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

[Eli Friedman]

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?

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.

> 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