Add "nowrshmsk" attribute, fix shift-and-mask bit slice write for signed offset, fixes #1990

Signed-off-by: Claire Wolf <claire@symbioticeda.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/README.md b/README.md
index 1e486c3ac..c17c0c3b1 100644
--- a/README.md
+++ b/README.md
@@ -281,6 +281,9 @@ Verilog Attributes and non-standard features
   temporary variable within an always block. This is mostly used internally
   by Yosys to synthesize Verilog functions and access arrays.
 
+- The ``nowrshmsk`` attribute on a register prohibits the generation of
+  shift-and-mask type circuits for writing to bit slices of that register.
+
 - The ``onehot`` attribute on wires mark them as one-hot state register. This
   is used for example for memory port sharing and set by the fsm_map pass.