[libc++] Fix leading zeros in std::to_chars

Summary:
It is a bugfix proposal for https://bugs.llvm.org/show_bug.cgi?id=42166.

`std::to_chars` appends leading zeros if input 64-bit value has 9, 10 or 11 digits.
According to documentation `std::to_chars` must not append leading zeros:
https://en.cppreference.com/w/cpp/utility/to_chars

Changeset should not affect `std::to_chars` performance:
http://quick-bench.com/CEpRs14xxA9WLvkXFtaJ3TWOVAg

Unit test that `std::from_chars` supports compatibility for both `std::to_chars` outputs (previous and fixed one) already exists:
https://github.com/llvm-mirror/libcxx/blob/1f60111b597e5cb80a4513ec86f79b7e137f7793/test/std/utilities/charconv/charconv.from.chars/integral.pass.cpp#L63

Reviewers: lichray, mclow.lists, ldionne, EricWF

Reviewed By: lichray, mclow.lists

Subscribers: zoecarver, christof, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D63047

llvm-svn: 362967

2 files changed, 141 insertions, 116 deletions

diff --git a/libcxx/src/charconv.cpp b/libcxx/src/charconv.cpp
index 6ea473b25f8..d3035883a78 100644
--- a/libcxx/src/charconv.cpp
+++ b/libcxx/src/charconv.cpp
@@ -60,48 +60,50 @@ append4(char* buffer, T i)
     return append2(append2(buffer, (i) / 100), (i) % 100);
 }
 
-char*
-__u32toa(uint32_t value, char* buffer)
+template <typename T>
+inline _LIBCPP_INLINE_VISIBILITY char*
+append2_no_zeros(char* buffer, T v)
+{
+    if (v < 10)
+        return append1(buffer, v);
+    else
+        return append2(buffer, v);
+}
+
+template <typename T>
+inline _LIBCPP_INLINE_VISIBILITY char*
+append4_no_zeros(char* buffer, T v)
 {
-    if (value < 10000)
+    if (v < 100)
+        return append2_no_zeros(buffer, v);
+    else if (v < 1000)
+        return append3(buffer, v);
+    else
+        return append4(buffer, v);
+}
+
+template <typename T>
+inline _LIBCPP_INLINE_VISIBILITY char*
+append8_no_zeros(char* buffer, T v)
+{
+    if (v < 10000)
     {
-        if (value < 100)
-        {
-            if (value < 10)
-                buffer = append1(buffer, value);
-            else
-                buffer = append2(buffer, value);
-        }
-        else
-        {
-            if (value < 1000)
-                buffer = append3(buffer, value);
-            else
-                buffer = append4(buffer, value);
-        }
+        buffer = append4_no_zeros(buffer, v);
     }
-    else if (value < 100000000)
+    else
     {
-        // value = bbbbcccc
-        const uint32_t b = value / 10000;
-        const uint32_t c = value % 10000;
-
-        if (value < 1000000)
-        {
-            if (value < 100000)
-                buffer = append1(buffer, b);
-            else
-                buffer = append2(buffer, b);
-        }
-        else
-        {
-            if (value < 10000000)
-                buffer = append3(buffer, b);
-            else
-                buffer = append4(buffer, b);
-        }
-
-        buffer = append4(buffer, c);
+        buffer = append4_no_zeros(buffer, v / 10000);
+        buffer = append4(buffer, v % 10000);
+    }
+    return buffer;
+}
+
+char*
+__u32toa(uint32_t value, char* buffer)
+{
+    if (value < 100000000)
+    {
+        buffer = append8_no_zeros(buffer, value);
     }
     else
     {
@@ -109,11 +111,7 @@ __u32toa(uint32_t value, char* buffer)
         const uint32_t a = value / 100000000;  // 1 to 42
         value %= 100000000;
 
