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Sorin Baltateanu

2011-Jun-19 14:21 UTC

[LLVMdev] Question about IndVarSimplify

Hi guys,

I am trying to use the indvars pass, but i don't see any changes in the IL
representation.

For this simple function

void init_array(int k)
{
    int A[20];
    for (int i=2; i<10; i++)
        A[i] = 0;
}

i run *clang test-simple.c -S -emit-llvm -o test.il* and i obtain the
following il representation

define void @init_array(i32 %k) nounwind {
entry:
  %k.addr = alloca i32, align 4
  %A = alloca [20 x i32], align 4
  %i = alloca i32, align 4
  store i32 %k, i32* %k.addr, align 4
  store i32 2, i32* %i, align 4
  br label %for.cond

for.cond:                                         ; preds = %for.inc, %entry
  %tmp = load i32* %i, align 4
  %cmp = icmp slt i32 %tmp, 10
  br i1 %cmp, label %for.body, label %for.end

for.body:                                         ; preds = %for.cond
  %tmp1 = load i32* %i, align 4
  %arrayidx = getelementptr inbounds [20 x i32]* %A, i32 0, i32 %tmp1
  store i32 0, i32* %arrayidx, align 4
  br label %for.inc

for.inc:                                          ; preds = %for.body
  %tmp2 = load i32* %i, align 4
  %inc = add nsw i32 %tmp2, 1
  store i32 %inc, i32* %i, align 4
  br label %for.cond

for.end:                                          ; preds = %for.cond
  ret void
}

Running *opt -loop-simplify < test.il -S* gives me the same output, which is
what i would expect.
The problem is that running *opt -indvars < test.il -S* gives, again, the
same output, but i would expect that i starts from 0 and is, eventualy,
defined using a PHYNode.

What it's wrong with what i'm doing?

Thank you.
Have a nice day
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Nick Lewycky

2011-Jun-19 18:18 UTC

head link

[LLVMdev] Question about IndVarSimplify

Sorin Baltateanu wrote:> Hi guys,
>
> I am trying to use the indvars pass, but i don't see any changes in the
> IL representation.
>
> For this simple function
>
> void init_array(int k)
> {
>      int A[20];
>      for (int i=2; i<10; i++)
>          A[i] = 0;
> }
>
> i run *clang test-simple.c -S -emit-llvm -o test.il*
> and i obtain the following il representation
>
> define void @init_array(i32 %k) nounwind {
> entry:
>    %k.addr = alloca i32, align 4
>    %A = alloca [20 x i32], align 4
>    %i = alloca i32, align 4
>    store i32 %k, i32* %k.addr, align 4
>    store i32 2, i32* %i, align 4
>    br label %for.cond
>
> for.cond:                                         ; preds = %for.inc,
%entry
>    %tmp = load i32* %i, align 4
>    %cmp = icmp slt i32 %tmp, 10
>    br i1 %cmp, label %for.body, label %for.end
>
> for.body:                                         ; preds = %for.cond
>    %tmp1 = load i32* %i, align 4
>    %arrayidx = getelementptr inbounds [20 x i32]* %A, i32 0, i32 %tmp1
>    store i32 0, i32* %arrayidx, align 4
>    br label %for.inc
>
> for.inc:                                          ; preds = %for.body
>    %tmp2 = load i32* %i, align 4
>    %inc = add nsw i32 %tmp2, 1
>    store i32 %inc, i32* %i, align 4
>    br label %for.cond
>
> for.end:                                          ; preds = %for.cond
>    ret void
> }
>
> Running *opt -loop-simplify < test.il <http://test.il> -S* gives
me the
> same output, which is what i would expect.
> The problem is that running *opt -indvars < test.il
<http://test.il> -S*
> gives, again, the same output, but i would expect that i starts from 0
> and is, eventualy,  defined using a PHYNode.
>
> What it's wrong with what i'm doing?
Each optimization pass in LLVM does one job only. Indvars does induction 
variable canonicalization, not SSA construction, so it sees those loads 
and stores and assumes they can't be optimized because they already 
would have been by other optimizers. SSA construction is done by 
-mem2reg. You can run opt -mem2reg -indvars, or just opt -mem2reg then 
opt -indvars individually to see what effect each has.

