Small constants can make C compiler assume signing

[veripoolbot]

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Author Name: Clifford Wolf (@cliffordwolf)
Original Redmine Issue: 773 from https://www.veripool.org
Original Date: 2014-05-23
Original Assignee: Wilson Snyder (@wsnyder)

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This should return y=0 for a=31, but Verilator 06744b6 returns y=31 instead:

 module issue_050(a, y);
   input [4:0] a;
   output [4:0] y;
   assign y = a >> ((a ? 1 : 2) << a);
 endmodule

Self-contained test case:

http://svn.clifford.at/handicraft/2014/verilatortest/test017.v

http://svn.clifford.at/handicraft/2014/verilatortest/test017.cc

http://svn.clifford.at/handicraft/2014/verilatortest/test017.sh

[veripoolbot]

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Original Redmine Comment
Author Name: Wilson Snyder (@wsnyder)
Original Date: 2014-05-24T12:04:05Z

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Fixed in git towards 3.861.

Interesting as this precise use of small constants caused the emitted code to be such that the C compiler saw a small constant on both sides of a comparison, so it used a signed compare instead of the intended unsigned compare. Now forcing unsigned.

[veripoolbot]

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Original Redmine Comment
Author Name: Wilson Snyder (@wsnyder)
Original Date: 2014-06-11T00:58:32Z

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In 3.862.