; or
> (3) I'm completely off and don't know what I do and don't understand all about Chisel/Scala ;-)

i've never seen this error before because once you know that you can't use "when" for an expression then you just don't do this.

> Anyway, if one would like to look into this, here is a small example to reproduce the issue
> and look it up with gtkwave:

where did you get the idea that when returns a value?
let me know and i'll correct the documentation.

> git clone git://github.com/schoeberl/comphdl.git
> cd comphdl/chisel
> make test MAIN=error.CombErrMain
> gtkwave ErrorContainer.vcd
>
> Cheers,
> Martin
>

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[Martin Schoeberl]

On 16 Feb 2013, at 05:08, Jonathan Bachrach <jackba...@gmail.com> wrote:

> On Feb 15, 2013, at 5:56 PM, Martin Schoeberl <mar...@jopdesign.com> wrote:
>
>> Hi all,
>>
>> I have following Chisel code:
>>
>> val limit = r1 === CNT_MAX
>> val tickb = when(limit) { Bits(1) } .otherwise { Bits(0) }
>>
>> The intention is that this resembles the C/Java expression: val = cond ? trueVal : falseVal.
>> I know that one can use a Mux or in this case directly tick = limit. However, this code
>> compiles and it does something.
>
> "when" is not an expression but a statement and is not supposed to be used to return a value.
> instead it is meant for doing conditional updates (e.g., a := b).
> you need to use Mux for expressions.

I understand. I was just playing around to get this ? : expression.

So is there a chance to actually make

val x = when ...

a compile error instead of 'interesting' behavior?

> you can use when to do a counter by doing:
>
> val limit = r1 === CNT_MAX
> when (limit) { c := c + Bits(1) }
>
> or
>
> val limit = r1 === CNT_MAX
> val tickb = Mux(limit, Bits(1), Bit(0))
> c := c + tickb
>
>> More interesting is looking at two versions:
>>
>> val limit = r1 === CNT_MAX
>>
>> val ticka = when(limit) { Bits(0) } .otherwise { Bits(1) }
>> val tickb = when(limit) { Bits(1) } .otherwise { Bits(0) }
>>
>> Both variables (ticka and tickb) get the same value: 0 when limit is true, 1 when false.
>>
>> So either:
>> (1) this is code that should not be legal Chisel code, then it should not be accepted by the compiler; or
>> (2) this is an error in Chisel; or
>> (3) I'm completely off and don't know what I do and don't understand all about Chisel/Scala ;-)
>
> i've never seen this error before because once you know that you can't use "when" for an expression then you just don't do this.

Ok, then it is (3) and I did something completely odd ;-)

I think it came from the Scala way that you have just an expression for
a return value from a function and you can write:
if xxx then y else z

>> Anyway, if one would like to look into this, here is a small example to reproduce the issue
>> and look it up with gtkwave:
>
> where did you get the idea that when returns a value?
> let me know and i'll correct the documentation.

I just wanted to avoid the Mux as it is not so clear which
value is returned when. First when the expression is true, or
when it is 0.

Martin