ARM Assembler does not support [pc, #offset] syntax for Thumb narrow LDR instructions.

[RichBarton-Arm]

Bugzilla Link	13259
Resolution	FIXED
Resolved on	Dec 09, 2015 10:51
Version	trunk
OS	All
Blocks	llvm/llvm-bugzilla-archive#18926
CC	@rengolin

Extended Description

LDR ,[pc, #​0]

Should form a valid syntax for a pc-relative load. Currently the Thumb assembler only generates the wide encoding for this instruction, when the narrow encoding should be available.

Reproduce with:

echo "LDR r0,[pc, #​4]" | ./llvm-oss/build-dbg/bin/llvm-mc -triple thumbv7 -show-encoding -show-inst

MC appears to treat immediates as offsets form the pc in this context (which I think is more than a little questionable). Assembling "LDR r0,#4" gives the narrow encoding.

[rengolin]

Hi Richard,

Seems to be working with Thumb1:

$ echo "LDR r0,[pc, #​4]" | llvm-mc -triple thumbv7 -show-encoding -show-inst
.text
ldr r0, [pc, #​4] @ encoding: [0x01,0x48]
@ <MCInst #​2776 tLDRpci
@
@
@
@ >

[rengolin]

Both encodings are identical.

[rengolin]

mentioned in issue llvm/llvm-bugzilla-archive#18926