[libc][docs] Printf behavior doc

[michaelrj]

Summary

In the document on undefined behavior, I noted that writing down your
decisions is very important. This document contains all the information
for compile flags and undefined behavior for our printf.

[michaelrj]

update doc to match the behavior from the new patch https://reviews.llvm.org/D159458

libc/docs/dev/index.rst

.. toctree::
    entrypoints
    cmake_build_rules
    clang_tidy_checks
    fuzzing
    ground_truth_specification
    header_generation
    implementation_standard
    undefined_behavior
+   printf_behavior
    api_test
    mechanics_of_public_api

libc/docs/dev/printf_behavior.rst

+====================================
+Printf Behavior Under All Conditions
+====================================
+
+Introduction: 
+=============
+On the "defining undefined behavior" page, I said you should write down your
+decisions regarding undefined behavior in your functions. This is that document
+for my printf implementation.
+
+Unless otherwise specified, the functionality described is aligned with the ISO
+C standard and POSIX standard. If any behavior is not mentioned here, it should
+be assumed to follow the behavior described in those standards.
+
+The LLVM-libc codebase is under active development, and may change. This
+document was last updated [August 18, 2023] by [michaelrj] and may
+not be accurate after this point.
+
+The behavior of LLVM-libc's printf is heavily influenced by compile-time flags.
+Make sure to check what flags are defined before filing a bug report. It is also
+not relevant to any other libc implementation of printf, which may or may not
+share the same behavior.
+
+This document assumes familiarity with the definition of the printf function and
+is intended as a reference, not a replacement for the original standards.
+
+--------------
+General Flags:
+--------------
+These compile-time flags will change the behavior of LLVM-libc's printf when it
+is compiled. Combinations of flags that are incompatible will be marked.
+
+LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+--------------------------------
+When set, this flag changes fprintf and printf to use the FILE API from the
+system's libc, instead of LLVM-libc's internal FILE API. This is set by default
+when LLVM-libc is built in overlay mode.
+
+LIBC_COPT_PRINTF_DISABLE_INDEX_MODE
+-----------------------------------
+When set, this flag disables support for the POSIX "%n$" format, hereafter
+referred to as "index mode"; conversions using the index mode format will be
+treated as invalid. This reduces code size.
+
+LIBC_COPT_PRINTF_INDEX_ARR_LEN
+------------------------------
+This flag takes a positive integer value, defaulting to 128. This flag
+determines the number of entries the parser's type descriptor array has. This is
+used in index mode to avoid re-parsing the format string to determine types when
+an index lower than the previously specified one is requested. This has no
+effect when index mode is disabled.
+
+LIBC_COPT_PRINTF_DISABLE_WRITE_INT
+----------------------------------
+When set, this flag disables support for the C Standard "%n" conversion; any
+"%n" conversion will be treated as invalid. This is set by default to improve
+security.
+
+LIBC_COPT_PRINTF_DISABLE_FLOAT
+------------------------------
+When set, this flag disables support for floating point numbers and all their
+conversions (%a, %f, %e, %g); any floating point number conversion will be
+treated as invalid. This reduces code size.
+
+LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
+----------------------------------
+When set, this flag disables the nullptr checks in %n and %s.
+
+LIBC_COPT_PRINTF_CONV_ATLAS
+---------------------------
+When set, this flag changes the include path for the "converter atlas" which is
+a header that includes all the files containing the conversion functions. This
+is not recommended to be set without careful consideration.
+
+LIBC_COPT_PRINTF_HEX_LONG_DOUBLE
+--------------------------------
+When set, this flag replaces all decimal long double conversions (%Lf, %Le, %Lg)
+with hexadecimal long double conversions (%La). This will improve performance
+significantly, but may cause some tests to fail. This has no effect when float
+conversions are disabled.
+
+--------------------------------
+Float Conversion Internal Flags:
+--------------------------------
+The following floating point conversion flags are provided for reference, but
+are not recommended to be adjusted except by persons familiar with the Printf
