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help: update the text documenting the -gc MODE option (#20179)

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vlib/v/help/build/build-c.txt
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@ -115,7 +115,7 @@ see also `v help build`.
-os <os>, -target-os <os>
Change the target OS that V tries to compile for.
By default, the target OS is the host system.
Here is a list of the operating systems, supported by V:
(CI tests runs on every commit/PR for each of these):
`windows`, `linux`, `macos`
@ -180,36 +180,46 @@ see also `v help build`.
more performant without autofree.
-gc <mode>
Use and link an optional garbage collector. Only the Boehm–Demers–Weiser
garbage collector is supported currently with the following sub-options:
V uses a garbage collector by default. Only the Boehm–Demers–Weiser garbage collector
is supported currently with the following sub-options:
`-gc boehm` ........... selects the default mode for the architecture
`-gc boehm` ........... use default garbage collector (same as not specifying option)
`-gc boehm_full` ...... full garbage collection mode
`-gc boehm_incr` ...... incremental/generational garbage collection mode
`-gc boehm_full_opt` .. optimized full garbage collection mode
`-gc boehm_incr_opt` .. optimized incremental/generational garbage collection mode
`-gc boehm_leak` ...... leak detection mode
`-gc none` ............ no garbage collector
You need to install a `libgc-dev` package first, or install it manually from:
https://github.com/ivmai/bdwgc
On Mac OS, it can be installed using homebrew (https://homebrew.sh/) with
`brew install libgc`.
Note, `-gc boehm` is complementary to -autofree. The Boehm garbage
collector is conservative, and it may make your program significantly
slower if it does many small allocations in a loop. This option
is intended *mainly* for reducing the memory usage of programs, that
process large amounts of text in *batch mode* on low/limited memory
environments like small VPSes, and for which a few ms of garbage
collection pauses from time to time *do not matter much*.
Note: the garbage collector is complementary to -autofree. The Boehm garbage
collector is conservative, and it may affect program speed if it does many small
allocations in a loop.
The option `-gc boehm_leak` is intended for leak detection in
manual memory management. The function `gc_check_leaks()`
can be called to get detection results. This function is a no-op
when `-gc boehm_leak` is not supplied.
Use `-gc none` to completely disable the garbage collector, in case of
wanting to handle memory manually, or strictly using `-autofree`.
Note: some short lived applications, like compilers and other CLI tools can be
more performant, without autofree, and without a garbage collector, instead
relying on the OS to free the allocated memory automatically, after their
process ends.
On most supported platforms, the source of the used garbage collector is
located in `thirdparty/libgc/`, produced by post-processing
`https://github.com/ivmai/bdwgc`. If you have installed libgc by other means,
(through your distro's package manager or by compiling it yourself and
installing it), you can also tell V to use it instead of its own copy,
by adding `-d dynamic_boehm` to the command line while compiling your program.
On Debian you can use `sudo apt-get install libgc-dev`.
On macOS, you can install it, using homebrew (https://homebrew.sh/)
with `brew install libgc`.
# Miscellaneous:
-printfn <fn_name>
Print the content of the generated C function named fn_name.
@ -291,8 +301,8 @@ see also `v help build`.
Example sample of the content of that file:
system,linux,eval
system,amd64,eval
system,solaris,skip
system,solaris,skip
system,solaris,skip
system,solaris,skip
user,gcboehm,eval
user,gg_record_trace,skip
@ -343,5 +353,4 @@ see also `v help build`.
See https://en.wikipedia.org/wiki/Floating-point_arithmetic#%22Fast_math%22_optimization
https://learn.microsoft.com/en-us/cpp/build/reference/fp-specify-floating-point-behavior?view=msvc-170&redirectedfrom=MSDN
https://clang.llvm.org/docs/UsersManual.html#cmdoption-ffast-math
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-ffast-math
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-ffast-math