To properly represent a negation of a signed X-bit quantity we may, in
general, need a signed (X+1)-bit signal — for example, negation of
3-bit -4 is 4, which is not representable in signed 3 bits.
We don't have any other convenient shortcut for x[off*w:(off+1)*w],
but using word_select to extract a single static range would result
in severe bloat of emitted code through expansion to dead branches.
Recognize and simplify this pattern.
Although constructor methods can improve clarity, there are many
contexts in which it is useful to use range() as a shape: notably
Layout, but also Const and AnyConst/AnyValue. Instead of duplicating
these constructor methods everywhere (which is not even easily
possible for Layout), use casting to Shape, introduced in 6aabdc0a.
Fixes#225.
Shapes have long been a part of nMigen, but represented using tuples.
This commit adds a Shape class (using namedtuple for backwards
compatibility), and accepts anything castable to Shape (including
enums, ranges, etc) anywhere a tuple was accepted previously.
In addition, `signed(n)` and `unsigned(n)` are added as aliases for
`Shape(n, signed=True)` and `Shape(n, signed=False)`, transforming
code such as `Signal((8, True))` to `Signal(signed(8))`.
These aliases are also included in prelude.
Preparation for #225.
This can cause confusion:
* If the erroneous object is None, it is printed as 'None', which
appears as a string (and could be the result of converting None
to a string.)
* If the erroneous object is a string, it is printed as ''<val>'',
which is a rather strange combination of quotes.
Almost no code would specify Signal(_, name) as a positional argument
on purpose, but forgetting parens and accidentally placing signedness
into the name position is so common that we had a test for it.
Also, replace `bits, sign = x.shape()` with more idiomatic
`width, signed = x.shape()`.
This unifies all properties corresponding to `len(x)` to `x.width`.
(Not all values have a `width` property.)
Fixes#210.
This simplifies creation of related signals with nice names during
metaprogramming, e.g.
def make_ff(m, sig):
sig_ff = Signal.like(sig, name_suffix="_ff")
m.d.sync += sig_ff.eq(sig)
return sig_ff
