From: whitequark <whitequark@whitequark.org>
Date: Mon, 13 Apr 2020 15:56:39 +0000 (+0000)
Subject: back.rtlil: fix expansion of Part() for partial dummy writes.
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=7c3f7b46a6b0af56aedd5fbed7552cda0d9777da;p=nmigen.git

back.rtlil: fix expansion of Part() for partial dummy writes.

Before this commit, selecting a part that was fully out of bounds of
a value was correctly implemented as a write to a dummy wire, but
selecting a part that was only partially out of bounds resulted in
a crash.

Fixes #351.
---

diff --git a/nmigen/back/rtlil.py b/nmigen/back/rtlil.py
index c074a97..8433fe2 100644
--- a/nmigen/back/rtlil.py
+++ b/nmigen/back/rtlil.py
@@ -632,12 +632,14 @@ class _LHSValueCompiler(_ValueCompiler):
     def on_Part(self, value):
         offset = self.s.expand(value.offset)
         if isinstance(offset, ast.Const):
-            if offset.value == len(value.value):
-                dummy_wire = self.s.rtlil.wire(value.width)
-                return dummy_wire
-            return self(ast.Slice(value.value,
-                                  offset.value * value.stride,
-                                  offset.value * value.stride + value.width))
+            start = offset.value * value.stride
+            stop  = start + value.width
+            slice = self(ast.Slice(value.value, start, min(len(value.value), stop)))
+            if len(value.value) >= stop:
+                return slice
+            else:
+                dummy_wire = self.s.rtlil.wire(stop - len(value.value))
+                return "{{ {} {} }}".format(dummy_wire, slice)
         else:
             # Only so many possible parts. The amount of branches is exponential; if value.offset
             # is large (e.g. 32-bit wide), trying to naively legalize it is likely to exhaust