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Ch444: serialize gradient numerator bank — reclaim ~half the FPGA's DSP

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The per-triangle gradient engine already time-shared ONE divider across all
GRAD_STEPS attributes, but computed every grad_load_num[0:GRAD_STEPS-1] numerator
IN PARALLEL — ~44 wide multiplies (~100 physical DSP) for once-per-triangle setup
consumed one-at-a-time. Pure redundant hardware; the design was DSP-maxed
(187/188, 99%) so nothing new could fit (fog needed 191/188).

Replace the parallel bank + the grad_num_q[] pre-latch array with grad_num_step:
computes ONLY the current grad_step's numerator from ONE mux-selected pair of
signed multipliers (attribute triple by grad_step>>1, axis by grad_step[0]; shared
da1/da2, two shared products, signed subtract, <<<20). grad_word_q/grad_slot are
held stable the whole solve, so it is bit-identical to the old grad_num_q[grad_step].
Removed grad_num_dadx/dady (inlined once). FSM sequencing and throughput unchanged.

Width note: da1/da2 are 33-bit (products 50-bit), NOT operand-width 32-bit — the
original (a1-a0) lived in a signed-64-bit expression context and never wrapped;
full-32-bit Z with |a1-a0|>2^31 needs the wider intermediate. tb_gs_grad_num_equiv
(extreme signed corners + 200k random = 494770 checks, 0 errors) caught a 32-bit
first cut that f52's real data never exercised.

Resource (26.1 Seed-3 fit): DSP needed 168->83 / final placement 187->119, i.e.
99% -> 44%, ~85 blocks reclaimed (Codex gate >=70 met). ALM 40458->38784 (86->83%).
RAM 322/358 unchanged. Timing CLEAN: setup +0.077, all classes >=0, 0 violated.

Verification: tb_gs_grad_num_equiv 0/494770; f52 replay BYTE-IDENTICAL golden
d0047677 (drops=0, occupancy unchanged); gradient/perspective/texture regressions
(tri_interp, grad_divider, persp_uv, zbuffer, fog_persp, textured_triangle,
triangle/perspective/combined/gouraud demos) all PASS. Byte-identical => the
screen is unchanged; this is the resource unlock for fog + coverage.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
thejayman77
2026-07-25 05:28:12 -04:00
parent 7d68577c75
commit 4e8c3a16b8
3 changed files with 201 additions and 99 deletions
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sim/Makefile
+10
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@@ -1182,6 +1182,15 @@ tb_gs_grad_divider: dirs
@echo "=== run tb_gs_grad_divider ==="
@cd $(TRACE_DIR) && $(VVP) $(BUILD_DIR)/tb_gs_grad_divider.vvp
tb_gs_grad_num_equiv: dirs
@echo "=== build tb_gs_grad_num_equiv ==="
$(IVERILOG) $(IVERILOG_FLGS) \
-o $(BUILD_DIR)/tb_gs_grad_num_equiv.vvp \
-s tb_gs_grad_num_equiv \
$(TB_ROOT)/gif_gs/tb_gs_grad_num_equiv.sv
@echo "=== run tb_gs_grad_num_equiv ==="
@cd $(TRACE_DIR) && $(VVP) $(BUILD_DIR)/tb_gs_grad_num_equiv.vvp
tb_gs_lpddr_scanout_fb: dirs
@echo "=== build tb_gs_lpddr_scanout_fb ==="
$(IVERILOG) $(IVERILOG_FLGS) \
@@ -6331,6 +6340,7 @@ run: tb_top_psmct32_sh3_zs640b24_cap tb_top_psmct32_sh3_zint640b24
.PHONY: tb_top_psmct32_sh3_zs640c24c_cap tb_top_psmct32_sh3_zint640c24c sh3_zs640c24c_fixture sh3_zs640motionabc_bootlet
run: tb_top_psmct32_sh3_zs640c24c_cap tb_top_psmct32_sh3_zint640c24c tb_gs_axi_w_regbuf tb_gs_axi_aw_regbuf tb_gs_axi_r_regbuf
run: tb_gs_lpddr_scanout_lb tb_gs_scanout_binomial_lookahead tb_gs_scanout_diag tb_gs_scanout_restart tb_gs_scanout_cdc_qual
run: tb_gs_grad_num_equiv
run: tb_ee_fetch tb_gs tb_intc tb_platform_video tb_bgcolor_via_dma tb_sif_mailbox \
tb_sif_command_echo tb_sif_command_echo_rearm tb_sif_negative_path \