[myhdl-list] [PATCH 2 of 2 V2] toVHDL: add support for negative indexes

[Angel E. <ang...@gm...>]

# HG changeset patch
# User Angel Ezquerra <ang...@gm...>
# Date 1350086923 -7200
# Branch 0.8-dev
# Node ID b8fa9fcfe7f2f4712c64b98da346191f57fef193
# Parent  c641c1ed333d6d03d9cee4a63df5111fe98574c8
toVHDL: add support for negative indexes

This patch tries to detect negative indexes and converts them into indexes
relative to the highest bit position (with -1 corresponding to the highest bit).

This is similar to what Python does with negative indexes for regular lists.

Two types of "negative indexes" are supported with this patch:

- Explicit negative numbers (e.g. -1, -3, etc)
- Explicit negative unary operator surrounding any other operation
(e.g. -(2 * 3))

Other cases in which the index may be negative are not detected. Negative
indexes are not supported for slices either (yet), just for simple one bit
index accesses. Despite these limitations I think that this functionality can be
quite useful.

As an example, the following simple test case converts fine with this patch:

~~~
#!/usr/env python
from myhdl import *

def test_module(rst, clk):
    '''This block generates the radio frame synchronization signal'''
    s_output = Signal(intbv(0)[16:])
    s_input = Signal(intbv(0)[16:])

    @always(clk.posedge, rst.posedge)
    def p_process():
        if rst == 1:
            s_input[-1] = 1
            s_input[-(3*2)] = 1
            s_output.next[-2] = s_input[-3]
        else:
            pass


    return instances()

clk, rst = [Signal(bool()) for n in range(2)]

if __name__ == '__main__':
    sync_inst = toVHDL(test_module3, rst, clk)
~~~

diff --git a/myhdl/conversion/_toVHDL.py b/myhdl/conversion/_toVHDL.py
--- a/myhdl/conversion/_toVHDL.py
+++ b/myhdl/conversion/_toVHDL.py
@@ -1293,6 +1293,19 @@
         self.visit(node.value)
         self.write("(")
         #assert len(node.subs) == 1
+        
+        def isnegativeindex(astvalue):
+            if isinstance(astvalue, ast.Num):
+                if astvalue.value < 0:
+                    return True
+            elif isinstance(astvalue, ast.UnaryOp):
+                if isinstance(astvalue.op, ast.USub):
+                    return True
+            return False
+        if isnegativeindex(node.slice.value):
+            # convert negative indexes into indexes from the MSB
+            self.write('%d + ' % node.value.obj._nrbits)
+
         self.visit(node.slice.value)
         self.write(")")
         self.write(suf)