Nick

Eli Friedman

2011-Jun-19 18:22 UTC

head link

[LLVMdev] Question about IndVarSimplify

On Sun, Jun 19, 2011 at 7:21 AM, Sorin Baltateanu
<baltateanu.sorin at gmail.com> wrote:> Hi guys,
>
> I am trying to use the indvars pass, but i don't see any changes in the
IL
> representation.
>
> For this simple function
>
> void init_array(int k)
> {
>     int A[20];
>     for (int i=2; i<10; i++)
>         A[i] = 0;
> }
>
> i run clang test-simple.c -S -emit-llvm -o test.il and i obtain the
> following il representation
>
> define void @init_array(i32 %k) nounwind {
> entry:
>   %k.addr = alloca i32, align 4
>   %A = alloca [20 x i32], align 4
>   %i = alloca i32, align 4
>   store i32 %k, i32* %k.addr, align 4
>   store i32 2, i32* %i, align 4
>   br label %for.cond
>
> for.cond:                                         ; preds = %for.inc,
%entry
>   %tmp = load i32* %i, align 4
>   %cmp = icmp slt i32 %tmp, 10
>   br i1 %cmp, label %for.body, label %for.end
>
> for.body:                                         ; preds = %for.cond
>   %tmp1 = load i32* %i, align 4
>   %arrayidx = getelementptr inbounds [20 x i32]* %A, i32 0, i32 %tmp1
>   store i32 0, i32* %arrayidx, align 4
>   br label %for.inc
>
> for.inc:                                          ; preds = %for.body
>   %tmp2 = load i32* %i, align 4
>   %inc = add nsw i32 %tmp2, 1
>   store i32 %inc, i32* %i, align 4
>   br label %for.cond
>
> for.end:                                          ; preds = %for.cond
>   ret void
> }
>
> Running opt -loop-simplify < test.il -S gives me the same output, which
is
> what i would expect.
> The problem is that running opt -indvars < test.il -S gives, again, the
same
> output, but i would expect that i starts from 0 and is, eventualy,  defined
> using a PHYNode.
>
> What it's wrong with what i'm doing?
You aren't running a normal pass pipeline; in particular, not running
mem2reg/scalarrepl first prevents a lot of optimizations from working
as expected.

-Eli

Sorin Baltateanu

2011-Jun-20 10:11 UTC

head link

[LLVMdev] Question about IndVarSimplify

Thanks. Works fine now and made some other things clear too.

On Sun, Jun 19, 2011 at 9:22 PM, Eli Friedman <eli.friedman at
gmail.com>wrote:
> On Sun, Jun 19, 2011 at 7:21 AM, Sorin Baltateanu
> <baltateanu.sorin at gmail.com> wrote:
> > Hi guys,
> >
> > I am trying to use the indvars pass, but i don't see any changes
in the
> IL
> > representation.
> >
> > For this simple function
> >
> > void init_array(int k)
> > {
> >     int A[20];
> >     for (int i=2; i<10; i++)
> >         A[i] = 0;
> > }
> >
> > i run clang test-simple.c -S -emit-llvm -o test.il and i obtain the
> > following il representation
> >
> > define void @init_array(i32 %k) nounwind {
> > entry:
> >   %k.addr = alloca i32, align 4
> >   %A = alloca [20 x i32], align 4
> >   %i = alloca i32, align 4
> >   store i32 %k, i32* %k.addr, align 4
> >   store i32 2, i32* %i, align 4
> >   br label %for.cond
> >
> > for.cond:                                         ; preds = %for.inc,
> %entry
> >   %tmp = load i32* %i, align 4
> >   %cmp = icmp slt i32 %tmp, 10
> >   br i1 %cmp, label %for.body, label %for.end
> >
> > for.body:                                         ; preds = %for.cond
> >   %tmp1 = load i32* %i, align 4
> >   %arrayidx = getelementptr inbounds [20 x i32]* %A, i32 0, i32 %tmp1
> >   store i32 0, i32* %arrayidx, align 4
> >   br label %for.inc
> >
> > for.inc:                                          ; preds = %for.body
> >   %tmp2 = load i32* %i, align 4
> >   %inc = add nsw i32 %tmp2, 1
> >   store i32 %inc, i32* %i, align 4
> >   br label %for.cond
> >
> > for.end:                                          ; preds = %for.cond
> >   ret void
> > }
> >
> > Running opt -loop-simplify < test.il -S gives me the same output,
which
> is
> > what i would expect.
> > The problem is that running opt -indvars < test.il -S gives, again,
the
> same
> > output, but i would expect that i starts from 0 and is, eventualy,
> defined
> > using a PHYNode.
> >
> > What it's wrong with what i'm doing?
>
> You aren't running a normal pass pipeline; in particular, not running
> mem2reg/scalarrepl first prevents a lot of optimizations from working
> as expected.
>
> -Eli
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