llvm dev - Oct 2013 - [LLVMdev] Question About Function Cloning

Hello all;

I have been writing this LLVM pass that extracts loops in a function
and then duplicates the function so the loop is executed twice.  The
pass executes fine but the produced bit code does not behave correctly
as it should (both the original and duplicated function in which the
loop is extracted is called, but from the output it seems only one of
them is executed.)

Here is the source:

#include<stdio.h>


int main()

{

   int i,j;

   int num_iter = 2;

   int val = 56;

for(int i = 0; i<num_iter; i++)

{

val++;

printf("Hello World\n");

}

printf("val is %d\n",val);
return 1;

}

Here's the bit code my pass produces:

; ModuleID = '<stdin>'

target datalayout
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"

target triple = "x86_64-unknown-linux-gnu"


@.str = private unnamed_addr constant [13 x i8] c"Hello World\0A\00",
align 1

@.str1 = private unnamed_addr constant [11 x i8] c"val is %d\0A\00",
align 1


; Function Attrs: nounwind uwtable

define i32 @main() #0 {

entry:

  %retval = alloca i32, align 4

  %i = alloca i32, align 4

  %j = alloca i32, align 4

  %num_iter = alloca i32, align 4

  %val = alloca i32, align 4

  %i1 = alloca i32, align 4

  store i32 0, i32* %retval

  store i32 2, i32* %num_iter, align 4

  store i32 56, i32* %val, align 4

  store i32 0, i32* %i1, align 4

  br label %codeRepl


codeRepl:                                         ; preds = %entry

  call void @main_for.condarnamoy(i32* %i1, i32* %num_iter, i32* %val)

  call void @main_for.cond(i32* %i1, i32* %num_iter, i32* %val)

  br label %for.end


for.end:                                          ; preds = %codeRepl

  %0 = load i32* %val, align 4

  %call3 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds
([11 x i8]* @.str1, i32 0, i32 0), i32 %0)

  ret i32 1

}


declare i32 @printf(i8*, ...) #1


; Function Attrs: nounwind

define internal void @main_for.cond(i32* %i1, i32* %num_iter, i32* %val) #2 {

newFuncRoot:

  br label %for.cond


for.end.exitStub:                                 ; preds = %for.cond

  ret void


for.cond:                                         ; preds = %for.inc,
%newFuncRoot

  %0 = load i32* %i1, align 4

  %1 = load i32* %num_iter, align 4

  %cmp = icmp slt i32 %0, %1

  br i1 %cmp, label %for.body, label %for.end.exitStub


for.body:                                         ; preds = %for.cond

  %2 = load i32* %val, align 4

  %inc = add nsw i32 %2, 1

  store i32 %inc, i32* %val, align 4

  %call = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([13
x i8]* @.str, i32 0, i32 0))

  br label %for.inc


for.inc:                                          ; preds = %for.body

  %3 = load i32* %i1, align 4

  %inc2 = add nsw i32 %3, 1

  store i32 %inc2, i32* %i1, align 4

  br label %for.cond

}


; Function Attrs: nounwind

define internal void @main_for.condarnamoy(i32* %i1, i32* %num_iter,
i32* %val) #2 {

newFuncRoot_arnamoy:

  br label %for.cond_arnamoy


for.end.exitStub_arnamoy:                         ; preds = %for.cond_arnamoy

  ret void


for.cond_arnamoy:                                 ; preds %for.inc_arnamoy,
%newFuncRoot_arnamoy

  %0 = load i32* %i1, align 4

  %1 = load i32* %num_iter, align 4

  %cmp_arnamoy = icmp slt i32 %0, %1

  br i1 %cmp_arnamoy, label %for.body_arnamoy, label %for.end.exitStub_arnamoy


for.body_arnamoy:                                 ; preds = %for.cond_arnamoy

  %2 = load i32* %val, align 4

  %inc_arnamoy = add nsw i32 %2, 1

  store i32 %inc_arnamoy, i32* %val, align 4

  %call_arnamoy = call i32 (i8*, ...)* @printf(i8* getelementptr
inbounds ([13 x i8]* @.str, i32 0, i32 0))
    br label %for.inc_arnamoy


for.inc_arnamoy:                                  ; preds = %for.body_arnamoy

  %3 = load i32* %i1, align 4
  %inc2_arnamoy = add nsw i32 %3, 1
  store i32 %inc2_arnamoy, i32* %i1, align 4
  br label %for.cond_arnamoy

}


attributes #0 = { nounwind uwtable
"less-precise-fpmad"="false"
"no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf"="true"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"unsafe-fp-math"="false"
"use-soft-float"="false" }

attributes #1 = { "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf"="true"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"unsafe-fp-math"="false"
"use-soft-float"="false" }

attributes #2 = { nounwind }

####################################
Any idea, why the output looks like -

Hello World

Hello World

val is 58

While the it should print 4 times?

Thanks a lot;





-- 
Arnamoy Bhattacharyya
Athabasca Hall 143
Department of Computing Science - University of Alberta
Edmonton, Alberta, Canada, T6G 2E8
587-710-7073

Hi Arnamoy,
> Any idea, why the output looks like -
> Hello World
> Hello World
> val is 58
>
> While the it should print 4 times?
Both your loops are sharing the counter variable (%i1) without
resetting it to 0 in between calls. I think that accounts for the
behaviour.

Cheers.

Tim.

Thanks Tim for the pointer.

Now I am trying to create two global variables, and call functions
with the pointer to them.

Though I can create two global integers, could anyone please tell me
how do I create a pointer to a global variable (not an array) in the
IR?  I know GetElementPtr can be used in case of array but what about
I want a pointer to just an integer and call my function with it?

Sorry, I used to use the LLVM demo page a lot for getting these done,
but now that is down. and also march=cpp does not work for me :-(

Any help will be appreciated.

Thanks a lot;

On Fri, Oct 25, 2013 at 5:35 PM, Tim Northover <t.p.northover at
gmail.com> wrote:
> Hi Arnamoy,
>
>> Any idea, why the output looks like -
>
>> Hello World
>> Hello World
>> val is 58
>>
>> While the it should print 4 times?
>
> Both your loops are sharing the counter variable (%i1) without
> resetting it to 0 in between calls. I think that accounts for the
> behaviour.
>
> Cheers.
>
> Tim.
>


-- 
Arnamoy Bhattacharyya
Athabasca Hall 143
Department of Computing Science - University of Alberta
Edmonton, Alberta, Canada, T6G 2E8
587-710-7073

Hi Arnamoy,

in case the loop iteration variables are local (probably most of the times),
run the mem2reg pass before your cloning pass. In case they are not (or you
don't
like the mem2reg solution), you could introduce loads in front of the
'original function' which then serve as input for the 'duplicated
function'.
However, you have to consider loop variant upper bounds and strides too.

Best regards,
  Johannes

On Fri, Oct 25, 2013 at 04:35:20PM +0100, Tim Northover
wrote:
> Hi Arnamoy,
> 
> > Any idea, why the output looks like -
> 
> > Hello World
> > Hello World
> > val is 58
> >
> > While the it should print 4 times?
> 
> Both your loops are sharing the counter variable (%i1) without
> resetting it to 0 in between calls. I think that accounts for the
> behaviour.
> 
> Cheers.
> 
> Tim.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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