From 6de80bc6b388a7d294fd711c3a34226f8b1a9acb Mon Sep 17 00:00:00 2001
From: Alain Dargelas <alain@silimate.com>
Date: Mon, 10 Mar 2025 13:47:06 -0700
Subject: [PATCH 1/2] neg sub pass

---
 passes/opt/Makefile.inc        |   1 +
 passes/opt/peepopt.cc          |   1 +
 passes/opt/peepopt_sub_neg.pmg |  62 ++++++++++++++++++++
 tests/peepopt/neg_sub.ys       | 102 +++++++++++++++++++++++++++++++++
 4 files changed, 166 insertions(+)
 create mode 100644 passes/opt/peepopt_sub_neg.pmg
 create mode 100644 tests/peepopt/neg_sub.ys

diff --git a/passes/opt/Makefile.inc b/passes/opt/Makefile.inc
index 9dd90072d..8b973c9a8 100644
--- a/passes/opt/Makefile.inc
+++ b/passes/opt/Makefile.inc
@@ -34,6 +34,7 @@ PEEPOPT_PATTERN += passes/opt/peepopt_muldiv.pmg
 PEEPOPT_PATTERN += passes/opt/peepopt_muldiv_c.pmg
 PEEPOPT_PATTERN += passes/opt/peepopt_muxorder.pmg
 PEEPOPT_PATTERN += passes/opt/peepopt_formal_clockgateff.pmg
+PEEPOPT_PATTERN += passes/opt/peepopt_sub_neg.pmg
 
 passes/opt/peepopt_pm.h: passes/pmgen/pmgen.py $(PEEPOPT_PATTERN)
 	$(P) mkdir -p $(dir $@) && $(PYTHON_EXECUTABLE) $< -o $@ -p peepopt $(filter-out $<,$^)
diff --git a/passes/opt/peepopt.cc b/passes/opt/peepopt.cc
index 8448b0485..cb6174d66 100644
--- a/passes/opt/peepopt.cc
+++ b/passes/opt/peepopt.cc
@@ -121,6 +121,7 @@ struct PeepoptPass : public Pass {
 					pm.run_shiftmul_left();
 					pm.run_muldiv();
 					pm.run_muldiv_c();
+					pm.run_sub_neg();
 					if (muxorder)
 						pm.run_muxorder();
 				}
diff --git a/passes/opt/peepopt_sub_neg.pmg b/passes/opt/peepopt_sub_neg.pmg
new file mode 100644
index 000000000..0b23c1ea4
--- /dev/null
+++ b/passes/opt/peepopt_sub_neg.pmg
@@ -0,0 +1,62 @@
+pattern sub_neg
+//
+// Authored by Akash Levy and Alain Dargelas of Silimate, Inc. under ISC license.
+// Transforms -(a-b)  ---->  (b-a)
+
+state <SigSpec> a b sub_y
+
+match sub
+	// Select sub
+	select sub->type == $sub
+endmatch
+
+code a b sub_y
+	// Get sub signals
+	a = port(sub, \A);
+	b = port(sub, \B);
+	sub_y = port(sub, \Y);
+
+	bool a_signed = sub->getParam(ID::A_SIGNED).as_bool();
+	bool b_signed = sub->getParam(ID::B_SIGNED).as_bool();
+
+	// Fanout of each sub Y bit should be 1 (no bit-split)
+  if (nusers(sub_y) != 2)
+ 		reject;
+
+	// Both operands need to be signed to be swapped
+	if (!a_signed || !b_signed)
+		reject;
+
+	// A and B can be interchanged
+	branch;
+	std::swap(a, b);
+endcode
+
+match neg
+	// Select neg of form -(a-b)
+	select neg->type == $neg
+
+	index <SigSpec> remove_bottom_padding(port(neg, \A)) === sub_y
+endmatch
+
+code
+	// Get neg signals
+	SigSpec neg_a = port(neg, \A);
+	SigSpec neg_y = port(neg, \Y);
+
+	bool a_signed = neg->getParam(ID::A_SIGNED).as_bool();
+	if (!a_signed)
+		reject;
+
+	// Rewire to only keep sub
+	sub->setPort(\A, b);
+	sub->setPort(\B, a);
+	sub->setPort(\Y, neg_y);
+
+	// Remove neg
+	autoremove(neg);
+
+	// Log, fixup, accept
+	log("sub_neg pattern in %s: neg=%s, sub=%s\n", log_id(module), log_id(neg), log_id(sub));
+	accept;
+endcode
diff --git a/tests/peepopt/neg_sub.ys b/tests/peepopt/neg_sub.ys
new file mode 100644
index 000000000..17980f632
--- /dev/null
+++ b/tests/peepopt/neg_sub.ys
@@ -0,0 +1,102 @@
+log -header "Test simple positive case"
+log -push
+design -reset
+read_verilog <<EOF
+module equation_example (
+  input signed [7:0] a,
+  input signed [7:0] b,
+  output signed [8:0] result
+);
+  assign result = - (a - b);
+endmodule
+EOF
+check -assert
+equiv_opt -assert -post peepopt
+select -assert-none t:$neg
+select -assert-any t:$sub
+design -reset
+log -pop
+
+log -header "Test positive case with intermediate signal"
+log -push
+design -reset
+read_verilog <<EOF
+module equation_example (
+  input signed [7:0] a,
+  input signed [7:0] b,
+  output signed [8:0] result
+);
+  wire signed [8:0] difference;
+  assign difference = a - b;
+  assign result = - difference;
+endmodule
+EOF
+check -assert
+equiv_opt -assert -post peepopt
+select -assert-none t:$neg
+select -assert-any t:$sub
+design -reset
+log -pop
+
+log -header "Test negative case (fanout)"
+log -push
+design -reset
+read_verilog <<EOF
+module equation_example (
+  input signed [7:0] a,
+  input signed [7:0] b,
+  output signed [8:0] result,
+  output signed [8:0] not_diff
+);
+  wire signed [8:0] difference;
+  assign difference = a - b;
+  assign result = -difference;
+  assign not_diff = ~difference;
+endmodule
+EOF
+check -assert
+equiv_opt -assert -post peepopt
+select -assert-any t:$neg
+select -assert-any t:$sub
+design -reset
+log -pop
+
+log -header "Test negative case (unsigned intermediate signal)"
+log -push
+design -reset
+read_verilog <<EOF
+module equation_example (
+  input signed [7:0] a,
+  input signed [7:0] b,
+  output signed [8:0] result
+);
+  wire [8:0] difference;
+  assign difference = a - b;
+  assign result = -difference;
+endmodule
+EOF
+check -assert
+equiv_opt -assert -post peepopt
+select -assert-any t:$neg
+select -assert-any t:$sub
+design -reset
+log -pop
+
+log -header "Test negative case, inputs are not signed"
+log -push
+design -reset
+read_verilog <<EOF
+module equation_example (
+  input [7:0] a,
+  input [7:0] b,
+  output signed [8:0] result
+);
+  assign result = - (a - b);
+endmodule
+EOF
+check -assert
+equiv_opt -assert -post peepopt
+select -assert-any t:$neg
+select -assert-any t:$sub
+design -reset
+log -pop

