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[Bug gas/18009] New: LDR macro on arm can't generate relative address

From: goswin-v-b at web dot de
Subject: [Bug gas/18009] New: LDR macro on arm can't generate relative address
Date: Mon, 23 Feb 2015 15:44:12 +0000


            Bug ID: 18009
           Summary: LDR macro on arm can't generate relative address
           Product: binutils
           Version: 2.25
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: goswin-v-b at web dot de

The arm assembly has a LDR macro to load constants:

    ldr r0, =symbol

This works for symbols (or constants, but that isn't the use case I want) in
the same file and section and also for symbols in other sections or files. For
external symbols it generates an R_ARM_ABS32 linker reference in the next
literals block and adds PC relative addressing to load that.

One can also generate a R_ARM_REL32 linker reference writing

1:  .word symbol - 1b

This correctly stores the relative (to the 1 label) offset of the symbol and
fills that in at link time (and generates a .rel.dyn entry for shared code).

What does not work is writing

1:  ldr r0, =(symbol - 1b)

boot.S:49: Error: constant expression expected -- `ldr r0,=(symbol-1b)'

this works fine with clangs internal assembler and should work here too. There
seems to be no other way to add a R_ARM_REL32 linker reference to the literals
block and load that into a register.

An alternate this could be something like

    ldr r0, [pc, =symbol]

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