[AArch64, 3/6] Restrict indirect tail calls to x16 and x17

[AArch64, 3/6] Restrict indirect tail calls to x16 and x17

This patch is part of a series that enables ARMv8.5-A in GCC and
adds Branch Target Identification Mechanism.

This patch changes the registers that are allowed for indirect tail calls.
We are choosing to restrict these to only x16 or x17.

Indirect tail calls are special in a way that they convert a call statement
(BLR instruction) to a jump statement (BR instruction). For the best possible
use of Branch Target Identification Mechanism, we would like to place a
"BTI C" (call) at the beginning of the function which is only
compatible with BLRs and BR X16/X17. In order to make indirect tail calls
compatible with this scenario, we are restricting the TAILCALL_ADDR_REGS.

In order to use x16/x17 for this purpose, we also had to change the use
of these registers in the epilogue/prologue handling. For this purpose
we are now using x12 and x13 named as EP0_REGNUM and EP1_REGNUM as
scratch registers for epilogue and prologue.

*** gcc/ChangeLog***

2018-01-09  Sudakshina Das  <sudi.das@arm.com>

	* config/aarch64/aarch64.c (aarch64_expand_prologue): Use new
	epilogue/prologue scratch registers EP0_REGNUM and EP1_REGNUM.
	(aarch64_expand_epilogue): Likewise.
	(aarch64_output_mi_thunk): Likewise
	* config/aarch64/aarch64.h (REG_CLASS_CONTENTS): Change
	TAILCALL_ADDR_REGS to x16 and x17.
	* config/aarch64/aarch64.md: Define EP0_REGNUM and EP1_REGNUM.

*** gcc/testsuite/ChangeLog ***

2018-01-09  Sudakshina Das  <sudi.das@arm.com>

	* gcc.target/aarch64/test_frame_17.c: Update to check for EP0_REGNUM
	instead of IP0_REGNUM and add test case.

From-SVN: r267767