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Support C++17 over-aligned allocation

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Summary:
Add support for C++17 over-aligned allocation:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0035r4.html

Supporting all 10 operators means we avoid thunking thru libstdc++-v3/libsupc++ and just call jemalloc directly.

It's also worth noting that there is now an aligned *and sized* operator delete:
```
void operator delete(void* ptr, std::size_t size, std::align_val_t al) noexcept;
```

If JeMalloc did not provide this, the default implementation would ignore the size parameter entirely:
https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/libsupc%2B%2B/del_opsa.cc#L30-L33

(I must also update ax_cxx_compile_stdcxx.m4 to a newer version with C++17 support.)

Test Plan:
Wrote a simple test that allocates and then deletes an over-aligned type:
```
struct alignas(32) Foo {};
Foo *f;

int main()
{
  f = new Foo;
  delete f;
}
```

Before this change, both new and delete go thru PLT, and we end up calling regular old free:
```
(gdb) disassemble
Dump of assembler code for function main():
...
   0x00000000004029b7 <+55>:    call   0x4022d0 <_ZnwmSt11align_val_t@plt>
...
   0x00000000004029d5 <+85>:    call   0x4022e0 <_ZdlPvmSt11align_val_t@plt>
...
(gdb) s
free (ptr=0x7ffff6408020) at /home/engshare/third-party2/jemalloc/master/src/jemalloc.git-trunk/src/jemalloc.c:2842
2842            if (!free_fastpath(ptr, 0, false)) {
```

After this change, we directly call new/delete and ultimately call sdallocx:
```
(gdb) disassemble
Dump of assembler code for function main():
...
   0x0000000000402b77 <+55>:    call   0x496ca0 <operator new(unsigned long, std::align_val_t)>
...
   0x0000000000402b95 <+85>:    call   0x496e60 <operator delete(void*, unsigned long, std::align_val_t)>
...
(gdb) s
116             je_sdallocx_noflags(ptr, size);
```
This commit is contained in:
Mark Santaniello
2019-10-26 23:28:42 -07:00
committed by Qi Wang
parent 9a3c738009
commit 8b2c2a596d
4 changed files with 536 additions and 42 deletions
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7
configure.ac
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@@ -290,8 +290,11 @@ if test "x$enable_cxx" = "x1" ; then
dnl Require at least c++14, which is the first version to support sized
dnl deallocation. C++ support is not compiled otherwise.
m4_include([m4/ax_cxx_compile_stdcxx.m4])
AX_CXX_COMPILE_STDCXX([14], [noext], [optional])
if test "x${HAVE_CXX14}" = "x1" ; then
AX_CXX_COMPILE_STDCXX([17], [noext], [optional])
if test "x${HAVE_CXX17}" != "x1"; then
AX_CXX_COMPILE_STDCXX([14], [noext], [optional])
fi
if test "x${HAVE_CXX14}" = "x1" -o "x${HAVE_CXX17}" = "x1"; then
JE_CXXFLAGS_ADD([-Wall])
JE_CXXFLAGS_ADD([-Wextra])
JE_CXXFLAGS_ADD([-g3])