via llvm-dev
2016-May-21 04:22 UTC
[llvm-dev] Using_an_MCStreamer_Directly_to_produce_an_o bject_file?
<html><body><span style="font-family:Verdana; color:#000;
font-size:10pt;"><div>llvm-dev,</div><div><br></div><div>There
were a couple of issues, but I think I have them fixed. The lack of all output
was related to needing the following at the end of my
function:</div><div><br></div><div>
Out->keep();<br>
Str->Finish();</div><div><br></div><div>I was
missing the finish call. Also, while I did make sure to add a .text section, I
wasn't adding symbol attributes to make functions global, so when I got
around to linking, the linker couldn't find things. This was solved with a
simple call
to:</div><div><br></div><div>Str->EmitSymbolAttribute(mcsn,attr);</div><div><br></div><div>Once
with attr as llvm::MCSymbolAttr::MCSA_Global and once as
llvm::MCSymbolAttr::MCSA_ELF_TypeFunction. Everything seems to work now.
Thanks if you took a look, but this issue no longer needs
attention.</div><div><br></div><div><br></div><div>Thanks
much,</div><div>~Aaron
Vose<br></div><div><br></div>
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-------- Original Message --------<br>
Subject: [llvm-dev] Using_an_MCStreamer_Directly_to_produce_an_o<br>
bject_file?<br>
From: via llvm-dev <<a
href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
Date: Fri, May 20, 2016 8:34 pm<br>
To: <a
href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><br>
<br>
llvm-dev,<br>
<br>
<br>
Thanks so much in advance for any help, tips, or advice you may be
able<br>
to offer me. I'm going to try to avoid the big-picture description
of<br>
the project I'm working on, and only talk about the parts that I
have<br>
trouble with / currently need to implement. -- I've been starting
by<br>
taking the source code from the "llvm-mc" tool, and working that
down<br>
into a smaller form that does the kinds of things I want to do at the
MC<br>
"layer".<br>
<br>
<br>
So, I'd like to be able to use a MCStreamer object directly to make
some<br>
output file, and object ".o" would be just fine to start
with.<br>
<br>
I seem to be able to create an aarch64 target without too much issue:<br>
<br>
llvm::InitializeAllTargetInfos();<br>
llvm::InitializeAllTargetMCs();<br>
llvm::InitializeAllAsmParsers();<br>
llvm::InitializeAllDisassemblers();<br>
<br>
std::string Error;<br>
std::string TripleName("aarch64-unknown-linux-gnu");<br>
Triple TheTriple(Triple::normalize(TripleName));<br>
const Target *TheTarget = TargetRegistry::lookupTarget(TripleName,<br>
Error);<br>
if (!TheTarget) {<br>
std::cerr << "llvm_insts_to_binary(): " ><<
Error;<br>
return 1;<br>
}<br>
<br>
<br>
Then, I move on to creating some of the needed ASM / REG info and an<br>
MCContext, etc.:<br>
<br>
std::unique_ptr><MCRegisterInfo><br>
MRI(TheTarget->createMCRegInfo(TripleName));<br>
assert(MRI && "Unable to create target register
info!");<br>
std::unique_ptr<MCAsmInfo>
MAI(TheTarget->createMCAsmInfo(*MRI,<br>
TripleName));<br>
assert(MAI && "Unable to create target asm
info!");<br>
std::string MCPU("generic");<br>
std::string FeaturesStr("");<br>
MCObjectFileInfo MOFI;<br>
MCContext Ctx(<a href="http://MAI.get">MAI.get</a>(),
MRI.get(), &MOFI);<br>
MOFI.InitMCObjectFileInfo(TheTriple, llvm::Reloc::Model::PIC_,<br>
llvm::CodeModel::Model::Default, Ctx);<br>
std::unique_ptr<MCInstrInfo>
MCII(TheTarget->createMCInstrInfo());<br>
std::unique_ptr<MCSubtargetInfo><br>
STI(TheTarget->createMCSubtargetInfo(TripleName, MCPU,
FeaturesStr));<br>
<br>
<br>
