From ffd52a0d8e43aba34d8d87def77b073c9fd90a07 Mon Sep 17 00:00:00 2001
From: Robert O'Callahan <rocallahan@google.com>
Date: Thu, 31 Jul 2025 10:19:44 +0000
Subject: [PATCH] Making `stringf()` use the format conversion specs as-is
 without widening them.

And make sure our fast-path for `%d` and `%u` narrows to `int` correctly.

Resolves #5260
---
 kernel/io.cc                | 34 ++----------------
 kernel/io.h                 |  2 +-
 tests/unit/kernel/ioTest.cc | 72 +++++++++++++++++++++++++++++++++++++
 3 files changed, 76 insertions(+), 32 deletions(-)
 create mode 100644 tests/unit/kernel/ioTest.cc

diff --git a/kernel/io.cc b/kernel/io.cc
index 3deb54d86..9811919ba 100644
--- a/kernel/io.cc
+++ b/kernel/io.cc
@@ -405,37 +405,9 @@ std::string unescape_format_string(std::string_view fmt)
 
 static std::string string_view_stringf(std::string_view spec, ...)
 {
-	std::string fmt(spec);
-	char format_specifier = fmt[fmt.size() - 1];
-	switch (format_specifier) {
-	case 'd':
-	case 'i':
-	case 'o':
-	case 'u':
-	case 'x':
-	case 'X': {
-		// Strip any length modifier off `fmt`
-		std::string long_fmt;
-		for (size_t i = 0; i + 1 < fmt.size(); ++i) {
-			char ch = fmt[i];
-			if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z')) {
-				break;
-			}
-			long_fmt.push_back(ch);
-		}
-		// Add `lld` or whatever
-		long_fmt += "ll";
-		long_fmt.push_back(format_specifier);
-		fmt = long_fmt;
-		break;
-	}
-	default:
-		break;
-	}
-
 	va_list ap;
 	va_start(ap, spec);
-	std::string result = vstringf(fmt.c_str(), ap);
+	std::string result = vstringf(std::string(spec).c_str(), ap);
 	va_end(ap);
 	return result;
 }
@@ -464,7 +436,7 @@ void format_emit_long_long(std::string &result, std::string_view spec, int *dyna
 {
 	if (spec == "%d") {
 		// Format checking will have guaranteed num_dynamic_ints == 0.
-		result += std::to_string(arg);
+		result += std::to_string(static_cast<int>(arg));
 		return;
 	}
 	format_emit_stringf(result, spec, dynamic_ints, num_dynamic_ints, arg);
@@ -475,7 +447,7 @@ void format_emit_unsigned_long_long(std::string &result, std::string_view spec,
 {
 	if (spec == "%u") {
 		// Format checking will have guaranteed num_dynamic_ints == 0.
-		result += std::to_string(arg);
+		result += std::to_string(static_cast<unsigned int>(arg));
 		return;
 	}
 	if (spec == "%c") {
diff --git a/kernel/io.h b/kernel/io.h
index ea2499e43..dafef8bfa 100644
--- a/kernel/io.h
+++ b/kernel/io.h
@@ -62,7 +62,7 @@ enum ConversionSpecifier : uint8_t
 	CONVSPEC_UNSIGNED_INT,
 	// Consumes a "double"
 	CONVSPEC_DOUBLE,
-	// Consumes a "const char*"
+	// Consumes a "const char*" or other string type
 	CONVSPEC_CHAR_PTR,
 	// Consumes a "void*"
 	CONVSPEC_VOID_PTR,
diff --git a/tests/unit/kernel/ioTest.cc b/tests/unit/kernel/ioTest.cc
new file mode 100644
index 000000000..7cb8498cb
--- /dev/null
+++ b/tests/unit/kernel/ioTest.cc
@@ -0,0 +1,72 @@
+#include <gtest/gtest.h>
+
+#include "kernel/io.h"
+
+YOSYS_NAMESPACE_BEGIN
+
+TEST(KernelStringfTest, integerTruncation)
+{
+	EXPECT_EQ(stringf("%d", 1LL << 32), "0");
+	EXPECT_EQ(stringf("%u", 1LL << 32), "0");
+	EXPECT_EQ(stringf("%x", 0xff12345678LL), "12345678");
+	EXPECT_EQ(stringf("%hu", 0xff12345678LL), "22136");
+}
+
+TEST(KernelStringfTest, charFormat)
+{
+        EXPECT_EQ(stringf("%c", 256), std::string_view("\0", 1));
+        EXPECT_EQ(stringf("%c", -1), "\377");
+}
+
+TEST(KernelStringfTest, floatFormat)
+{
+	EXPECT_EQ(stringf("%g", 1.0), "1");
+}
+
+TEST(KernelStringfTest, intToFloat)
+{
+        EXPECT_EQ(stringf("%g", 1), "1");
+}
+
+TEST(KernelStringfTest, floatToInt)
+{
+        EXPECT_EQ(stringf("%d", 1.0), "1");
+        EXPECT_EQ(stringf("%d", -1.6), "-1");
+}
+
+TEST(KernelStringfTest, stringParam)
+{
+        EXPECT_EQ(stringf("%s", std::string("hello")), "hello");
+}
+
+TEST(KernelStringfTest, stringViewParam)
+{
+        EXPECT_EQ(stringf("%s", std::string_view("hello")), "hello");
+}
+
+TEST(KernelStringfTest, escapePercent)
+{
+	EXPECT_EQ(stringf("%%"), "%");
+}
+
+TEST(KernelStringfTest, trailingPercent)
+{
+	EXPECT_EQ(stringf("%"), "%");
+}
+
+TEST(KernelStringfTest, dynamicWidth)
+{
+        EXPECT_EQ(stringf("%*s", 8, "hello"), "   hello");
+}
+
+TEST(KernelStringfTest, dynamicPrecision)
+{
+        EXPECT_EQ(stringf("%.*f", 4, 1.0), "1.0000");
+}
+
+TEST(KernelStringfTest, dynamicWidthAndPrecision)
+{
+        EXPECT_EQ(stringf("%*.*f", 8, 4, 1.0), "  1.0000");
+}
+
+YOSYS_NAMESPACE_END