From 0d117eabc863cbabc286b8ce47162259c7bf0f33 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 14 Apr 2026 23:49:45 +0300
Subject: [PATCH] cgen: optimize string interpolation performance (fixes #7516)

---
 vlib/v/gen/c/str_intp.v                       | 78 +++++++++++++++++++
 ...erpolation_simple_optimization.c.must_have |  3 +
 ...tring_interpolation_simple_optimization.vv | 17 ++++
 3 files changed, 98 insertions(+)
 create mode 100644 vlib/v/gen/c/testdata/string_interpolation_simple_optimization.c.must_have
 create mode 100644 vlib/v/gen/c/testdata/string_interpolation_simple_optimization.vv

diff --git a/vlib/v/gen/c/str_intp.v b/vlib/v/gen/c/str_intp.v
index cd5d73434..f7828072f 100644
--- a/vlib/v/gen/c/str_intp.v
+++ b/vlib/v/gen/c/str_intp.v
@@ -629,6 +629,9 @@ fn (mut g Gen) string_inter_literal(node ast.StringInterLiteral) {
 			else {}
 		}
 	}
+	if g.gen_simple_string_inter_literal(node_, fmts) {
+		return
+	}
 	g.write2('builtin__str_intp(', node.vals.len.str())
 	g.write(', _MOV((StrIntpData[]){')
 	for i, val in node.vals {
@@ -709,3 +712,78 @@ fn (mut g Gen) string_inter_literal(node ast.StringInterLiteral) {
 	}
 	g.write('}))')
 }
+
+const simple_string_interpolation_default_precision = 987698
+
+fn (mut g Gen) gen_simple_string_inter_literal(node ast.StringInterLiteral, fmts []u8) bool {
+	if node.exprs.len == 0 || node.expr_types.len < node.exprs.len {
+		return false
+	}
+	for i in 0 .. node.exprs.len {
+		if i >= node.need_fmts.len || node.need_fmts[i] || i >= fmts.len || fmts[i] == `_` {
+			return false
+		}
+		if node.expr_types[i].is_any_kind_of_pointer() || node.expr_types[i].is_int_valptr()
+			|| node.expr_types[i].is_float_valptr() {
+			return false
+		}
+		if i < node.fwidths.len && node.fwidths[i] != 0 {
+			return false
+		}
+		if i < node.fwidth_exprs.len && node.fwidth_exprs[i] !is ast.EmptyExpr {
+			return false
+		}
+		if i < node.precisions.len
+			&& node.precisions[i] != simple_string_interpolation_default_precision {
+			return false
+		}
+		if i < node.precision_exprs.len && node.precision_exprs[i] !is ast.EmptyExpr {
+			return false
+		}
+		if i < node.pluss.len && node.pluss[i] {
+			return false
+		}
+		if i < node.fills.len && node.fills[i] {
+			return false
+		}
+	}
+	if node.exprs.len == 1 && node.vals.len == 2 && node.vals[0].len == 0 && node.vals[1].len == 0 {
+		g.gen_expr_to_string(node.exprs[0], node.expr_types[0])
+		return true
+	}
+	mut part_count := 0
+	for i, val in node.vals {
+		if val.len > 0 {
+			part_count++
+		}
+		if i < node.exprs.len {
+			part_count++
+		}
+	}
+	if part_count <= 0 {
+		return false
+	}
+	g.write('builtin__string_plus_many(${part_count}, _MOV((string[${part_count}]){')
+	mut written_parts := 0
+	for i, val in node.vals {
+		if val.len > 0 {
+			if written_parts > 0 {
+				g.write(', ')
+			}
+			mut escaped_val := cescape_nonascii(util.smart_quote(val, false))
+			escaped_val = escaped_val.replace('\0', '\\0')
+			g.write2('_S("', escaped_val)
+			g.write('")')
+			written_parts++
+		}
+		if i < node.exprs.len {
+			if written_parts > 0 {
+				g.write(', ')
+			}
+			g.gen_expr_to_string(node.exprs[i], node.expr_types[i])
+			written_parts++
+		}
+	}
+	g.write('}))')
+	return true
+}
diff --git a/vlib/v/gen/c/testdata/string_interpolation_simple_optimization.c.must_have b/vlib/v/gen/c/testdata/string_interpolation_simple_optimization.c.must_have
new file mode 100644
index 000000000..ba07c248f
--- /dev/null
+++ b/vlib/v/gen/c/testdata/string_interpolation_simple_optimization.c.must_have
@@ -0,0 +1,3 @@
+return builtin__int_str(n);
+builtin__string_plus_many(3, _MOV((string[3]){prefix, builtin__int_str(n), suffix}))
+builtin__string_plus_many(4, _MOV((string[4]){_S("n="), builtin__int_str(n), _S(":"), suffix}))
diff --git a/vlib/v/gen/c/testdata/string_interpolation_simple_optimization.vv b/vlib/v/gen/c/testdata/string_interpolation_simple_optimization.vv
new file mode 100644
index 000000000..2d310cca0
--- /dev/null
+++ b/vlib/v/gen/c/testdata/string_interpolation_simple_optimization.vv
@@ -0,0 +1,17 @@
+fn interp_number(n int) string {
+	return '${n}'
+}
+
+fn interp_chain(prefix string, n int, suffix string) string {
+	return '${prefix}${n}${suffix}'
+}
+
+fn interp_static(n int, suffix string) string {
+	return 'n=${n}:${suffix}'
+}
+
+fn main() {
+	assert interp_number(42) == '42'
+	assert interp_chain('a', 7, 'b') == 'a7b'
+	assert interp_static(9, 'z') == 'n=9:z'
+}
-- 
2.39.5