llvm dev - Dec 2011 - [LLVMdev] Adding option to LLVM opt to disable a specific pass from command line

David,

I think my explanation is not clear, my front-end did NOTt generate
'llvm.memcpy' it generate LL code that after use of LLVM 'opt'
get
transformed by 'loop-idom' pass into an 'llvm.memcpy' for an
overlapping
loop:

static void
t0(int n)
{
    int i;
    for (i=0; i<n; i++)
    result[i+1] = result[i];
}

Then 'llc' expanded llvm.memcpy into a sequence of load/store that where
performed out-of-order and thus the final code was incorrect.
So to sumarize, BUG was not in my front-end, it was in LLVM 'opt'
loop-idom
pass, it seems now fixed into 3.0. However I think I would be a good idea
to add more control from command line on pass we want to disable.

Best Regards
Seb

2011/12/8 David Blaikie <dblaikie at gmail.com>
> > For instance, I figured out that loop-idiom pass has a BUG in
> > LLVM 2.9, a llvm.memcpy is generated for an overlapping memory region
and
> > then x86 backend reorder loads/store thus generating a BUG.
>
> Just for the record it seems this is a bug in your frontend, not in
> the LLVM backend. The memcpy intrinsic, like the standard memcpy
> function, requires that the regions be non-overlapping:
> http://llvm.org/docs/LangRef.html#int_memcpy By violating this
> contract it's possible you'll encounter all sorts of issues in
other
> passes too.
>
> - David
>
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On Fri, Dec 09, 2011 at 10:03:37AM +0100, Seb wrote:
> I think my explanation is not clear, my front-end did NOTt generate
> 'llvm.memcpy' it generate LL code that after use of LLVM
'opt' get
> transformed by 'loop-idom' pass into an 'llvm.memcpy' for
an overlapping
> loop:
> 
> static void
> t0(int n)
> {
>     int i;
>     for (i=0; i<n; i++)
>     result[i+1] = result[i];
> }
Do you really want to assign result[0] to everything?

I wonder how much work it is to each the loop-idiom pass to handle this
and the case of reverse indices correctly, if result is char *. E.g.
create either memset or memmove...

Joerg

2011/12/9 Joerg Sonnenberger <joerg at britannica.bec.de>
> On Fri, Dec 09, 2011 at 10:03:37AM +0100, Seb wrote:
> > I think my explanation is not clear, my front-end did NOTt generate
> > 'llvm.memcpy' it generate LL code that after use of LLVM
'opt' get
> > transformed by 'loop-idom' pass into an 'llvm.memcpy'
for an overlapping
> > loop:
> >
> > static void
> > t0(int n)
> > {
> >     int i;
> >     for (i=0; i<n; i++)
> >     result[i+1] = result[i];
> > }
>
> Do you really want to assign result[0] to everything?
>
> I wonder how much work it is to each the loop-idiom pass to handle this
> and the case of reverse indices correctly, if result is char *. E.g.
> create either memset or memmove...
>
> Joerg
>
This thread is not to discuss how relevant this example is. I just would
like to know:
a) If people think that adding an option to disable a specific pass is
useful.
b) To discuss implementation details (disable all pass invocations, what to
do when there are dependencies between passes invocations).
c) If I implement it, what's the process to get my change merge with trunk.

Now for my own purpose, I commented out loop-idiom invocation in LLVM 2.9
sources and it worked well. I just wanted to develop something generic that
could benefit LLVM community.

Best Regards
Seb
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