[cfe-users] PIC in ARM assembly

Jeff Schenck Jeff.Schenck at dts.com
Mon Feb 24 12:10:34 PST 2014


I'm trying to create position-independent ARM assembly code.  For armcc I do the following:

        IMPORT  foo

L_PIC   ldr     r0, L_FOO
        add     r0, pc, r0
L_FOO   DCD     foo - (L_PIC + 8)

which for gcc looks similar:

        .extern foo
L_PIC:  ldr     r0, L_FOO
        add     r0, pc, r0
L_FOO:  .word   foo - (L_PIC + 8)

This works fine when using the GNU assembler.  But when using Clang I get the following weird error: "symbol '_foo' can not be undefined in a subtraction expression".  Looking at assembly code produced from C by the Clang compiler, it does things differently, using the directive ".indirect_symbol", which gcc and armcc don't support:

        movw    r0, :lower16:(L_FOO-(L_PIC+8))
        movt    r0, :upper16:(L_FOO-(L_PIC+8))
L_PIC:  ldr     r0, [pc, r0]
L_FOO:  .indirect_symbol    _foo

This would be fine except that I want to have one code base that can be built on multiple architectures with different tools.  In the short term, I'm looking for an approach that will work with both gcc and Clang.  In the longer term, it would be nice if this could be "fixed" in Clang.  (I put "fixed" in quotes since I don't know if Clang has compatibility with armcc/gcc as a goal.)  Thanks in advance for any suggestions.

Jeff Schenck


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