[RFC PATCH, i386]: Convert TLS location to DEFAULT_TLS_SEG_REG address space

[Uros Bizjak <ubizjak at gmail dot com>]

Hello!

Using following testcase:

--cut here--
__thread int a;

int foo (void)
{
  return a;
}
--cut here--

Attached patch converts TLS location in the form of:

        (mem/c:SI (plus:DI (unspec:DI [
                        (const_int 0 [0])
                    ] UNSPEC_TP)
                (const:DI (unspec:DI [
                            (symbol_ref:DI ("a") [flags 0x2a]
<var_decl 0x7f8c3acd5e10 a>)
                        ] UNSPEC_NTPOFF))) [1 a+0 S4 A32]))
"thread.c":5 82 {*movsi_internal}
to:

        (mem/c:SI (const:DI (unspec:DI [
                        (symbol_ref:DI ("a") [flags 0x2a]  <var_decl
0x7f8c3acd5e10 a>)
                    ] UNSPEC_NTPOFF)) [1 a+0 S4 A32 AS1])) "thread.c":5 -1

avoiding the UNSPEC_TP tag and instead mark the location with AS.

This way, address space becomes the property of the memory location,
not of the address itself, so we don't need ugly hacks when the
address is moved to a register (LEA ignores segment overrides, c.f.
split_long_move function in i386.c).

Please note that some instructions (e.g. prefetchX) operate on
prefixed *address*, so we can't just rip out non-AS code from
print_operand. The above amended example:

--cut here--
__thread int a;

int foo (void)
{
  __builtin_prefetch (&a, 0);
  return a;
}
--cut here--

compiles to:

        prefetcht0      %fs:a@tpoff
        movl    %fs:a@tpoff, %eax

where prefetch operand remains:

(insn 7 6 16 2 (prefetch (plus:DI (unspec:DI [
                    (const_int 0 [0])
                ] UNSPEC_TP)
            (const:DI (unspec:DI [
                        (symbol_ref:DI ("a") [flags 0x2a]  <var_decl
0x7fe994a14e10 a>)
                    ] UNSPEC_NTPOFF)))
        (const_int 0 [0])
        (const_int 3 [0x3])) "thread.c":5 1010 {*prefetch_sse}
     (nil))

2017-08-31  Uros Bizjak  <ubizjak@gmail.com>

    * config/i386/i386-protos.h (ix86_tls_address_pattern_p) New prototype.
    (ix86_rewrite_tls_address): Ditto.
    * config/i386/i386.c (ix86_tls_address_pattern_p) New function.
    (ix86_rewrite_tls_address_1): Ditto.
    (ix86_rewrite_tls_address): Ditto.
    * config/i386/predicates.md (tls_address_pattern): New predicate.
    * config/i386/i386.md (TLS address splitter): New splitter.

Patch was bootstrapped and regression tested on x86_64-linux-gnu
{,-m32}, all default languages plus go.

I plan to commit the patch to the mainline early next week.

Uros

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