-        if (a < 10)
-            buffer = append1(buffer, a);
-        else
-            buffer = append2(buffer, a);
-
+        buffer = append2_no_zeros(buffer, a);
         buffer = append4(buffer, value / 10000);
         buffer = append4(buffer, value % 10000);
     }
@@ -127,71 +125,14 @@ __u64toa(uint64_t value, char* buffer)
     if (value < 100000000)
     {
         uint32_t v = static_cast<uint32_t>(value);
-        if (v < 10000)
-        {
-            if (v < 100)
-            {
-                if (v < 10)
-                    buffer = append1(buffer, v);
-                else
-                    buffer = append2(buffer, v);
-            }
-            else
-            {
-                if (v < 1000)
-                    buffer = append3(buffer, v);
-                else
-                    buffer = append4(buffer, v);
-            }
-        }
-        else
-        {
-            // value = bbbbcccc
-            const uint32_t b = v / 10000;
-            const uint32_t c = v % 10000;
-
-            if (v < 1000000)
-            {
-                if (v < 100000)
-                    buffer = append1(buffer, b);
-                else
-                    buffer = append2(buffer, b);
-            }
-            else
-            {
-                if (v < 10000000)
-                    buffer = append3(buffer, b);
-                else
-                    buffer = append4(buffer, b);
-            }
-
-            buffer = append4(buffer, c);
-        }
+        buffer = append8_no_zeros(buffer, v);
     }
     else if (value < 10000000000000000)
     {
         const uint32_t v0 = static_cast<uint32_t>(value / 100000000);
         const uint32_t v1 = static_cast<uint32_t>(value % 100000000);
 
-        const uint32_t b0 = v0 / 10000;
-        const uint32_t c0 = v0 % 10000;
-
-        if (v0 < 1000000)
-        {
-            if (v0 < 100000)
-                buffer = append1(buffer, b0);
-            else
-                buffer = append2(buffer, b0);
-        }
-        else
-        {
-            if (v0 < 10000000)
-                buffer = append3(buffer, b0);
-            else
-                buffer = append4(buffer, b0);
-        }
-
-        buffer = append4(buffer, c0);
+        buffer = append8_no_zeros(buffer, v0);
         buffer = append4(buffer, v1 / 10000);
         buffer = append4(buffer, v1 % 10000);
     }
@@ -201,20 +142,7 @@ __u64toa(uint64_t value, char* buffer)
             static_cast<uint32_t>(value / 10000000000000000);  // 1 to 1844
         value %= 10000000000000000;
 
-        if (a < 100)
-        {
-            if (a < 10)
-                buffer = append1(buffer, a);
-            else
-                buffer = append2(buffer, a);
-        }
-        else
-        {
-            if (a < 1000)
-                buffer = append3(buffer, a);
-            else
-                buffer = append4(buffer, a);
-        }
+        buffer = append4_no_zeros(buffer, a);
 
         const uint32_t v0 = static_cast<uint32_t>(value / 100000000);
         const uint32_t v1 = static_cast<uint32_t>(value % 100000000);
diff --git a/libcxx/test/std/utilities/charconv/charconv.to.chars/integral.pass.cpp b/libcxx/test/std/utilities/charconv/charconv.to.chars/integral.pass.cpp
index 4bfe27d6dac..e30d3c84909 100644
--- a/libcxx/test/std/utilities/charconv/charconv.to.chars/integral.pass.cpp
+++ b/libcxx/test/std/utilities/charconv/charconv.to.chars/integral.pass.cpp
@@ -45,6 +45,56 @@ struct test_basics : to_chars_test_base<T>
         test(0xdeadbeaf, "deadbeaf", 16);
         test(0755, "755", 8);
 
