Synthesis attributes are directives passed to sythesis tools, such as Quartus. An example of such an attribute in VHDL:

  attribute chip_pin : string;
  attribute chip_pin of sel : signal is "C4";
  attribute chip_pin of data : signal is "D1, D2, D3, D4";

This would instruct the synthesis tool to map the wire sel to pin C4, and wire data to pins D1, D2, D3, and D4. To achieve this in Clash, Attrs are used. An example of the same annotation:

import Clash.Annotations.SynthesisAttributes (Attr (..), Annotate )

myFunc
    :: (Signal System Bool `Annotate` 'StringAttr "chip_pin" "C4")
    -> (Signal System Int4 `Annotate` 'StringAttr "chip_pin" "D1, D2, D3, D4")
    -> ...
myFunc sel data = ...
{-# NOINLINE myFunc #-}

To ensure this function will be rendered as its own module, do not forget a NOINLINE pragma.

Multiple attributes for the same argument can be specified by using a list. For example:

Signal System Bool `Annotate`
  [ 'StringAttr "chip_pin" "C4"
  , 'BoolAttr "direct_enable" 'True
  , 'IntegerAttr "max_depth" 512
  , 'Attr "keep"
  ]

For Verilog see: https://www.intel.com/content/www/us/en/programmable/quartushelp/current/index.htm#hdl/vlog/vlog_file_dir.htm

For VHDL, see: https://www.intel.com/content/www/us/en/programmable/quartushelp/current/index.htm#hdl/vhdl/vhdl_file_dir.htm

Warnings

When using annotations, it is important that annotated arguments are not eta-reduced, as this may result in the annotation being stripped by GHC. For example

f :: Signal System Bool `Annotate` 'StringAttr "chip_pin" "C4"
  -> Signal System Bool
f x = id x -- Using a lambda, i.e. f = x -> id x also works

will reliably show the annotation in the generated HDL, but

g :: Signal System Bool `Annotate` 'StringAttr "chip_pin" "C4"
  -> Signal System Bool
g = id

will not work.

This is an experimental feature, please report any unexpected or broken behavior to Clash's GitHub page (https://github.com/clash-lang/clash-compiler/issues).