From 51a5c3aa60764e2ec7521d240ee1a7587183d352 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Thu, 26 Feb 2026 11:08:36 +0300
Subject: [PATCH] cgen: fix incorrect f64 -> u64 conversions (fixes #20944)

---
 vlib/v/gen/c/cgen.v                                |  6 ++++++
 vlib/v/gen/c/cheaders.v                            | 14 ++++++++++++++
 vlib/v/tests/casts/cast_f64_to_u64_boundary_test.v | 14 ++++++++++++++
 3 files changed, 34 insertions(+)
 create mode 100644 vlib/v/tests/casts/cast_f64_to_u64_boundary_test.v

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index bda8e3c7c..218d2dd4f 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -1036,6 +1036,7 @@ pub fn (mut g Gen) init() {
 		} else {
 			g.cheaders.writeln(c_headers)
 		}
+		g.cheaders.writeln(c_float_to_unsigned_conversion_functions)
 		if !g.pref.skip_unused || g.table.used_features.safe_int {
 			g.cheaders.writeln(c_unsigned_comparison_functions)
 		}
@@ -6009,6 +6010,11 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 			}
 			g.expr(node.expr)
 		}
+	} else if !node_typ_is_option && final_sym.kind == .u64
+		&& final_expr_sym.kind in [.f32, .f64, .float_literal] {
+		g.write('_v_f64_to_u64((double)(')
+		g.expr(node.expr)
+		g.write('))')
 	} else if (expr_type == ast.bool_type && node_typ.is_int()) || node_typ == ast.bool_type {
 		if node_typ_is_option {
 			g.expr_with_opt(node.expr, expr_type, node_typ)
diff --git a/vlib/v/gen/c/cheaders.v b/vlib/v/gen/c/cheaders.v
index d760f53ae..d68adf1cb 100644
--- a/vlib/v/gen/c/cheaders.v
+++ b/vlib/v/gen/c/cheaders.v
@@ -281,6 +281,20 @@ static inline bool _us64_le(uint64_t a, int64_t b) { return a <= INT64_MAX && (i
 static inline bool _us64_lt(uint64_t a, int64_t b) { return a < INT64_MAX && (int64_t)a < b; }
 '
 
+const c_float_to_unsigned_conversion_functions = '
+// deterministic float -> u64 conversions for explicit V casts
+// direct C casts are undefined for out-of-range values
+static inline uint64_t _v_f64_to_u64(double x) {
+	if (!(x >= 0.0)) {
+		return 0;
+	}
+	if (x >= 18446744073709551616.0) {
+		return UINT64_MAX;
+	}
+	return (uint64_t)x;
+}
+'
+
 const c_helper_macros = '//============================== HELPER C MACROS =============================*/
 // _SLIT0 is used as NULL string for literal arguments
 // `"" s` is used to enforce a string literal argument
diff --git a/vlib/v/tests/casts/cast_f64_to_u64_boundary_test.v b/vlib/v/tests/casts/cast_f64_to_u64_boundary_test.v
new file mode 100644
index 000000000..e3ca552b3
--- /dev/null
+++ b/vlib/v/tests/casts/cast_f64_to_u64_boundary_test.v
@@ -0,0 +1,14 @@
+import math
+
+fn test_f64_to_u64_boundary_values() {
+	max_u64 := u64(-1)
+	pow_63 := math.pow(2, 63)
+	pow_64 := math.pow(2, 64)
+	high_step := pow_63 + 2048.0
+
+	assert u64(-1.0) == 0
+	assert u64(pow_63) == u64(1) << 63
+	assert u64(high_step) == (u64(1) << 63) + u64(2048)
+	assert u64(pow_64) == max_u64
+	assert u64(f64(max_u64)) == max_u64
+}
-- 
2.39.5