llvm dev - Nov 2015 - How does -fPIE get passed from clang to llc when run on a .ll file?

On Fri, Nov 6, 2015 at 10:46 AM, Tim Northover <t.p.northover at
gmail.com>
wrote:
> On 6 November 2015 at 10:32, Phil Tomson via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > So if I were to pass this .ll file (global_dat_x86_pie.ll) to llc how
> would
> > llc know that the PIE Level is 2? Is this an oversight, bug, or
expetcted
> > behaviour with a different workaround?
>
> It looks like it's the "-enable-pie" command line option,
which gets
> hooked up to TargetOptions::PositionIndependentExecutable if you're
> doing it programmatically. I'm not sure of the history behind why
> they're different though, I'm afraid.
>
>
With 3.6 I get an unused argument warning with -enable-pie:

$ clang -v -emit-llvm -enable-pie -O0 -S global_dat.c -o
global_dat_x86_pie.ll

clang: warning: argument unused during compilation: '-e nable-pie'
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On 6 November 2015 at 11:00, Phil Tomson <phil.a.tomson at gmail.com>
wrote:
> With 3.6 I get an unused argument warning with -enable-pie:
>
> $ clang -v -emit-llvm -enable-pie -O0 -S global_dat.c -o
> global_dat_x86_pie.ll
Yes, it's an llc option not a Clang option. It actually looks like
Clang doesn't forward it to LLVM at all (almost certainly a bug, if
so). But that's probably OK because the only place LLVM actually uses
it at all is to determine what kind of TLS model to use
(TargetMachine.cpp), which linkers can mostly relax anyway.

It does seem to get passed onto the linker by Clang though.

Cheers.

Tim.

On Fri, Nov 6, 2015 at 11:10 AM, Tim Northover <t.p.northover at
gmail.com>
wrote:
> On 6 November 2015 at 11:00, Phil Tomson <phil.a.tomson at gmail.com>
wrote:
> > With 3.6 I get an unused argument warning with -enable-pie:
> >
> > $ clang -v -emit-llvm -enable-pie -O0 -S global_dat.c -o
> > global_dat_x86_pie.ll
>
> Yes, it's an llc option not a Clang option. It actually looks like
> Clang doesn't forward it to LLVM at all (almost certainly a bug, if
> so). But that's probably OK because the only place LLVM actually uses
> it at all is to determine what kind of TLS model to use
> (TargetMachine.cpp), which linkers can mostly relax anyway.
>
In my case I'm actually working on a backend for a new architecture and
need this to show up in LLVM because certain pseudo ops need to be expanded
in a certain way if we're using PIE (vs. just a default static link). So
are these options accessible in the Target-specific code?
>
> It does seem to get passed onto the linker by Clang though.
>
Not sure how that can work.

> Cheers.
>
> Tim.
>
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