The asker wrote a parameterized signed saturating truncator: take a WIN-bit signed input, produce a WOUT-bit signed output that clamps to MAX/MIN if the input falls outside the representable range, and sign-extends when WOUT >= WIN. Icarus and Verilator simulate the module correctly, but SynplifyPro synthesizes hardware whose behavior diverges from both simulators — a classic "passes sim, fails on the board" trap.

Why this is interesting: Saturation logic is one of the most error-prone idioms in RTL because it mixes three sharp edges of the Verilog language at once:

• Signedness propagation — comparison and concatenation operators silently drop the signed attribute if any operand is unsigned. A single literal like 1'b1 can promote the whole expression to unsigned and invert the meaning of a < against a negative value.
• Width extension rules — RHS expressions are extended to the context width, but the extension is sign-extension only when every operand is signed. Parameterized widths (WIN, WOUT) make this hard to eyeball.
• Synthesis vs. simulation interpretation — IEEE 1364/1800 leaves a few corners loosely specified, and vendor synthesizers historically take different shortcuts than event-driven simulators, particularly around constant-folding signed comparisons with mixed-width operands.

Direction toward an answer:

• Reproduce by writing a small directed testbench that drives boundary values: ±(2^(WOUT-1)), ±(2^(WOUT-1))-1, and the extreme values of WIN. Compare RTL sim against a Synplify post-synthesis netlist simulation (not just on-hardware behavior) — that pins down whether the bug is in synthesis or in P&R.
• Audit every operand of every comparison: add explicit $signed(...) casts on each side of < and >. Verify literals are written as WIN'sd... rather than bare decimals.
• Check Synplify's log for warnings like "comparison between signed and unsigned" or "width mismatch". Synplify is generally noisy here and the warning often points at the exact line.
• As a sanity check, rewrite the saturation as two ranges checks on the upper bits being dropped: upper_bits == {WIN-WOUT+1{in[WIN-1]}} — a sign-replication check that avoids comparisons entirely.

Gotchas: when WOUT >= WIN the saturation branch must be unreachable, but generate-time conditionals often still synthesize the dead arm and can introduce unsigned literals into the always block. Wrap the saturation in a generate if so the dead logic vanishes from the netlist. Also: if any intermediate is declared reg rather than reg signed, sign info is lost across the assignment regardless of the RHS.