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Add experimental API: smallocx_return_t smallocx(size, flags)

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---

Motivation:

This new experimental memory-allocaction API returns a pointer to
the allocation as well as the usable size of the allocated memory
region.

The `s` in `smallocx` stands for `sized`-`mallocx`, attempting to
convey that this API returns the size of the allocated memory region.

It should allow C++ P0901r0 [0] and Rust Alloc::alloc_excess to make
use of it.

The main purpose of these APIs is to improve telemetry. It is more accurate
to register `smallocx(size, flags)` than `smallocx(nallocx(size), flags)`,
for example. The latter will always line up perfectly with the existing
size classes, causing a loss of telemetry information about the internal
fragmentation induced by potentially poor size-classes choices.

Instrumenting `nallocx` does not help much since user code can cache its
result and use it repeatedly.

---

Implementation:

The implementation adds a new `usize` option to `static_opts_s` and an `usize`
variable to `dynamic_opts_s`. These are then used to cache the result of
`sz_index2size` and similar functions in the code paths in which they are
unconditionally invoked. In the code-paths in which these functions are not
unconditionally invoked, `smallocx` calls, as opposed to `mallocx`, these
functions explicitly.

---

[0]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0901r0.html
This commit is contained in:
gnzlbg
2017-11-15 18:26:49 +01:00
committed by Jason Evans
parent 325e3305fc
commit 08260a6b94
5 changed files with 108 additions and 2 deletions
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19
configure.ac
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@@ -850,7 +850,7 @@ AC_ARG_WITH([export],
fi]
)
public_syms="aligned_alloc calloc dallocx free mallctl mallctlbymib mallctlnametomib malloc malloc_conf malloc_message malloc_stats_print malloc_usable_size mallocx nallocx posix_memalign rallocx realloc sallocx sdallocx xallocx"
public_syms="aligned_alloc calloc dallocx free mallctl mallctlbymib mallctlnametomib malloc malloc_conf malloc_message malloc_stats_print malloc_usable_size mallocx smallocx nallocx posix_memalign rallocx realloc sallocx sdallocx xallocx"
dnl Check for additional platform-specific public API functions.
AC_CHECK_FUNC([memalign],
[AC_DEFINE([JEMALLOC_OVERRIDE_MEMALIGN], [ ])
@@ -1043,6 +1043,22 @@ if test "x$enable_stats" = "x1" ; then
fi
AC_SUBST([enable_stats])
dnl Do not enable smallocx by default.
AC_ARG_ENABLE([experimental_smallocx],
[AS_HELP_STRING([--enable-experimental-smallocx], [Enable experimental smallocx API])],
[if test "x$enable_experimental_smallocx" = "xno" ; then
enable_experimental_smallocx="0"
else
enable_experimental_smallocx="1"
fi
],
[enable_experimental_smallocx="0"]
)
if test "x$enable_experimental_smallocx" = "x1" ; then
AC_DEFINE([JEMALLOC_EXPERIMENTAL_SMALLOCX_API])
fi
AC_SUBST([enable_experimental_smallocx])
dnl Do not enable profiling by default.
AC_ARG_ENABLE([prof],
[AS_HELP_STRING([--enable-prof], [Enable allocation profiling])],
@@ -2281,6 +2297,7 @@ AC_MSG_RESULT([malloc_conf : ${config_malloc_conf}])
AC_MSG_RESULT([autogen : ${enable_autogen}])
AC_MSG_RESULT([debug : ${enable_debug}])
AC_MSG_RESULT([stats : ${enable_stats}])
AC_MSG_RESULT([experimetal_smallocx : ${enable_experimental_smallocx}])
AC_MSG_RESULT([prof : ${enable_prof}])
AC_MSG_RESULT([prof-libunwind : ${enable_prof_libunwind}])
AC_MSG_RESULT([prof-libgcc : ${enable_prof_libgcc}])