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legend xx via llvm-dev

2020-May-23 16:15 UTC

[llvm-dev] Loop Unroll

This is my example (for.c):

#include <stdio.h>

int add(int a, int b) {
    return a + b;
}

int main() {
   int a, b, c, d;
   a = 5;
   b = 15;
   c = add(a, b);
   d = 0;
   for(int i=0;i<16;i++)
       d = add(c, d);
}

I run:
$ clang -O0 -Xclang -disable-O0-optnone -emit-llvm for.c -S -o forO0.ll
$ opt -O0 -S --loop-unroll --unroll-count=4 -view-cfg forO0.ll -o
for-opt00-unroll4.ll

And this is the LLVM IR code that I get:

; ModuleID = 'forO0.ll'
source_filename = "for.c"
target datalayout
"e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: noinline nounwind uwtable
define dso_local i32 @add(i32 %a, i32 %b) #0 {
entry:
  %a.addr = alloca i32, align 4
  %b.addr = alloca i32, align 4
  store i32 %a, i32* %a.addr, align 4
  store i32 %b, i32* %b.addr, align 4
  %0 = load i32, i32* %a.addr, align 4
  %1 = load i32, i32* %b.addr, align 4
  %add = add nsw i32 %0, %1
  ret i32 %add
}

; Function Attrs: noinline nounwind uwtable
define dso_local i32 @main() #0 {
entry:
  %retval = alloca i32, align 4
  %a = alloca i32, align 4
  %b = alloca i32, align 4
  %c = alloca i32, align 4
  %d = alloca i32, align 4
  %i = alloca i32, align 4
  store i32 0, i32* %retval, align 4
  store i32 5, i32* %a, align 4
  store i32 15, i32* %b, align 4
  %0 = load i32, i32* %a, align 4
  %1 = load i32, i32* %b, align 4
  %call = call i32 @add(i32 %0, i32 %1)
  store i32 %call, i32* %c, align 4
  store i32 0, i32* %d, align 4
  store i32 0, i32* %i, align 4
  br label %for.cond

for.cond:                                         ; preds = %for.inc.3,
%entry
  %2 = load i32, i32* %i, align 4
  %cmp = icmp slt i32 %2, 16
  br i1 %cmp, label %for.body, label %for.end

for.body:                                         ; preds = %for.cond
  %3 = load i32, i32* %c, align 4
  %4 = load i32, i32* %d, align 4
  %call1 = call i32 @add(i32 %3, i32 %4)
  store i32 %call1, i32* %d, align 4
  br label %for.inc

for.inc:                                          ; preds = %for.body
  %5 = load i32, i32* %i, align 4
  %inc = add nsw i32 %5, 1
  store i32 %inc, i32* %i, align 4
  %6 = load i32, i32* %i, align 4
  %cmp.1 = icmp slt i32 %6, 16
  br i1 %cmp.1, label %for.body.1, label %for.end

for.end:                                          ; preds = %for.inc.2,
%for.inc.1, %for.inc, %for.cond
  %7 = load i32, i32* %d, align 4
  %call2 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([20 x
i8], [20 x i8]* @.str, i64 0, i64 0), i32 %7)
  %8 = load i32, i32* %retval, align 4
  ret i32 %8

for.body.1:                                       ; preds = %for.inc
  %9 = load i32, i32* %c, align 4
  %10 = load i32, i32* %d, align 4
  %call1.1 = call i32 @add(i32 %9, i32 %10)
  store i32 %call1.1, i32* %d, align 4
  br label %for.inc.1

for.inc.1:                                        ; preds = %for.body.1
  %11 = load i32, i32* %i, align 4
  %inc.1 = add nsw i32 %11, 1
  store i32 %inc.1, i32* %i, align 4
  %12 = load i32, i32* %i, align 4
  %cmp.2 = icmp slt i32 %12, 16
  br i1 %cmp.2, label %for.body.2, label %for.end

for.body.2:                                       ; preds = %for.inc.1
  %13 = load i32, i32* %c, align 4
  %14 = load i32, i32* %d, align 4
  %call1.2 = call i32 @add(i32 %13, i32 %14)
  store i32 %call1.2, i32* %d, align 4
  br label %for.inc.2

for.inc.2:                                        ; preds = %for.body.2
  %15 = load i32, i32* %i, align 4
  %inc.2 = add nsw i32 %15, 1
  store i32 %inc.2, i32* %i, align 4
  %16 = load i32, i32* %i, align 4
  %cmp.3 = icmp slt i32 %16, 16
  br i1 %cmp.3, label %for.body.3, label %for.end

for.body.3:                                       ; preds = %for.inc.2
  %17 = load i32, i32* %c, align 4
  %18 = load i32, i32* %d, align 4
  %call1.3 = call i32 @add(i32 %17, i32 %18)
  store i32 %call1.3, i32* %d, align 4
  br label %for.inc.3

for.inc.3:                                        ; preds = %for.body.3
  %19 = load i32, i32* %i, align 4
  %inc.3 = add nsw i32 %19, 1
  store i32 %inc.3, i32* %i, align 4
  br label %for.cond, !llvm.loop !2
}

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

attributes #0 = { noinline nounwind uwtable
"correctly-rounded-divide-sqrt-fp-math"="false"
"disable-tail-calls"="false"
"frame-pointer"="all"
"less-precise-fpmad"="false"
"min-legal-vector-width"="0"
"no-infs-fp-math"="false"
"no-jump-tables"="false"
"no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false"
"no-trapping-math"="false"
"stack-protector-buffer-size"="8"
"target-cpu"="x86-64"
"target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87"
"unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #1 = {
"correctly-rounded-divide-sqrt-fp-math"="false"
"disable-tail-calls"="false"
"frame-pointer"="all"
"less-precise-fpmad"="false"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false"
"no-trapping-math"="false"
"stack-protector-buffer-size"="8"
"target-cpu"="x86-64"
"target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87"
"unsafe-fp-math"="false"
"use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"clang version 11.0.0 (https://github.com/llvm/llvm-project.git
a3485301d4870f57590d7b69eed7959134a694ab)"}
!2 = distinct !{!2, !3}
!3 = !{!"llvm.loop.unroll.disable"}


So my problem is:
With unroll 4 on the loop with 16 bounds I should see one single block for
the incrementation i=i+4, then 4 instructions for each previous one
instruction, and the condition should check if i<16. This is the intuitive
code. However, the incrementation that I get is i=i+1 and there are only 4
blocks.

