[LLVMdev] Possible error in LegalizeDAG

[Villmow, Micah]

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
On Behalf Of Eli Friedman
Sent: Wednesday, February 18, 2009 3:01 PM
To: LLVM Developers Mailing List
Subject: Re: [LLVMdev] Possible error in LegalizeDAG

On Wed, Feb 18, 2009 at 10:14 AM, Villmow, Micah <Micah.Villmow at amd.com>
wrote:
> I'm still trying to track down some alignment issues with loads(i.e.
8/16
> bit loads being turned into 32bit sign extending loads) and I cannot
for the
> life of me seem to figure out how to enter this section of code:
>
> // If this is an unaligned load and the target doesn't support it,
>
>             // expand it.

>Why do you expect to enter this section of code?  It's impossible for
>an i8 load to be unaligned.

On the hardware that I am targeting, which is not a CPU, I must support
i8 loads, however the hardware only supports natively 32bit aligned
loads, therefore I have to read in 4 i8's and unpack them and shift them
based on the read address.  So any i8 load has a 75% chance of being
unaligned on my hardware, so I need a way to tell LLVM not to generate
sext_loads, or if it does to expand them. Everything that has been
suggested so far has not worked.

> Any hints would be greatly appreciated, this is a blocking issue that
I just
> cannot seem to resolve without modifying the LLVM codebase to remove
the
> extend + load -> extload combining step.

>LLVM will "uncombine" it for you if you use setLoadExtAction with the
>appropriate arguments.

>-Eli

I've tried setting setLoadXAction to Custom, Legal, Expand and Promote.
I assert somewhere when I try to custom expanding this operation because
it expects it to be a certain form, but my custom load instruction has a
different form. Setting it to Legal generates the sext_load in the first
dag combine pass, because it never checks if it should make this
combination. Since it doesn't enter the section of code I mentioned
earlier, it never uncombines it. When I set it to promote, it asserts on
"not yet implemented". Setting it to Expand does not expand it to
sign_extend and load but to  extload and sign_extend, but I don't
support extload either.

Please correct me if I am wrong, but I've been looking at this issue for
awhile now and I cannot see where it uncombines the sextload to a load
and sign_extension.

My current solution is to just comment out that combination so that it
never occurs.

Thanks,
Micah