OK, that seems, OK. Now I want to setup an output buffer:<br>
<br>
std::string
ofile("/home/users/avose/hpcgp/hpcgp_rundir/tmp.o");<br>
std::unique_ptr<tool_output_file> Out =
GetOutputStream(ofile);<br>
if (!Out) {<br>
return 1;<br>
}<br>
std::unique_ptr<buffer_ostream> BOS;<br>
raw_pwrite_stream *OS = &Out->os();<br>
if (!Out->os().supportsSeeking()) {<br>
BOS = make_unique<buffer_ostream>(Out->os());<br>
OS = <a href="http://BOS.get">BOS.get</a>();<br>
}<br>
<br>
<br>
This also seems to do OK. Now I would think I can finally get around
to<br>
making the code emitter and the MCStreamer:<br>
<br>
MCCodeEmitter *CE = TheTarget->createMCCodeEmitter(*MCII, *MRI,
Ctx);<br>
MCAsmBackend *MAB = TheTarget->createMCAsmBackend(*MRI,
TripleName,<br>
MCPU);<br>
MCStreamer *Str = TheTarget->createMCObjectStreamer(TheTriple,
Ctx,<br>
*MAB, *OS, CE, *STI, true, false);<br>
<br>
<br>
So, like, wow, I would think I should be good to go here! I should be<br>
able to just use functions like:<br>
<br>
Str->EmitInstruction();<br>
Str->EmitLabel();<br>
<br>
<br>
These should then result in everything I've tried to emit coming out
in<br>
my output object file "tmp.o", you know, as long as I'm careful to
be<br>
sure to make a call to keep my output file from being deleted. So,
when<br>
I'm done making "emit" calls like the above, I be sure to do the
keep<br>
call:<br>
<br>
Out->keep();<br>
<br>
<br>
I'm aware that the calls to EmitInstruction(); and EmitLabel(); need
to<br>
have their arguments properly constructed first, and I do try to do
that<br>
using the context I had set up before, so, something like:<br>
<br>
const llvm::Twine tname("my_label_name");<br>
llvm::MCSymbol* mcs = Ctx.getOrCreateSymbol(tname);<br>
Str->EmitLabel(mcs);<br>
<br>
<br>
Or even something more complicated like setting up an MCInst and all<br>
it's operands:<br>
<br>
llvm::MCInst *llinst = new MCInst();<br>
llinst->setOpcode(input_opcode);<br>
llinst->addOperand(llvm::MCOperand::createReg(input_reg0));<br>
llinst->addOperand(llvm::MCOperand::createReg(input_reg1));<br>
const llvm::Twine tname("label_name");<br>
const llvm::MCSymbol* mcs = Ctx.getOrCreateSymbol(tname);<br>
const llvm::MCSymbolRefExpr *msre =
llvm::MCSymbolRefExpr::create(mcs,<br>
Ctx);<br>
llinst->addOperand(llvm::MCOperand::createExpr(msre));<br>
<br>
<br>
However, while I don't get any compiler errors, and everything runs
to<br>
completion without printing any errors, my output file "tmp.o" is
always<br>
created, but just empty with zero bytes. I must be doing something<br>
silly here.. Does anyone have any ideas about what I'm doing wrong,
or<br>
perhaps have example code that directly uses the MCStreamer object to<br>
write out object binary files? Some examples showing how to make this<br>
work would _really_ make my week.<br>
<br>
This has been hard to debug, as I don't get any errors, just an
empty<br>
output .o file...<br>
<br>
I was thinking it may be because of section issues, so I did add some<br>
initial sections, hoping that might help:<br>
<br>
Str->InitSections(false);<br>
MCSection *textsect = Ctx.getELFSection(".text",
ELF::SHT_PROGBITS,<br>
ELF::SHF_EXECINSTR | ELF::SHF_ALLOC);<br>
Str->SwitchSection(textsect);<br>
<br>
<br>
This did not seem to help at all. I'm running out of ideas.
Anyone<br>
know how to use an MCStreamer object directly to make and object file?<br>
<br>
<br>
Thanks so much for your time,<br>
~Aaron Vose<br>
<br>
<br>
(Software Engineer)<br>
(Cray Inc.)<br>
<br>
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