[sv-bc] sign/width casting semantics

Hi,

There is unclearness about the semantics of sign casting and width
casting.

For casting to a full type, the semantics are clear:

"If the expression is assignment compatible with the casting type, then
the cast shall return the value that a variable of the casting type
would hold after being assigned the expression."

However, how do sign casting and width casting work?

The LRM says,

"When changing the size, the signing shall pass through unchanged and
the result type shall be a one-dimensional packed array with a right
bound of zero. When changing the signing, the type of the expression to
be cast shall pass through unchanged, except for the signing."

Let's take an example:

logic signed [7:0] regS;
regS = signed'(4'b1100);

My understanding is that "signed'(4'b1100)" turns into 4'sb1100, which
is then sign-extended to the width of regS, giving 8'sb11111100. This
means it works the same as $signed().

However, some people (e.g., Greg Jaxon) say that the context-determined
width of the entire expression, 8 bits, is propagated into the cast
operand, while the sign cast determines the signedness of the
size-extended expression, giving 8'sb00001100, giving a different result
from $signed().

Look at 11.7, which contains these examples:

logic signed [7:0] regS;
regS = $signed (4'b1100); // regS = -4
regS = signed'(4'b1100); // regS = -4

The $signed() example comes from 1364-2005. The signed'() example was
added by the editor during the LRM merge. He assumed it is identical to
$signed(). Is this correct?

Another example, from the $clog2() discussion recently: Greg wrote
(slightly edited),

     (expr) | signed'(1'b1)
...
Note that 1'sb1 would be sign-extended to match width (bad!), whereas
signed'(1'b1) will 0-extend the literal before cloaking it with
signedness (so as to not poison expr with unsignedness).

and I disagree with Greg.

The question about width casting is similar. Is the expression inside
the width-cast affected by the signedness of the surrounding expression
before changing its width (e.g., changing it from signed to unsigned,
causing zero-extension instead of sign-extension)?

The committee needs to decide on the correct semantics and clarify them
in the LRM. Tools are divergent on this already.

Some relevant links:

http://www.eda-stds.org/sv-bc/hm/1346.html
http://www.eda-stds.org/sv-bc/hm/1714.html
http://www.eda.org/sv-bc/display_issue.cgi?issue_num=38
http://www.eda-stds.org/sv-bc/hm/1777.html
http://www.eda-stds.org/sv-bc/hm/1780.html
http://www.eda-stds.org/sv-bc/hm/1790.html
http://www.eda-stds.org/sv-bc/hm/1794.html
http://www.eda-stds.org/sv-bc/hm/1796.html
http://www.eda-stds.org/sv-bc/hm/1800.html


Thanks,
Shalom

Shalom Bresticker
Intel Jerusalem LAD DA
+972 2 589-6582
+972 54 721-1033

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