[libcxx-commits] [libcxx] 06e0472 - [libcxx][NFC] Tidy up calculation of __nbuf in num_put::do_put, and add comments

[Abhina Sreeskantharajan via libcxx-commits]

Author: Daniel McIntosh
Date: 2021-06-02T11:46:17-04:00
New Revision: 06e047227d71b2c1c477e280172abed0f1eed2c8

URL: https://github.com/llvm/llvm-project/commit/06e047227d71b2c1c477e280172abed0f1eed2c8
DIFF: https://github.com/llvm/llvm-project/commit/06e047227d71b2c1c477e280172abed0f1eed2c8.diff

LOG: [libcxx][NFC] Tidy up calculation of __nbuf in num_put::do_put, and add comments

In 07ef8e679621 and 3ed9f6ebdeeb, `__nbuf` started to diverge from the amount
of space that was actually needed for the buffer. For 32-bit longs for example,
we allocate a buffer that is one larger than needed. Moreover, it is no longer
clear exactly where the extra +1 or +2 comes from - they're just numbers pulled
from thin air. This PR cleans up how `__nbuf` is calculated, and adds comments
to further clarify where each part comes from.

Specifically, it corrects the underestimation of the max size buffer needed
that the above two commits had to compensate for. The root cause looks to be
the use of signed type parameters to numeric_limits<>::digits. Since digits
only counts non-sign bits, the calculation was acting as though (for a signed
64-bit type) the longest value we would print was 2^63 in octal. However,
printing in octal treats values as unsigned, so it is actually 2^64. Thus,
using unsigned types and changing the final +2 to a +1 is probably a better
option.

Reviewed By: #libc, ldionne, Mordante

Differential Revision: https://reviews.llvm.org/D103339

Added: 
    

Modified: 
    libcxx/include/locale

Removed: 
    


################################################################################
diff  --git a/libcxx/include/locale b/libcxx/include/locale
index 85155a88300f..49d76839d9c9 100644
--- a/libcxx/include/locale
+++ b/libcxx/include/locale
@@ -1458,10 +1458,12 @@ num_put<_CharT, _OutputIterator>::do_put(iter_type __s, ios_base& __iob,
     char __fmt[6] = {'%', 0};
     const char* __len = "l";
     this->__format_int(__fmt+1, __len, true, __iob.flags());
-    const unsigned __nbuf = (numeric_limits<long>::digits / 3)
-                          + ((numeric_limits<long>::digits % 3) != 0)
-                          + ((__iob.flags() & ios_base::showbase) != 0)
-                          + 2;
+    // Worst case is octal, with showbase enabled. Note that octal is always
+    // printed as an unsigned value.
+    const unsigned __nbuf = (numeric_limits<unsigned long>::digits / 3)          // 1 char per 3 bits
+                            + ((numeric_limits<unsigned long>::digits % 3) != 0) // round up
+                            + ((__iob.flags() & ios_base::showbase) != 0)        // base prefix
+                            + 1;                                                 // terminating null character
     char __nar[__nbuf];
     int __nc = __libcpp_snprintf_l(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);
     char* __ne = __nar + __nc;
@@ -1485,10 +1487,12 @@ num_put<_CharT, _OutputIterator>::do_put(iter_type __s, ios_base& __iob,
     char __fmt[8] = {'%', 0};
     const char* __len = "ll";
     this->__format_int(__fmt+1, __len, true, __iob.flags());
-    const unsigned __nbuf = (numeric_limits<long long>::digits / 3)
-                          + ((numeric_limits<long long>::digits % 3) != 0)
-                          + ((__iob.flags() & ios_base::showbase) != 0)
-                          + 2;
+    // Worst case is octal, with showbase enabled. Note that octal is always
+    // printed as an unsigned value.
+    const unsigned __nbuf = (numeric_limits<unsigned long long>::digits / 3)          // 1 char per 3 bits
+                            + ((numeric_limits<unsigned long long>::digits % 3) != 0) // round up
+                            + ((__iob.flags() & ios_base::showbase) != 0)             // base prefix
+                            + 1;                                                      // terminating null character
     char __nar[__nbuf];
     int __nc = __libcpp_snprintf_l(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);
     char* __ne = __nar + __nc;
@@ -1512,10 +1516,11 @@ num_put<_CharT, _OutputIterator>::do_put(iter_type __s, ios_base& __iob,
     char __fmt[6] = {'%', 0};
     const char* __len = "l";
     this->__format_int(__fmt+1, __len, false, __iob.flags());
-    const unsigned __nbuf = (numeric_limits<unsigned long>::digits / 3)
-                          + ((numeric_limits<unsigned long>::digits % 3) != 0)
-                          + ((__iob.flags() & ios_base::showbase) != 0)
-                          + 1;
+    // Worst case is octal, with showbase enabled.
+    const unsigned __nbuf = (numeric_limits<unsigned long>::digits / 3)          // 1 char per 3 bits
+                            + ((numeric_limits<unsigned long>::digits % 3) != 0) // round up
+                            + ((__iob.flags() & ios_base::showbase) != 0)        // base prefix
+                            + 1;                                                 // terminating null character
     char __nar[__nbuf];
     int __nc = __libcpp_snprintf_l(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);
     char* __ne = __nar + __nc;
@@ -1539,10 +1544,11 @@ num_put<_CharT, _OutputIterator>::do_put(iter_type __s, ios_base& __iob,
     char __fmt[8] = {'%', 0};
     const char* __len = "ll";
     this->__format_int(__fmt+1, __len, false, __iob.flags());
-    const unsigned __nbuf = (numeric_limits<unsigned long long>::digits / 3)
-                          + ((numeric_limits<unsigned long long>::digits % 3) != 0)
-                          + ((__iob.flags() & ios_base::showbase) != 0)
-                          + 1;
+    // Worst case is octal, with showbase enabled.
+    const unsigned __nbuf = (numeric_limits<unsigned long long>::digits / 3)          // 1 char per 3 bits
+                            + ((numeric_limits<unsigned long long>::digits % 3) != 0) // round up
+                            + ((__iob.flags() & ios_base::showbase) != 0)             // base prefix
+                            + 1;                                                      // terminating null character
     char __nar[__nbuf];
     int __nc = __libcpp_snprintf_l(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);
     char* __ne = __nar + __nc;