+        // Test each len till len of UINT64_MAX = 20 because to_chars algorithm
+        // makes branches based on decimal digits count in the value string
+        // representation.
+        // Test driver automatically skips values not fitting into source type.
+        test(1UL, "1");
+        test(12UL, "12");
+        test(123UL, "123");
+        test(1234UL, "1234");
+        test(12345UL, "12345");
+        test(123456UL, "123456");
+        test(1234567UL, "1234567");
+        test(12345678UL, "12345678");
+        test(123456789UL, "123456789");
+        test(1234567890UL, "1234567890");
+        test(12345678901UL, "12345678901");
+        test(123456789012UL, "123456789012");
+        test(1234567890123UL, "1234567890123");
+        test(12345678901234UL, "12345678901234");
+        test(123456789012345UL, "123456789012345");
+        test(1234567890123456UL, "1234567890123456");
+        test(12345678901234567UL, "12345678901234567");
+        test(123456789012345678UL, "123456789012345678");
+        test(1234567890123456789UL, "1234567890123456789");
+        test(12345678901234567890UL, "12345678901234567890");
+
+        // Test special cases with zeros inside a value string representation,
+        // to_chars algorithm processes them in a special way and should not
+        // skip trailing zeros
+        // Test driver automatically skips values not fitting into source type.
+        test(0UL, "0");
+        test(10UL, "10");
+        test(100UL, "100");
+        test(1000UL, "1000");
+        test(10000UL, "10000");
+        test(100000UL, "100000");
+        test(1000000UL, "1000000");
+        test(10000000UL, "10000000");
+        test(100000000UL, "100000000");
+        test(1000000000UL, "1000000000");
+        test(10000000000UL, "10000000000");
+        test(100000000000UL, "100000000000");
+        test(1000000000000UL, "1000000000000");
+        test(10000000000000UL, "10000000000000");
+        test(100000000000000UL, "100000000000000");
+        test(1000000000000000UL, "1000000000000000");
+        test(10000000000000000UL, "10000000000000000");
+        test(100000000000000000UL, "100000000000000000");
+        test(1000000000000000000UL, "1000000000000000000");
+        test(10000000000000000000UL, "10000000000000000000");
+
         for (int b = 2; b < 37; ++b)
         {
             using xl = std::numeric_limits<T>;
@@ -73,6 +123,53 @@ struct test_signed : to_chars_test_base<T>
         test(-2647, "-101001010111", 2);
         test(-0xcc1, "-cc1", 16);
 
+        // Test each len till len of INT64_MAX = 19 because to_chars algorithm
+        // makes branches based on decimal digits count in the value string
+        // representation.
+        // Test driver automatically skips values not fitting into source type.
+        test(-1L, "-1");
+        test(-12L, "-12");
+        test(-123L, "-123");
+        test(-1234L, "-1234");
+        test(-12345L, "-12345");
+        test(-123456L, "-123456");
+        test(-1234567L, "-1234567");
+        test(-12345678L, "-12345678");
+        test(-123456789L, "-123456789");
+        test(-1234567890L, "-1234567890");
+        test(-12345678901L, "-12345678901");
+        test(-123456789012L, "-123456789012");
+        test(-1234567890123L, "-1234567890123");
+        test(-12345678901234L, "-12345678901234");
+        test(-123456789012345L, "-123456789012345");
+        test(-1234567890123456L, "-1234567890123456");
+        test(-12345678901234567L, "-12345678901234567");
+        test(-123456789012345678L, "-123456789012345678");
+        test(-1234567890123456789L, "-1234567890123456789");
+
+        // Test special cases with zeros inside a value string representation,
+        // to_chars algorithm processes them in a special way and should not
+        // skip trailing zeros
+        // Test driver automatically skips values not fitting into source type.
+        test(-10L, "-10");
+        test(-100L, "-100");
+        test(-1000L, "-1000");
+        test(-10000L, "-10000");
+        test(-100000L, "-100000");
+        test(-1000000L, "-1000000");
+        test(-10000000L, "-10000000");
+        test(-100000000L, "-100000000");
+        test(-1000000000L, "-1000000000");
+        test(-10000000000L, "-10000000000");
+        test(-100000000000L, "-100000000000");
+        test(-1000000000000L, "-1000000000000");
+        test(-10000000000000L, "-10000000000000");
+        test(-100000000000000L, "-100000000000000");
+        test(-1000000000000000L, "-1000000000000000");
+        test(-10000000000000000L, "-10000000000000000");
+        test(-100000000000000000L, "-100000000000000000");
+        test(-1000000000000000000L, "-1000000000000000000");
+
         for (int b = 2; b < 37; ++b)
         {
             using xl = std::numeric_limits<T>;