+Ryu Algorithm. Additionally they have no effect when float conversions are
+disabled.
+
+LIBC_COPT_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE
+-------------------------------------------------
+When set, the float to string decimal conversion algorithm will use a larger
+table to accelerate long double conversions. This larger table is around 5MB of 
+size when compiled. This flag is enabled by default in the CMake.
+
+LIBC_COPT_FLOAT_TO_STR_USE_DYADIC_FLOAT(_LD)
+--------------------------------------------
+When set, the float to string decimal conversion algorithm will use dyadic
+floats instead of a table when performing floating point conversions. This
+results in ~50 digits of accuracy in the result, then zeroes for the remaining
+values. This may improve performance but may also cause some tests to fail. The
+flag ending in _LD is the same, but only applies to long double decimal
+conversions.
+
+LIBC_COPT_FLOAT_TO_STR_USE_INT_CALC
+-----------------------------------
+When set, the float to string decimal conversion algorithm will use wide
+integers instead of a table when performing floating point conversions. This
+gives the same results as the table, but is very slow at the extreme ends of
+the long double range. If no flags are set this is the default behavior for
+long double conversions.
+
+LIBC_COPT_FLOAT_TO_STR_NO_TABLE
+-------------------------------
+When set, the float to string decimal conversion algorithm will not use either
+the mega table or the normal table for any conversions. Instead it will set
+algorithmic constants to improve performance when using calculation algorithms.
+If this flag is set without any calculation algorithm flag set, an error will
+occur.
+
+--------
+Parsing:
+--------
+
+When printf encounters an invalid conversion specification, the entire
+conversion specification will be passed literally to the output string.
+As an example, printf("%Z") would display "%Z".
+
+If an index mode conversion is requested for index "n" and there exists a number
+in [1,n) that does not have a conversion specified in the format string, then
+the conversion for index "n" is considered invalid.
+
+If a non-index mode (also referred to as sequential mode) conversion is
+specified after an index mode conversion, the next argument will be read but the
+current index will not be incremented. From this point on, the arguments
+selected by each conversion may or may not be correct. This is considered
+dangerously undefined and may change without warning.
+
+If a conversion specification is provided an invalid type modifier, that type
+modifier will be ignored, and the default type for that conversion will be used.
+In the case of the length modifier "L" and integer conversions, it will be
+treated as if it was "ll" (lowercase LL). For this purpose the list of integer
+conversions is d, i, u, o, x, X, n.
+
+If a conversion specification ending in % has any options that consume arguments
+(e.g. "%*.*%") those arguments will be consumed as normal, but their values will
+be ignored.
+
+If a conversion specification ends in a null byte ('\0') then it shall be
+treated as an invalid conversion followed by a null byte.
+
+If a number passed as a min width or precision value is out of range for an int,
+then it will be treated as the largest or smallest value in the int range
+(e.g. "%-999999999999.999999999999s" is the same as "%-2147483648.2147483647s").
+
+----------
+Conversion
+----------
+Any conversion specification that contains a flag or option that it does not
+have defined behavior for will ignore that flag or option (e.g. %.5c is the same
+as %c).
+
+If a conversion specification ends in %, then it will be treated as if it is
+"%%", ignoring all options.
+
+If a null pointer is passed to a %s conversion specification and null pointer
+checks are enabled, it will be treated as if the provided string is "null".
+
+If a null pointer is passed to a %n conversion specification and null pointer
+checks are enabled, the conversion will fail and printf will return a negative
+value.
+
+If a null pointer is passed to a %p conversion specification, the string
+"(nullptr)" will be returned instead of an integer value.
+
+The %p conversion will display any non-null pointer as if it was a uintptr value
+passed to a "%#tx" conversion, with all other options remaining the same as the
+original conversion.
+
+The %p conversion will display a null pointer as if it was the string
+"(nullptr)" passed to a "%s" conversion, with all other options remaining the
+same as the original conversion.