Cross-module TLS imports are broken on MIPS

[alexcrichton]

This is a tracking issue for this debugging story which concludes with:

$ cat foo.ll
@foo = external hidden thread_local global i32

define i32 @bar() {
  %a = load i32, i32* @foo
  ret i32 %a
}
$ llc foo.ll -filetype=obj -o foo.o -mtriple=mipsel-unknown-linux-gnu
$ readelf -Ws foo.o

Symbol table '.symtab' contains 5 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 FILE    LOCAL  DEFAULT  ABS foo.ll
     2: 00000000     0 SECTION LOCAL  DEFAULT    2
     3: 00000000    24 FUNC    GLOBAL DEFAULT    2 bar
     4: 00000000     0 TLS     GLOBAL HIDDEN   UND foo
$ llc foo.ll -filetype=obj -o foo.o -mtriple=mipsel-unknown-linux-gnu -relocation-model=pic
$ readelf -Ws foo.o

Symbol table '.symtab' contains 7 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 FILE    LOCAL  DEFAULT  ABS foo.ll
     2: 00000000     0 SECTION LOCAL  DEFAULT    2
     3: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND __tls_get_addr
     4: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND _gp_disp
     5: 00000000    56 FUNC    GLOBAL DEFAULT    2 bar
     6: 00000000     0 NOTYPE  GLOBAL HIDDEN   UND foo

Notably the import of foo on MIPS is not flagged as "TLS" under the "Type" column when the import is both hidden visibility and compiled with -relocation-model=pic. This is most likely an LLVM bug but we're starting to implement workarounds for this, and those workarounds will reference this issue.

[alexcrichton]

A fix has landed upstream, I'm now testing to see whether it works.