llvm dev - Dec 2012 - [LLVMdev] [MC] [llvm-mc] Getting target specific information to <target>ELFObjectWriter

Older targets like Mips had/have assemblers and ABIs that carry a lot of
baggage.

The small bit of baggage that is giving me fits is that MipsELFObjectWriter
needs to know the relocation model (static,pic,cpic), whether we are using xgot
(-mgot), which abi (old,new), which architecture (32r[123],64[123]), which if
any coprocessor or extention instructions are used (mips16,micromips,etc.).

I shouldn't have to muck with base classes to handle esoteric target
specific issues such as these.

<target>ELFObjectWriteris used for direct object output whether directly
from the codegen or from the llvm-mc assembler.

The grand idea was to use MipsSubtarget because it has almost everything I need,
but the big glitch is that is only used by llc and when used directly by
llvm-mc, we only get the base class MCSubtargetInfo.

AsmPrinter has access to the codegen variation (<target>Subtarget) and
thus doesn't have this issue.

Is the answer for every create<target>MCSubtargetInfo() call to mimic
<target>TargetMachine and create a <target>Subtarget? This would
give a target a common class object to work from. The mechanism for filling the
information doesn't need to be the same, but the subclass does.

This seems a bit heavy handed. I am also looking at Features to see if that
could be abused into passing the information. The Features string is stored in
the MCSubtarget base class.

I believe a clean solution to this could lead to much simpler parameter lists
for creating other MC class objects as well, but currently I just want my
e_header hurt to go away.

Feedback please

Jack



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On 6 December 2012 17:49, Carter, Jack <jcarter at mips.com>
wrote:
> Older targets like Mips had/have assemblers and ABIs that carry a lot of
> baggage.
>
> The small bit of baggage that is giving me fits is that MipsELFObjectWriter
> needs to know the relocation model (static,pic,cpic), whether we are using
> xgot (-mgot), which abi (old,new), which architecture (32r[123],64[123]),
> which if any coprocessor or extention instructions are used
> (mips16,micromips,etc.).
So the mips assembler has command line options like -fPIC?

Cheers,
Rafael

Hi Rafael,

There are a lot of flags. Here are the ones you ask about:

-KPIC, -call_shared	generate SVR4 position independent code
-call_nonpic		generate non-PIC code that can operate with DSOs
-mvxworks-pic	generate VxWorks position independent code
-non_shared		do not generate code that can operate with DSOs
-xgot			assume a 32 bit GOT

Just to make things fun, the SGI notion of cpic (call pic) fits gnu's
-call_nonpic.

Remember, this issue is not whether the direct or standalone assembler can deal
with it in their code. The problem is how to convey it to the
<target>ELFObjectWriter in a clean manor because both use it. Some flags
are picked by being the default, some from the commandline and others from the
run of play. It is not a difficult issue except we require the ELF portion to
work from the same sketch book for the 2 assemblers.

I am experimenting putting in a std::set into the MCSubtargetInfo base class
that is for target use and see if I can end my ordeal ;-) Probably only useful
for the older targets which have a lot of baggage to support. If that works I
can convert it to work with Feature, but that is a bit of overkill at this
point.

Cheers,

Jack
________________________________________
From: Rafael Espíndola [rafael.espindola at gmail.com]
Sent: Friday, December 07, 2012 1:52 PM
To: Carter, Jack
Cc: List
Subject: Re: [LLVMdev] [MC] [llvm-mc] Getting target specific information to
<target>ELFObjectWriter

On 6 December 2012 17:49, Carter, Jack <jcarter at mips.com>
wrote:
> Older targets like Mips had/have assemblers and ABIs that carry a lot of
> baggage.
>
> The small bit of baggage that is giving me fits is that MipsELFObjectWriter
> needs to know the relocation model (static,pic,cpic), whether we are using
> xgot (-mgot), which abi (old,new), which architecture (32r[123],64[123]),
> which if any coprocessor or extention instructions are used
> (mips16,micromips,etc.).
So the mips assembler has command line options like -fPIC?

Cheers,
Rafael