Re: Fix assembly of Thumb pcrel LDRs against global symbols

[Dave Martin <dave dot martin at linaro dot org>]

Hi,

On Mon, Feb 28, 2011 at 12:59:04PM +0000, Richard Sandiford wrote:
> The PC-relative LDR instructions have no associated relocation,
> so even LDRs for global symbols should be resolved by the assembler.
> We currently handle this correctly for single-register ARM loads,
> but we're missing the associated relocation types for LDRD and Thumb.
> This leads to errors like:
> 
>   Error: invalid offset, value too big (0xFFFFFFFFFFFFFFFC)
> 
> or:
> 
>   Error: cannot represent T32_OFFSET_IMM relocation in this object file format
> 
> Tested on arm-linux-gnueabi.  OK to install?

This seems to work for me for some simple ldrs on locally-
defined global symbols in Thumb.

Note that the annotation of ldrd instructions in Thumb during
disassembly appears incomplete: the target label is not
annotated, whereas for ldr, or for ldr or ldrd in ARM, the
instruction is annotated with the target label in the
disassembly.

I believe the generated instruction encoding is right in the case
shown below, but it might be a good idea to double-check.

Cheers
---Dave

binutils$ cat <<EOF >ldr-tst.s
.syntax unified

.globl d
d:	.quad	0

.type f, %function
f:	ldrd	r0, r1, d
	nop
	ldrd	r0, r1, d
	ldr	r0, d
	nop
	ldr	r0, d
EOF

binutils$ binutils/objdump -mthumb -o ldr-tst.o ldr-tst.s && arm-linux-gnueabi-objdump -dr ldr-tst.o

[...]

Disassembly of section .text:

00000000 <d>:
	...

00000008 <f>:
   8:	e95f 0103 	ldrd	r0, r1, [pc, #-12]
   c:	46c0      	nop			; (mov r8, r8)
   e:	e95f 0104 	ldrd	r0, r1, [pc, #-16]
  12:	f85f 0014 	ldr.w	r0, [pc, #-20]	; 0 <d>
  16:	46c0      	nop			; (mov r8, r8)
  18:	f85f 001c 	ldr.w	r0, [pc, #-28]	; 0 <d>