[Bug 221337] -fsanitize=address (asan) fails on i386

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221337

--- Comment #3 from Ed Maste <emaste at freebsd.org> ---
Looking at libclang_rt.asan-i386.so it does have PT_TLS with 4-byte alignment,
and perhaps that should be changed on general principle:

% readelf -l /usr/lib/clang/5.0.0/lib/freebsd/libclang_rt.asan-i386.so

Elf file type is DYN (Shared object file)
Entry point 0xbb80
There are 6 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x000000 0x00000000 0x00000000 0xc21c4 0xc21c4 R E 0x1000
  LOAD           0x0c21c4 0x000c31c4 0x000c31c4 0x02f08 0x49a58c RW  0x1000
  DYNAMIC        0x0c2470 0x000c3470 0x000c3470 0x000f8 0x000f8 RW  0x4
  TLS            0x0c21c4 0x000c31c4 0x000c31c4 0x00000 0x00018 R   0x4
  GNU_EH_FRAME   0x0b5a50 0x000b5a50 0x000b5a50 0x02934 0x02934 R   0x4
  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4

 Section to Segment mapping:
  Segment Sections...
   00     .hash .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rel.dyn
.rel.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame .comment
.gnu_debuglink .shstrtab 
   01     .tbss .ctors .dtors .jcr .data.rel.ro .dynamic .got .got.plt .data
.bss 
   02     .dynamic 
   03     .tbss .ctors 
   04     .eh_frame_hdr 
   05     
ref12-i386% 

but note that asan is not asserting that one of its symbols is not 8-byte
aligned but rather (some #ifdefs trimmed, showing only the FreeBSD/i386 path):

static void **ThreadSelfSegbase() {
  void **segbase = 0;
  // sysarch(I386_GET_GSBASE, segbase);
  __asm __volatile("mov %%gs:0, %0" : "=r" (segbase));
  return segbase;
}

uptr ThreadSelf() {
  return (uptr)ThreadSelfSegbase()[2];
}

static void GetTls(uptr *addr, uptr *size) {
  void** segbase = ThreadSelfSegbase();
  *addr = 0;
  *size = 0;
  if (segbase != 0) {
    // tcbalign = 16
    // tls_size = round(tls_static_space, tcbalign);
    // dtv = segbase[1];
    // dtv[2] = segbase - tls_static_space;
    void **dtv = (void**) segbase[1];
    *addr = (uptr) dtv[2];
    *size = (*addr == 0) ? 0 : ((uptr) segbase[0] - (uptr) dtv[2]);
  }
}

and later asserts that the addr returned from GetTls is 8-byte aligned

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