verilator/docs/TODO

// DESCRIPTION: Verilator: List of To Do issues.
//
// Copyright 2004-2019 by Wilson Snyder. This program is free software; you can
// redistribute it and/or modify it under the terms of either the GNU
// Lesser General Public License Version 3 or the Perl Artistic License
// Version 2.0.

* Language support:
** Fix ordering of each bit separately in a signal (mips)
   assign b[3:0] = b[7:4];  assign b[7:4] = in;
** Support UDP gate primitives/ cell libraries
   (have code for combos - problem is sequential udps)
** Function to eval combo logic after /*verilator public*/ functions [gwaters]
** Support generated clocks (correctness)
** Recursive functions
** Verilog configuration files
** Expression coverage (see notes)
** Better tristate support
** UVM

* Long-term Features
** Assertions
** Tristate support

* Configure/Make/Install
** Distribute with flex/bison already expanded?
   Flex library not needed.  Probably too difficult to be worth it.

* Testing:
** Capture all inputs into global "rerun it" file
** Code to make wrapper that sets signals, so can do comparison checks
** New random program generator
** Better graph viewer with search and zoom
** Port and test against opencores.org code
** // verilator debug in code so can see only tree affecting those nodes

* Usability:
** Detect and pre-remove most UNOPTFLATs
** Better reporting of unopt problems, including what lines of code
** Report more errors (all of them?) before exiting [Eugene Weber]
** Auto-create scons config files
** Print version/etc message at runtime. (4.000?)
   Include number of lines of code, percent comments, code complexity measurement
   <-80chars------------------------------------------------------------------->
   Verilator 3.600 - The fast free open-sourced simulator.  Copyright 2001-2013.
   Verilated #### modules, #### instances, ##### sigs,
         #### non-comment lines, ##### ops, ### KB model size

* Lint:
** CDCRSTLOGIC should allow filtering with paths
   "waive CDCRSTLOGIC --from a.b.sig  --to a.c.sig --via OR"

* Internal Code:
** A Visitor class that understands how to traverse data types
** V3Graph should be templated container type, taking in Vertex + Edge types
** Instead of string, have an VEncodedString/VIdString which contains __DOT__ish
   things, to reduce bugs.  Also add _20 trailing space to \ encoded names.

* Runtime:
** New evalulation loop   ~/src/verilator/notes/event_loop.txt (4.000?)
** Remove all private internal functions from top level wrapper header, move
   to new level
** Completely standalone simulation
    main() records arguments for $test$plusvars
    instantiates top,
    does tracing  (support $dump?)
    calls top->simulateForever()
    exits

* Performance:
** Latch optimizations
** Constant propagation
   Extra cleaning AND:  1 & ((VARREF >> 1) | ((&VARREF >> 1) & VARREF))
   Extra shift (perhaps due to clean): if (1 & CAST (VARREF >> #))
** Gated clock and latch conversion to flops.  [JeanPaul Vanitegem]
   Could propagate the AND into pos/negedges and let domaining optimize.
** Negedge reset
   Switch to remove negedges that don't matter
   Can't remove async resets from control flops (like in syncronizers)
** If all references to array have a constant index, blow up into separate signals-per-index
** Bit-multiply for faster bit swapping and a=b[1,3,2] random bit reorderings.
** Move _last sets and all other combo logic inside master
   if() that triggers on all possible sense items
** Rewrite and combine V3Life, V3Subst
   If block temp only ever set in one place to constant, propagate it
     Used in t_mem for array delayed assignments
   Replace variables if set later in same cfunc branch
     See for example duplicate sets of _narrow in cycle 90/91 of t_select_plusloop
** Same assignment on both if branches
   "if (a) { ... b=2; } else { ... b=2;}" -> "b=2; if ..."
   Careful though, as b could appear in the statement or multiple times in statement
   (Could just require exatly two 'b's in statement)
** Simplify XOR/XNOR/AND/OR bit selection trees
   Foo = A[1] ^ A[2] ^ A[3] etc are better as ^ ( A & 32'b...1110 )
** Combine variables into wider elements
   Parallel statements on different bits should become single signal
   Variables that are always consumed in "parallel" can be joined
** Duplicate assignments in gate optimization
   Common to have many separate posedge blocks, each with identical
   reset_r <= rst_in
** If signal is used only once (not counting trace), always gate substitute
   Don't merge if any combining would form circ logic (out goes back to in)
** Multiple assignments each bit can become single assign with concat
   Make sure a SEL of a CONCAT can get the single bit back.
** Usually blocks/values
   Enable only after certain time, so VL_TIME_I(32) > 0x1e gets eliminated out
** Better ordering of a<=b, b<=c, put all refs to 'b' next to each other to optimize caching
** I-cache packing improvements (what/how?)
** Data cache organization (order of vars in class)
   First have clocks,
   then bools instead of uint32_t's
   then based on what sense list they come from, all outputs, then all inputs
   finally have any signals part of a "usually" block, or constant.
** Rather then tracking widths, have a MSB...LSB of this expression
   (or better, a bitmask of bits relevant in this expression)
** Track recirculation and convert into clock-enables
** Clock enables should become new clocking domains for speed
** If floped(a) & flopped(b) and no other a&b, then instead flop(a&b).
** Sort by output bitselects so can combine more assignments (see DDP example dx_dm signal)