From 855ef52cb2aba94774773ea342b21d221ecef0c3 Mon Sep 17 00:00:00 2001
From: TomAFrench <tom@tomfren.ch>
Date: Wed, 15 Nov 2023 08:02:21 +0000
Subject: [PATCH 1/2] fix: apply predicate to over/underflow checks

---
 compiler/noirc_evaluator/src/ssa/ir/printer.rs   |  2 +-
 .../noirc_evaluator/src/ssa/opt/flatten_cfg.rs   | 16 ++++++++++++++++
 .../conditional_regression_underflow/Nargo.toml  |  6 ++++++
 .../conditional_regression_underflow/Prover.toml |  1 +
 .../conditional_regression_underflow/src/main.nr |  6 ++++++
 5 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/Nargo.toml
 create mode 100644 tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/Prover.toml
 create mode 100644 tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/src/main.nr

diff --git a/compiler/noirc_evaluator/src/ssa/ir/printer.rs b/compiler/noirc_evaluator/src/ssa/ir/printer.rs
index 51e436643ab..c6b1f3c7528 100644
--- a/compiler/noirc_evaluator/src/ssa/ir/printer.rs
+++ b/compiler/noirc_evaluator/src/ssa/ir/printer.rs
@@ -173,7 +173,7 @@ pub(crate) fn display_instruction(
             )
         }
         Instruction::RangeCheck { value, max_bit_size, .. } => {
-            write!(f, "range_check {} to {} bits", show(*value), *max_bit_size,)
+            writeln!(f, "range_check {} to {} bits", show(*value), *max_bit_size,)
         }
     }
 }
diff --git a/compiler/noirc_evaluator/src/ssa/opt/flatten_cfg.rs b/compiler/noirc_evaluator/src/ssa/opt/flatten_cfg.rs
index f35e8023584..d2ed21c60d7 100644
--- a/compiler/noirc_evaluator/src/ssa/opt/flatten_cfg.rs
+++ b/compiler/noirc_evaluator/src/ssa/opt/flatten_cfg.rs
@@ -662,6 +662,22 @@ impl<'f> Context<'f> {
                     self.remember_store(address, value);
                     Instruction::Store { address, value }
                 }
+                Instruction::RangeCheck { value, max_bit_size, assert_message } => {
+                    // Replace value with `value * predicate` to zero out value when predicate is inactive.
+
+                    // Condition needs to be cast to argument type in order to multiply them together.
+                    let argument_type = self.inserter.function.dfg.type_of_value(value);
+                    let casted_condition = self.insert_instruction(
+                        Instruction::Cast(condition, argument_type),
+                        call_stack.clone(),
+                    );
+
+                    let value = self.insert_instruction(
+                        Instruction::binary(BinaryOp::Mul, value, casted_condition),
+                        call_stack.clone(),
+                    );
+                    Instruction::RangeCheck { value, max_bit_size, assert_message }
+                }
                 other => other,
             }
         } else {
diff --git a/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/Nargo.toml b/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/Nargo.toml
new file mode 100644
index 00000000000..54a082081f7
--- /dev/null
+++ b/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/Nargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "conditional_underflow"
+type = "bin"
+authors = [""]
+
+[dependencies]
diff --git a/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/Prover.toml b/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/Prover.toml
new file mode 100644
index 00000000000..ca65e8b9428
--- /dev/null
+++ b/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/Prover.toml
@@ -0,0 +1 @@
+x = "4"
\ No newline at end of file
diff --git a/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/src/main.nr b/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/src/main.nr
new file mode 100644
index 00000000000..5b35d854b9e
--- /dev/null
+++ b/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/src/main.nr
@@ -0,0 +1,6 @@
+// Regression test for https://github.com/noir-lang/noir/issues/3493
+fn main(x: u4) {
+    if x == 10 {
+        x + 15;
+    }
+}
\ No newline at end of file

From 8540b55c9ed2fc9f183ec6b070dc3511ef0bd0a2 Mon Sep 17 00:00:00 2001
From: TomAFrench <tom@tomfren.ch>
Date: Wed, 15 Nov 2023 08:14:47 +0000
Subject: [PATCH 2/2] chore: cover all affected instructions in regression test

---
 .../conditional_regression_underflow/src/main.nr         | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/src/main.nr b/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/src/main.nr
index 5b35d854b9e..be70b67be5e 100644
--- a/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/src/main.nr
+++ b/tooling/nargo_cli/tests/execution_success/conditional_regression_underflow/src/main.nr
@@ -3,4 +3,13 @@ fn main(x: u4) {
     if x == 10 {
         x + 15;
     }
+    if x == 9 {
+        x << 3;
+    }
+    if x == 8 {
+        x * 3;
+    }
+    if x == 7 {
+        x - 8;
+    }
 }
\ No newline at end of file