New submission from STINNER Victor <vstin...@redhat.com>:
On x86-64, clang -O3 compiles the following function:
PyCArgObject *
PyCArgObject_new(void)
{
PyCArgObject *p;
p = PyObject_New(PyCArgObject, &PyCArg_Type);
if (p == NULL)
return NULL;
p->pffi_type = NULL;
p->tag = '\0';
p->obj = NULL;
memset(&p->value, 0, sizeof(p->value));
return p;
}
like that:
0x00007fffe9c6acb0 <+0>: push rax
0x00007fffe9c6acb1 <+1>: mov rdi,QWORD PTR [rip+0xe308] #
0x7fffe9c78fc0
0x00007fffe9c6acb8 <+8>: call 0x7fffe9c5e8a0 <_PyObject_New@plt>
0x00007fffe9c6acbd <+13>: test rax,rax
0x00007fffe9c6acc0 <+16>: je 0x7fffe9c6acdf <PyCArgObject_new+47>
0x00007fffe9c6acc2 <+18>: mov QWORD PTR [rax+0x20],0x0
0x00007fffe9c6acca <+26>: mov BYTE PTR [rax+0x28],0x0
0x00007fffe9c6acce <+30>: xorps xmm0,xmm0
0x00007fffe9c6acd1 <+33>: movaps XMMWORD PTR [rax+0x30],xmm0
0x00007fffe9c6acd5 <+37>: mov QWORD PTR [rax+0x40],0x0
0x00007fffe9c6acdd <+45>: pop rcx
0x00007fffe9c6acde <+46>: ret
0x00007fffe9c6acdf <+47>: xor eax,eax
0x00007fffe9c6ace1 <+49>: pop rcx
0x00007fffe9c6ace2 <+50>: ret
The problem is that movaps requires the memory address to be aligned on 16
bytes, whereas PyObject_New() uses pymalloc allocator (the requested size is 80
bytes, pymalloc supports allocations up to 512 bytes) and pymalloc only
provides alignment on 8 bytes.
If PyObject_New() returns an address not aligned on 16 bytes,
PyCArgObject_new() crash immediately with a segmentation fault (SIGSEGV).
CPython must be compiled using -fmax-type-align=8 to avoid such alignment
crash. Using this compiler flag, clag emits expected machine code:
0x00007fffe9caacb0 <+0>: push rax
0x00007fffe9caacb1 <+1>: mov rdi,QWORD PTR [rip+0xe308] #
0x7fffe9cb8fc0
0x00007fffe9caacb8 <+8>: call 0x7fffe9c9e8a0 <_PyObject_New@plt>
0x00007fffe9caacbd <+13>: test rax,rax
0x00007fffe9caacc0 <+16>: je 0x7fffe9caacdf <PyCArgObject_new+47>
0x00007fffe9caacc2 <+18>: mov QWORD PTR [rax+0x20],0x0
0x00007fffe9caacca <+26>: mov BYTE PTR [rax+0x28],0x0
0x00007fffe9caacce <+30>: xorps xmm0,xmm0
0x00007fffe9caacd1 <+33>: movups XMMWORD PTR [rax+0x30],xmm0
0x00007fffe9caacd5 <+37>: mov QWORD PTR [rax+0x40],0x0
0x00007fffe9caacdd <+45>: pop rcx
0x00007fffe9caacde <+46>: ret
0x00007fffe9caacdf <+47>: xor eax,eax
0x00007fffe9caace1 <+49>: pop rcx
0x00007fffe9caace2 <+50>: ret
"movaps" instruction becomes "movups" instruction: "a" stands for "aligned" in
movaps, whereas "u" stands for "unaligned" in movups.
----------
components: Build
messages: 340087
nosy: vstinner
priority: normal
severity: normal
status: open
title: clang expects memory aligned on 16 bytes, but pymalloc aligns to 8 bytes
versions: Python 3.8
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue36618>
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