[llvm-dev] Got stuck with PC-rel branching

Friedman, Eli via llvm-dev llvm-dev at lists.llvm.org
Thu Jan 19 10:06:35 PST 2017

On 1/19/2017 9:11 AM, Peter Bel via llvm-dev wrote:
> Hi all,
> I'm trying to make an LLVM backend for the Adapteva's Epiphany E16 CPU 
> (used in Parallella board), using CPU0 and some other backends as 
> examples, and I've got stuck with branching.
> When I'm printing out asm, all branch labels are printed as they 
> should be. But when I'm trying to generate obj file, I'm getting zeros 
> instead of PC-related offset in all branch instructions.
> In short, what I'm doing:
> - Pattern (br bb:$addr) is selected using EpiphanyInstrInfo.td as 
> BNONE32(ins jmptarget:$addr), Branch32 class
> - Branch32 class is defined in EpiphanyInstrFormats.td with bits<24> 
> addr, which should go into bits{31-8} of the MC instruction. Those 
> bits remain zeros after relaxation for some reason.
> - jmptarget operand has type OPERAND_PCREL, and uses EncoderMethod 
> "getJumpTargetOpValue" defined in EpiphanyMCCodeEmitter. If this 
> method gets MCExpr, it creates fixup, and I can see this fixup in debug.
> - Fixup is called fixup_Epiphany_PCREL24, and is defined in 
> EpiphanyFixupKinds and EpiphanyAsmBackend, with FKF_IsPCRel flag.
> Can someone please tell me if I am missing something?

Are you sure there's actually a problem?  On x86, for example:

$ echo "int f(); int g() { return f(); }" | clang -x c - -o /tmp/a.o -c 
&& objdump -d /tmp/a.o

/tmp/a.o:     file format elf64-x86-64

Disassembly of section .text:

0000000000000000 <g>:
    0:   55                      push   %rbp
    1:   48 89 e5                mov    %rsp,%rbp
    4:   b0 00                   mov    $0x0,%al
    6:   e8 00 00 00 00          callq  b <g+0xb>
    b:   5d                      pop    %rbp
    c:   c3                      retq

It looks like the call isn't calling the right function, but that's 
fine: it'll get fixed by the linker.


Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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