[patch][aarch64]: fix frame pointer setup before tlsdesc call

[Sylvia Taylor]

Greetings,

This patch fixes a bug with TLS in which the frame pointer is not
established until after the tlsdesc call, thus not conforming to
the aarch64 procedure call standard.

Changed the tlsdesc instruction patterns to set a dependency on the
x29 frame pointer. This helps the instruction scheduler to arrange
the tlsdesc call after the frame pointer is set.

Example of frame pointer (x29) set incorrectly after tlsdesc call:

stp	x29, x30, [sp, -16]!
adrp	x0, :tlsdesc:.LANCHOR0
ldr	x2, [x0, #:tlsdesc_lo12:.LANCHOR0]
add	x0, x0, :tlsdesc_lo12:.LANCHOR0
.tlsdesccall	.LANCHOR0
blr	x2
...
mov	x29, sp
...

After introducing dependency on x29, the scheduler does the frame
pointer setup before tlsdesc:

stp	x29, x30, [sp, -16]!
mov	x29, sp
adrp	x0, :tlsdesc:.LANCHOR0
ldr	x2, [x0, #:tlsdesc_lo12:.LANCHOR0]
add	x0, x0, :tlsdesc_lo12:.LANCHOR0
.tlsdesccall	.LANCHOR0
blr	x2
...

Testcase used with -O2 -fpic:

void foo()
{
  static __thread int x = 0;
  bar (&x);
}

I am not sure what would classify as an effective check for this
testcase. The only idea I received so far would be to write a regexp
inside a scan-assembler-not that would potentially look for this pattern:

<optional instructions>
.tlsdesccall <label>
blr <reg>
<optional instructions>
[mov x29, sp] OR [add x29, sp, 0]
<optional instructions>

(similar to what was attempted in gcc/testsuite/gcc.target/arm/pr85434.c)

I would like maintainers' input on whether such a testcase should be added
and if there are better ways of checking for the instruction order.

Bootstrapped and tested on aarch64-none-linux-gnu.

Ok for trunk? If yes, I don't have any commit rights, so can someone please
commit it on my behalf.

Cheers,
Syl

gcc/ChangeLog:

2019-06-25  Sylvia Taylor  <sylvia.taylor@arm.com>

	* config/aarch64/aarch64.md
	(tlsdesc_small_advsimd_<mode>): Update.
	(tlsdesc_small_sve_<mode>): Likewise.
	(FP_REGNUM): New.
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