| name | rtl-area-timing |
| description | Use when optimizing RTL microarchitecture for area or clock frequency (Fmax), a design is too big to fit or too slow to meet timing, a wide multiply or barrel shifter is the critical path, or a "compute everything and select" datapath is too large |
RTL Area and Timing Optimization
Overview
Making RTL smaller or faster is a sequence of structural decisions, each justified by a measurement. The wins are rarely where intuition points: the giant is often a structure you didn't think of (a "ROM" that is really 94k flops), and the critical path is usually one specific primitive, not "logic depth" in general.
Core principle: Diagnose with data, change one structure, re-measure. Optimize the actual critical path or the actual giant, and stop the moment it stops being the bottleneck. Guessing wastes builds and can place worse.
When to Use
- A design won't fit, or misses its timing constraint
- A wide multiply, barrel shifter, or big mux is suspected of dominating
- A microcoded or "compute all handlers and select" datapath is too large
- You're about to "optimize" something without having read the reports
This is the RTL-technique companion to fpga-synthesis-fit (the tool methodology for measuring). Measure there, transform here.
Pipeline A Wide Multiply Internally
A single-cycle NxN multiply (64x64) maps to DSP tiles plus a long partial-product carry chain, and that chain is usually the critical path.
Registering only the multiply's OUTPUT does not break the internal carry chain; the operands-to-output path is still essentially the whole multiply. You must pipeline INTERNALLY: decompose into smaller products (four 32x32), register the partial products, then sum the shifted partials in a second registered stage. Make the op multi-cycle with a small stall counter. On ECP5 this took a 64x64 from about 33 MHz to about 47 MHz.
Registering the multiply INPUTS too gave diminishing returns and placed worse. Stop once the multiply leaves the critical path; re-read the report to confirm.
"Compute Everything And Select" Is Area-Heavy
A microcoded exec or decoder that computes all handler datapaths in parallel and muxes the winner by opcode builds every handler's logic. Real area wins:
- Remove unreachable/dead arms. A memory-size case covering byte/half for atomics, when atomics only exist at word/dword, is dead logic. Provably correct, removes structurally-distinct logic, about 6% in one case.
- Share a single resource (one ALU, one memory port) routed by control signals.
- Do not source-level deduplicate identical operand reads. yosys already CSEs them, so it is a no-op for area. Only removing structurally-distinct logic (dead arms, different widths, a separate adder) actually shrinks the design.
Variable Barrel Shift: Replace With Fixed-Slice Mux, But Measure
addr >> (base + k*stride) is a 64-bit barrel shifter plus a multiply. If it extracts a FIXED field per k (a page-table VPN[level] slice), replace it with a mux of fixed slices.
But on ECP5 this can be post-pack neutral: barrel shifters and mux trees pack to similar slice counts. It reduces pre-pack LUT4 but may not move TRELLIS_COMB. Measure post-pack before believing the win (see the metric trap in fpga-synthesis-fit).
Share One Comparator Across Signed/Unsigned And Widths
Sign-extension from W to 2W bits is monotonic for BOTH signed and unsigned W-bit ordering (it maps the two halves of the W-bit range to two ordered ranges in the wider unsigned space). So one comparator on sign-extended operands computes signed min/max AND unsigned minu/maxu, useful for AMOs and ALUs.
Verify the stored RESULT width: compute at XLEN but store the low size.bits, and match the surrounding code's sign-vs-zero extension of the stored value. A test once caught a sign/zero-extend mismatch on the store side here.
Verification Discipline That Paid Off
- Build the optimized unit standalone first with a golden test (bit-for-bit vs a reference) before wiring it in. De-risks correctness and locks the interface.
- After every structural change, re-run the full functional matrix. A cycle-accurate vs-reference matrix catches FSM/timing regressions a hand-picked test misses; it caught an off-by-one stall, a stale read, and an AMO store-extend bug. See
differential-verification.
- When you change the read latency of a shared memory, give the sim model the same latency as the FPGA primitive so the matrix verifies real hardware behavior, not a faster sim variant.
Spec "May" vs "Must": Don't Call Permissible Behavior A Bug
Before making hardware stricter to "fix" it, check whether the old behavior is permitted by the spec. Pre-Svade RISC-V PERMITS hardware page-table A/D-bit update; "always update A/D" is a legal implementation, not a violation. Making it Svade-strict (fault on A=0) is an ISA-policy choice tied to what the core advertises (Svadu/Svade), and it needs the tests' page tables updated to set A/D. It is not a free correctness fix. One such change hung 12 tests for no clear gain and was reverted.
Process
- Diagnose with data before optimizing: per-module cell counts, the critical-path report, the generic-cell-type breakdown (
$mux vs $add vs $sdffe). Don't guess the bottleneck.
- The biggest area win is often a structural surprise (a flop-ROM), not the thing you assumed (interpreter logic depth).
- When a build thrashes or hangs, check whether it is converging (a trend) before killing it; conversely, don't wait hours on a flat-lined metric.
Midstall House Style
- River on ECP5 is the reference: internal multiply pipelining, microcode area trims, comparator sharing, all measured against the matrix and the post-pack reports.
- No em dashes, no emoji. Measure with
fpga-synthesis-fit; verify with differential-verification.
