llvm dev - Apr 2006 - [LLVMdev] Re: how to code a loop in llvm assembly

Simon Burton <simon at arrowtheory.com> writes:
> Hi,
>
> I've read over the "LLVM Language Reference Manual"
> a few times, and writing some ll code, but i'm stuck at
> a very basic point. How to decrement a counter variable ?
>
> int %count(int %n) {
> EntryBlock:
>   %cond = seteq int %n, 0
>   br bool %cond, label %Exit, label %Next
> Next:
> ; how to decrement n ?
>   %new_n = sub int %n, 1
>   br label %EntryBlock
> Exit:
>   ret int 0
> }
>
> I guess I could malloc a variable and use store/load. Is that the right way
?
>
> Also the above code is invalid:
> Entry block to function must not have predecessors!
> label %EntryBlock
> Broken module found, compilation aborted!
You need a phi node.

A good way to learn how to write LLVM assembler is to use the online
demo available at http://llvm.org/demo/ You feed a C/C++/Stacker
program and you get the equivalent LLVM code. In this case, something
like this C code is necessary:

int foo(int n, int x) {
  while(n > x) --n;
  return n;
}

The extra stuff is for circumventing the optimizer. If you write

int foo(int n) {
  while(n) --n;
  return 0;
}

the optimizer will reduce it to a ret int 0, which teach us nothing.

An option for disabing optimizations would be very helpful for
learners.

BTW, Simon, is there a reason for writing LLVM assembler and not
generating LLVM code directly? The later is simpler and relieves you
from some nasty burdens. It doesn't require you to generate SSA
conformant code, for instance. Relating to your present problem, it is
"natural" to code a loop without resorting to phi nodes.

-- 
Oscar

Oscar Fuentes <oscarfv at telefonica.net> writes:

[snip]
> BTW, Simon, is there a reason for writing LLVM assembler and not
> generating LLVM code directly? The later is simpler and relieves you
> from some nasty burdens. It doesn't require you to generate SSA
> conformant code, for instance. Relating to your present problem, it is
> "natural" to code a loop without resorting to phi nodes.
Clarification: it is incorrect to say that generating LLVM code by
instantiating LLVM instructions doesn't require to follow SSA
rules. The truth is that the API is designed in such a way that you
follow SSA rules without knowing about them, except for things like
entries/exits of basic blocks (at least, I have not found any other
issues with SSA so far).

-- 
Oscar

Hi Simon,
> I've read over the "LLVM Language Reference Manual" a few
times, and
> writing some ll code, but i'm stuck at a very basic point. How to
> decrement a counter variable ?
As Oscar pointed out, you need a phi-node.  I read some of the published
papers on LLVM before the Language Reference Manual and found them to be
of use.  You may also find Wikipedia's explanation of SSA useful,
especially the key paragraph

    http://en.wikipedia.org/wiki/Static_single_assignment_form

    "Note: the phi functions are not actually implemented; instead,
    they're just markers for the compiler to place the value of all the
    variables grouped together by the phi function, in the same location
    in memory (or same register)."

Cheers,


Ralph.

Hi,

I've read over the "LLVM Language Reference Manual"
a few times, and writing some ll code, but i'm stuck at
a very basic point. How to decrement a counter variable ?

int %count(int %n) {
EntryBlock:
  %cond = seteq int %n, 0
  br bool %cond, label %Exit, label %Next
Next:
; how to decrement n ?
  %new_n = sub int %n, 1
  br label %EntryBlock
Exit:
  ret int 0
}

I guess I could malloc a variable and use store/load. Is that the right way ?

Also the above code is invalid:
Entry block to function must not have predecessors!
label %EntryBlock
Broken module found, compilation aborted!

thanks for any hints,

Simon.

-- 
Simon Burton, B.Sc.
Licensed PO Box 8066
ANU Canberra 2601
Australia
Ph. 61 02 6249 6940
http://arrowtheory.com

On Sat, Apr 15, 2006 at 07:47:00AM +0200, Oscar Fuentes
wrote:
> Simon Burton <simon at arrowtheory.com> writes:
> > I've read over the "LLVM Language Reference Manual"
> > a few times, and writing some ll code, but i'm stuck at
> > a very basic point. How to decrement a counter variable ?
[snip]
> > I guess I could malloc a variable and use store/load. Is that the
right way ?
[snip]
> You need a phi node.
As others have pointed out, because LLVM is in SSA form, you will need
to use a phi node.  To see how it would be written, try using the demo
script as Oscar suggested, or you can do what you suggested (I would use
alloca instead of malloc, though I think either should work) and then
run it through "opt -mem2reg" to convert it to the phi-node form.

-- 
Misha Brukman :: http://misha.brukman.net

Using an alloca'd variable with loads and stores would work well  
too.  To date this
is how every llvm language front end works.  You would want to avoid  
malloc for
such a purpose generally.  mem2reg is nice enough to take your local  
load
and stores and promote them to registers with PHINodes :-)

On Apr 15, 2006, at 4:43 AM, Ralph Corderoy wrote:
>
> Hi Simon,
>
>> I've read over the "LLVM Language Reference Manual" a few
times, and
>> writing some ll code, but i'm stuck at a very basic point. How to
>> decrement a counter variable ?
>
> As Oscar pointed out, you need a phi-node.  I read some of the  
> published
> papers on LLVM before the Language Reference Manual and found them  
> to be
> of use.  You may also find Wikipedia's explanation of SSA useful,
> especially the key paragraph
>
>     http://en.wikipedia.org/wiki/Static_single_assignment_form
>
>     "Note: the phi functions are not actually implemented; instead,
>     they're just markers for the compiler to place the value of all  
> the
>     variables grouped together by the phi function, in the same  
> location
>     in memory (or same register)."
>
> Cheers,
>
>
> Ralph.
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

On Sat, 15 Apr 2006 07:47:00 +0200
Oscar Fuentes <oscarfv at telefonica.net> wrote:
> 
> BTW, Simon, is there a reason for writing LLVM assembler and not
> generating LLVM code directly?
You mean write C++ code that calls the LLVM library ?
I have a mild C++ allergy that I don't wish to aggravate.
> The later is simpler and relieves you
> from some nasty burdens.
Yes, i'm finding it quite hairy!

:)

Simon.

-- 
Simon Burton, B.Sc.
Licensed PO Box 8066
ANU Canberra 2601
Australia
Ph. 61 02 6249 6940
http://arrowtheory.com