[ARM] Add support for -mpure-code in thumb-1 (v6m)

[ARM] Add support for -mpure-code in thumb-1 (v6m)

This patch extends support for -mpure-code to all thumb-1 processors,
by removing the need for MOVT.

Symbol addresses are built using upper8_15, upper0_7, lower8_15 and
lower0_7 relocations, and constants are built using sequences of
movs/adds and lsls instructions.

The extension of the *thumb1_movhf pattern uses always the same size
(6) although it can emit a shorter sequence when possible. This is
similar to what *arm32_movhf already does.

CASE_VECTOR_PC_RELATIVE is now false with -mpure-code, to avoid
generating invalid assembly code with differences from symbols from
two different sections (the difference cannot be computed by the
assembler).

Tests pr45701-[12].c needed a small adjustment to avoid matching
upper8_15 when looking for the r8 register.

Test no-literal-pool.c is augmented with __fp16, so it now uses
-mfp16-format=ieee.

Test thumb1-Os-mult.c generates an inline code sequence with
-mpure-code and computes the multiplication by using a sequence of
add/shift rather than using the multiply instruction, so we skip it in
presence of -mpure-code.

With -mcpu=cortex-m0, the pure-code/no-literal-pool.c fails because
code like:
static char *p = "Hello World";
char *
testchar ()
{
  return p + 4;
}

generates 2 indirections (I removed non-essential directives/code)
          .section        .rodata
	  .LC0:
	  .ascii  "Hello World\000"
	  .data
	  p:
	  .word   .LC0
	  .section        .rodata
	  .LC2:
	  .word   p
	  .section .text,"0x20000006",%progbits
	  testchar:
	  push    {r7, lr}
	  add     r7, sp, #0
	  movs    r3, #:upper8_15:#.LC2
	  lsls    r3, #8
	  adds    r3, #:upper0_7:#.LC2
	  lsls    r3, #8
	  adds    r3, #:lower8_15:#.LC2
	  lsls    r3, #8
	  adds    r3, #:lower0_7:#.LC2
	  ldr     r3, [r3]
	  ldr     r3, [r3]
	  adds    r3, r3, #4
	  movs    r0, r3
	  mov     sp, r7
	  @ sp needed
	  pop     {r7, pc}

By contrast, when using -mcpu=cortex-m4, the code looks like:
        .section        .rodata
	.LC0:
	.ascii  "Hello World\000"
	.data
	p:
	.word   .LC0
	testchar:
	push    {r7}
	add     r7, sp, #0
	movw    r3, #:lower16:p
	movt    r3, #:upper16:p
	ldr     r3, [r3]
	adds    r3, r3, #4
	mov     r0, r3
	mov     sp, r7
	pop     {r7}
	bx      lr

I haven't found yet how to make code for cortex-m0 apply upper/lower
relocations to "p" instead of .LC2. The current code looks functional,
but could be improved.

2019-10-18  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/arm-protos.h (thumb1_gen_const_int): Add new prototype.
	* config/arm/arm.c (arm_option_check_internal): Remove restriction
	on MOVT for -mpure-code.
	(thumb1_gen_const_int): New function.
	(thumb1_legitimate_address_p): Support -mpure-code.
	(thumb1_rtx_costs): Likewise.
	(thumb1_size_rtx_costs): Likewise.
	(arm_thumb1_mi_thunk): Likewise.
	* config/arm/arm.h (CASE_VECTOR_PC_RELATIVE): Likewise.
	* config/arm/thumb1.md (thumb1_movsi_symbol_ref): New.
	(*thumb1_movhf): Support -mpure-code.

	gcc/testsuite/
	* gcc.target/arm/pr45701-1.c: Adjust for -mpure-code.
	* gcc.target/arm/pr45701-2.c: Likewise.
	* gcc.target/arm/pure-code/no-literal-pool.c: Add tests for
	__fp16.
	* gcc.target/arm/pure-code/pure-code.exp: Remove thumb2 and movt
	conditions.
	* gcc.target/arm/thumb1-Os-mult.c: Skip if -mpure-code is used.

From-SVN: r279463