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

In 07ef8e6796 and 3ed9f6ebde, `__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
This commit is contained in:
Daniel McIntosh 2021-06-02 11:44:07 -04:00 committed by Abhina Sreeskantharajan
parent 5ecfdb5123
commit 06e047227d
1 changed files with 22 additions and 16 deletions
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38
libcxx/include/locale
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@ -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;