From: Eddie Hung <eddie@fpgeh.com>
Date: Tue, 10 Sep 2019 21:11:56 +0000 (-0700)
Subject: Add comment for why opt_expr is necessary
X-Git-Tag: working-ls180~1039^2~152
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=f3a55d3f06a21af06a267b3f9da8bd4df3d4aa82;p=yosys.git

Add comment for why opt_expr is necessary
---

diff --git a/techlibs/xilinx/synth_xilinx.cc b/techlibs/xilinx/synth_xilinx.cc
index 2cf0bd4f9..c99b77905 100644
--- a/techlibs/xilinx/synth_xilinx.cc
+++ b/techlibs/xilinx/synth_xilinx.cc
@@ -325,6 +325,8 @@ struct SynthXilinxPass : public ScriptPass
 			if (!nodsp || help_mode) {
 				// NB: Xilinx multipliers are signed only
 				run("techmap -map +/mul2dsp.v -map +/xilinx/dsp_map.v -D DSP_A_MAXWIDTH=25 -D DSP_A_MAXWIDTH_PARTIAL=18 -D DSP_B_MAXWIDTH=18 -D DSP_SIGNEDONLY=1 -D DSP_NAME=$__MUL25X18", "(skip if '-nodsp')");
+				// Necessary for optimising away $shl cells, as well as $add cells
+				//   that have inputs with 1'b0 LSBs
 				run("opt_expr -fine", "                 (skip if '-nodsp')");
 				run("wreduce", "                        (skip if '-nodsp')");
 				run("xilinx_dsp", "                     (skip if '-nodsp')");