[LLVMdev] Resolving sizeof's; target triples; type optimizations

Harel Cain harel.cain at gmail.com
Thu Oct 27 10:55:15 PDT 2011

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
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

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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