llvm dev - Jan 2017 - Got stuck with PC-rel branching

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?

The source itself can be found at
https://github.com/upcFrost/Epiphany/tree/Call_support

Debug output on pastebin: http://pastebin.com/8uKRv0qK



Thanks,

Petr



P.S. sorry for messy code, I knew nothing about LLVM backends when I
started, so the code have copy-paste in more than one place.
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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.

-Eli

-- 
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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Hi,

For the function call - yes, probably. But what about branching inside one
function (standard if-then for example)?

For example:
echo "int g() { int a = 1; if (a > 3) return 1; return 0; }" |
clang -x c -
-o /tmp/a.o -c && objdump -d /tmp/a.o

Thanks,
Petr

On Thu, Jan 19, 2017 at 8:06 PM, Friedman, Eli <efriedma at
codeaurora.org>
wrote:
> 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.
>
> -Eli
>
> --
> 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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On Thu, Jan 19, 2017 at 10:06:35AM -0800, Friedman, Eli via llvm-dev
wrote:
> $ echo "int f(); int g() { return f(); }" | clang -x c - -o
/tmp/a.o -c &&
> objdump -d /tmp/a.o
It is easier to see what happens with objdump -dr :)

Joerg