llvm dev - Oct 2011 - [LLVMdev] Resolving sizeof's; target triples; type optimizations

Thanks for the answers.
> See http://llvm.org/docs/FAQ.html#platformindependent .  If you're
> dealing with C code, it isn't too hard to add a new target to clang;
> send an email to cfe-dev if you need help with that.  The "target
> datalayout" information is purely a hint to the optimizers.
My experience this far has been mainly with using llvm-gcc and llvmc,
especially using llvmc as a cross-compiler, with passing the correct flags
(mainly -march) to llc via llvmc so as to pick for example ARM, MIPS and C
backends.
In doing this, I'm essentially overriding any "target triple"
existing in
the IR, breaking up the process into a multi-stage operation (llvm-gcc, opt
and llc running independently) and picking whatever target I want, right?

What's the proper way to use clang as a cross-compiler (for example with the
ARM or C backends) then?

The reason I want to avoid optimizing 8 x i8 into i64 is that I use the C
backend (with the resultant C files as input to legacy compilers on esoteric
platforms), and i64s turn into long longs that don't exist on those
platforms.

Thanks again,
Harel Cain

On Thu, Oct 27, 2011 at 17:53, Eli Friedman <eli.friedman at gmail.com>
wrote:
> On Thu, Oct 27, 2011 at 7:31 AM, Harel Cain <harel.cain at gmail.com>
wrote:
> > Hi all,
> >
> > A few different though somewhat related questions here. I'm really
> grateful
> > for your answers!
> >
> > 1. From a previous question I know that sizeof's are resolved into
> literals
> > early in the front-end before IR is even emitted. It seems that they
are
> > resolved into whatever value is correct for the host machine. But if
one
> > wishes to then take the IR and emit assembly code for some other
platform
> > (in other words, to cross-compile), what's the correct way to go
about
> it?
> > How to make the front-end resolve the sizeof's according to some
other
> > target triple? For example, the sizeof of a C struct of one int and
one
> char
> > is resolved into 8 and not 5, but for some platforms out there 5 might
be
> > the correct answer. Will the "target datalayout" information
in the IR
> file
> > affect this in any way?
>
>
>
> > 2. What is that "target triple" as appears in the IR code?
Who inserts
> it?
> > Who takes note of it, if at all? Can the front-end be made to use some
> other
> > triple than the native one? Does it affect the backends at all?
>
> In the workflow for compiling C, the clang frontend sets it, and the
> backend uses it to pick the correct target the generate assembly for.
>
> > 3. In my code, I see an array of 8 x i8 being optimized into i64 (so
that
> > memcpy is then optimized into mov) by some optimization
> transformation(s). I
> > want to prevent this from happening. What optimization does this kind
of
> > type replacement? How can one prevent it alone from running (but
keeping
> all
> > other optmizations, i. e. not using -O0 or something similar).
>
> IIRC, instcombine does this; I'm not sure why you would want to
> disable it, though.
>
> -Eli
>


-- 
Quidquid latine dictum sit, altum videtur.
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Hello
> In doing this, I'm essentially overriding any "target triple"
existing in
> the IR, breaking up the process into a multi-stage operation (llvm-gcc, opt
> and llc running independently) and picking whatever target I want, right?
No. This won't work since, as indicated in the FAQ entry IR derived
from C/C++ is not target neutral, so you cannot 'just' override the
triple.
You have to build/use llvm-gcc/clang as cross-compiler.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

I guess that involves some special flags when running 'configure' before
building the front-end. Right? Is there any handy documentation on how to
build/use llvm-gcc as a cross-compiler?


Harel

On Thu, Oct 27, 2011 at 20:47, Anton Korobeynikov
<anton at korobeynikov.info>wrote:
> Hello
>
> > In doing this, I'm essentially overriding any "target
triple" existing in
> > the IR, breaking up the process into a multi-stage operation
(llvm-gcc,
> opt
> > and llc running independently) and picking whatever target I want,
right?
> No. This won't work since, as indicated in the FAQ entry IR derived
> from C/C++ is not target neutral, so you cannot 'just' override the
> triple.
> You have to build/use llvm-gcc/clang as cross-compiler.
>
> --
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University
>


-- 
Quidquid latine dictum sit, altum videtur.
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