[cfe-dev] Is it possible to select UnknownArch through command line in clang?

[Zvonimir Rakamaric]

Ok, yes, we should probably take a step back...

What SMACK currently does not like is type coercion when for example
{i32,i32} is packed into i64 before a function call, and so on. And
the reason is that this often requires byte-level reasoning that is
not supported in SMACK, and on top of that supporting it would almost
certainly add a performance overhead.

So what I learned after poking around clang source is that enabling
DefaultTargetCodeGenInfo solves this problem since it disables most of
such coercions. After learning that, I ventured into figuring out how
to select DefaultTargetCodeGenInfo from command line, and it appears
that is not possible. Is that right?

Now, I got to UnknownArch by tracking the source code back from where
DefaultTargetCodeGenInfo gets selected. If I remember correctly, clang
will select DefaultTargetCodeGenInfo if UnknownArch is selected in the
Triple. So then I started asking how to select UnknownArch...

One final question: why does DefaultTargetCodeGenInfo even exist if it
cannot be selected?

Thanks!


--
http://zvonimir.info
http://soarlab.org/


On Tue, May 27, 2014 at 7:31 PM, Reid Kleckner <rnk at google.com> wrote:
> I don't see how Clang can support targeting an unknown ISA.  How is it
> supposed to lower sizeof(void*) without real data layout information?  Or do
> any record layout at all?  The calling convention stuff is really just part
> of it.  Is there any reason you can't use the le32 target, or whatever PNaCl
> uses?  Why is byval struct expansion a problem for SMACK?
>
>
> On Tue, May 27, 2014 at 8:30 AM, Zvonimir Rakamaric <zvonimir at cs.utah.edu>
> wrote:
>>
>> Ok, I invested some time looking into this issue...
>>
>> Here is the update I propose in clang/lib/basic/Targets.cpp:
>> static TargetInfo *AllocateTarget(const llvm::Triple &Triple) {
>>   llvm::Triple::OSType os = Triple.getOS();
>>
>>   switch (Triple.getArch()) {
>>   default:
>>     return NULL;
>>
>>   // Add this case to support UnknownArch
>>   case llvm::Triple::UnknownArch:
>>     return new X86_64TargetInfo(Triple);
>>
>>   case llvm::Triple::xcore:
>>     return new XCoreTargetInfo(Triple);
>>
>>
>> Please comment... This works fine on my regressions, and I suspect it
>> should work fine on clang regressions too.
>>
>> Now, is there any chance this update could be pushed into the main
>> clang trunk so that unknown architecture is supported?
>>
>> This is really important to us, and releasing a patch for clang with
>> SMACK verifier would be a very inconvenient solution.
>>
>> Thanks a lot!
>>
>> Best,
>> -- Zvonimir
>>
>>
>> --
>> http://zvonimir.info
>> http://soarlab.org/
>>
>>
>> On Fri, May 23, 2014 at 6:36 PM, Nikola Smiljanic <popizdeh at gmail.com>
>> wrote:
>> > Looking at Driver.cpp, passing unknown as Arch is treated as an error.
>> > Not
>> > sure if hacking this check would solve your problem or explode later.
>> >
>> >
>> > On Sat, May 24, 2014 at 2:06 AM, Zvonimir Rakamaric
>> > <zvonimir at cs.utah.edu>
>> > wrote:
>> >>
>> >> Not really...
>> >>
>> >> My question is a follow-up on this:
>> >> http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-May/072986.html
>> >>
>> >> I figured out how to turn off coercion by hacking into clang and
>> >> adding one line that selects DefaultTargetCodeGenInfo (which does not
>> >> do coercion) by default. Now, this is a hacky solution that requires
>> >> distributing this weird patch for clang with my tool (SMACK verifier).
>> >>
>> >> So I wonder if the same thing could somehow be achieved through
>> >> command line. I tried something like:
>> >> clang --target=unknown-unknown-linux-gnu simple.c
>> >> but that throws an error.
>> >>
>> >> Thanks!
>> >>
>> >>
>> >> --
>> >> http://zvonimir.info
>> >> http://soarlab.org/
>> >>
>> >>
>> >> On Fri, May 23, 2014 at 2:13 AM, Jonathan Roelofs
>> >> <jonathan at codesourcery.com> wrote:
>> >> > This might be me answering the question you're trying to ask, but not
>> >> > the
>> >> > one you're specifically asking.... does '-S -emit-llvm' do what you
>> >> > want
>> >> > to
>> >> > do?
>> >> >
>> >> > Jon
>> >> >
>> >> >
>> >> > On 5/22/14, 6:12 PM, Zvonimir Rakamaric wrote:
>> >> >>
>> >> >> Hi all,
>> >> >>
>> >> >> Quick background story: I would like to select
>> >> >> DefaultTargetCodeGenInfo when generating LLVM IR for a project of
>> >> >> mine. And as far as I could figure out, that could be accomplished
>> >> >> by
>> >> >> selecting UnknownArch as the target architecture.
>> >> >>
>> >> >> What I could not figure out was how to set UnknownArch from the
>> >> >> command line using --target or something like that.
>> >> >>
>> >> >> Invoking clang like this:
>> >> >> clang --target=x86_64-unknown-linux-gnu simple.c
>> >> >> is fine.
>> >> >>
>> >> >> But for this:
>> >> >> clang --target=unknown-unknown-linux-gnu simple.c
>> >> >>
>> >> >> I get an error:
>> >> >> error: unknown target triple 'unknown-unknown-linux-gnu', please use
>> >> >> -triple or -arch
>> >> >>
>> >> >> It would really be awesome if UnknownArch could somehow be selected
>> >> >> from the command line. Is that possible? Am I missing something?
>> >> >>
>> >> >> Thanks!
>> >> >> -- Zvonimir
>> >> >> _______________________________________________
>> >> >> cfe-dev mailing list
>> >> >> cfe-dev at cs.uiuc.edu
>> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>> >> >>
>> >> >
>> >> > --
>> >> > Jon Roelofs
>> >> > jonathan at codesourcery.com
>> >> > CodeSourcery / Mentor Embedded
>> >> >
>> >> _______________________________________________
>> >> cfe-dev mailing list
>> >> cfe-dev at cs.uiuc.edu
>> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>> >
>> >
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