From e1671b45b68ed33d4e5b1ee9df23fc4575d7cdd0 Mon Sep 17 00:00:00 2001
From: Alain Dargelas <alain@silimate.com>
Date: Mon, 10 Mar 2025 14:44:14 -0700
Subject: [PATCH 2/2] Code review

---
 passes/opt/peepopt_sub_neg.pmg |  4 +--
 tests/peepopt/neg_sub.ys       | 46 ++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/passes/opt/peepopt_sub_neg.pmg b/passes/opt/peepopt_sub_neg.pmg
index 0b23c1ea4..24e617052 100644
--- a/passes/opt/peepopt_sub_neg.pmg
+++ b/passes/opt/peepopt_sub_neg.pmg
@@ -20,8 +20,8 @@ code a b sub_y
 	bool b_signed = sub->getParam(ID::B_SIGNED).as_bool();
 
 	// Fanout of each sub Y bit should be 1 (no bit-split)
-  if (nusers(sub_y) != 2)
- 		reject;
+	if (nusers(sub_y) != 2)
+		reject;
 
 	// Both operands need to be signed to be swapped
 	if (!a_signed || !b_signed)
diff --git a/tests/peepopt/neg_sub.ys b/tests/peepopt/neg_sub.ys
index 17980f632..5926f2b6b 100644
--- a/tests/peepopt/neg_sub.ys
+++ b/tests/peepopt/neg_sub.ys
@@ -100,3 +100,49 @@ select -assert-any t:$neg
 select -assert-any t:$sub
 design -reset
 log -pop
+
+log -header "Negative case with disconnected intermediate signal (operator in the middle)"
+log -push
+design -reset
+read_verilog <<EOF
+module equation_example (
+  input signed [7:0] a,
+  input signed [7:0] b,
+  output signed [8:0] result
+);
+  wire signed [8:0] difference;
+  wire signed [8:0] c;
+  assign difference = a - b;
+  assign c = ~difference;
+  assign result = - c;
+endmodule
+EOF
+check -assert
+equiv_opt -assert -post peepopt
+select -assert-any t:$neg
+select -assert-any t:$sub
+design -reset
+log -pop
+
+log -header "Negative case with disconnected intermediate signal"
+log -push
+design -reset
+read_verilog <<EOF
+module equation_example (
+  input signed [7:0] a,
+  input signed [7:0] b,
+  output signed [8:0] result
+);
+  wire signed [8:0] difference;
+  wire signed [8:0] c;
+  assign difference = a - b;
+  assign c = a + b ;
+  assign result = - c;
+endmodule
+EOF
+check -assert
+equiv_opt -assert -post peepopt
+select -assert-any t:$neg
+select -assert-any t:$sub
+design -reset
+log -pop