Do you know why this happen?

Thanks.

El vie., 22 may. 2020 a las 19:49, Florian Hahn (<florian_hahn at
apple.com>)
escribió:
>
>
> > On May 22, 2020, at 09:55, legend xx via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > I'm interesting in find a pass for loop unrolling in LLVM
compiler. I
> tried opt --loop-unroll --unroll-count=4, but it don't work well.
> >
> > What pass I can used and how?
> >
>
> -loop-unroll should be the right pass. There are multiple possible reasons
> why the loop is not unrolled and the pass has a bunch of options to
> enable/force unrolling for more cases (see
>
https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp#L81).
>
>
> Passing `-debug` should give you a better idea why the loop is not
> unrolled. If you would share the IR, someone might be able to provide
> additional insight.
>
> > I would also like to know if there is any way to mark the loops that I
> want them to be unroll
>
> Yes it is possible to explicitly mark loops for unrolling using metadata
> in LLVM IR: https://llvm.org/docs/LangRef.html#llvm-loop-unroll. But the
> metadata might not help, if the loop contains code the unroller does not
> support.
>
> Cheers,
> Florian
>
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Florian Hahn via llvm-dev

2020-May-24 12:36 UTC

head link

[llvm-dev] Loop Unroll

> On May 23, 2020, at 17:15, legend xx <legendaryxx7slh at gmail.com>
wrote:
> 
> This is my example (for.c):
> 
> #include <stdio.h>
> 
> int add(int a, int b) {
>     return a + b;
> }
> 
> int main() {
>    int a, b, c, d;
>    a = 5;
>    b = 15;
>    c = add(a, b);
>    d = 0;
>    for(int i=0;i<16;i++)
>        d = add(c, d);
> }
> 
> I run:
> $ clang -O0 -Xclang -disable-O0-optnone -emit-llvm for.c -S -o forO0.ll
> $ opt -O0 -S --loop-unroll --unroll-count=4 -view-cfg forO0.ll -o
for-opt00-unroll4.ll
> 
> And this is the LLVM IR code that I get: 
> 
> ; ModuleID = 'forO0.ll'
> source_filename = "for.c"
> target datalayout =
"e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-unknown-linux-gnu"
> 
> ; Function Attrs: noinline nounwind uwtable
> define dso_local i32 @add(i32 %a, i32 %b) #0 {
> entry:
>   %a.addr = alloca i32, align 4
>   %b.addr = alloca i32, align 4
>   store i32 %a, i32* %a.addr, align 4
>   store i32 %b, i32* %b.addr, align 4
>   %0 = load i32, i32* %a.addr, align 4
>   %1 = load i32, i32* %b.addr, align 4
>   %add = add nsw i32 %0, %1
>   ret i32 %add
> }
> 
> ; Function Attrs: noinline nounwind uwtable
> define dso_local i32 @main() #0 {
> entry:
>   %retval = alloca i32, align 4
>   %a = alloca i32, align 4
>   %b = alloca i32, align 4
>   %c = alloca i32, align 4
>   %d = alloca i32, align 4
>   %i = alloca i32, align 4
>   store i32 0, i32* %retval, align 4
>   store i32 5, i32* %a, align 4
>   store i32 15, i32* %b, align 4
>   %0 = load i32, i32* %a, align 4
>   %1 = load i32, i32* %b, align 4
>   %call = call i32 @add(i32 %0, i32 %1)
>   store i32 %call, i32* %c, align 4
>   store i32 0, i32* %d, align 4
>   store i32 0, i32* %i, align 4
>   br label %for.cond
> 
> for.cond:                                         ; preds = %for.inc.3,
%entry
>   %2 = load i32, i32* %i, align 4
>   %cmp = icmp slt i32 %2, 16
>   br i1 %cmp, label %for.body, label %for.end
> 
> for.body:                                         ; preds = %for.cond
>   %3 = load i32, i32* %c, align 4
>   %4 = load i32, i32* %d, align 4
>   %call1 = call i32 @add(i32 %3, i32 %4)
>   store i32 %call1, i32* %d, align 4
>   br label %for.inc
> 
> for.inc:                                          ; preds = %for.body
>   %5 = load i32, i32* %i, align 4
>   %inc = add nsw i32 %5, 1
>   store i32 %inc, i32* %i, align 4
>   %6 = load i32, i32* %i, align 4
>   %cmp.1 = icmp slt i32 %6, 16
>   br i1 %cmp.1, label %for.body.1, label %for.end
> 
> for.end:                                          ; preds = %for.inc.2,
%for.inc.1, %for.inc, %for.cond
>   %7 = load i32, i32* %d, align 4
>   %call2 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([20 x
i8], [20 x i8]* @.str, i64 0, i64 0), i32 %7)
>   %8 = load i32, i32* %retval, align 4
>   ret i32 %8
> 
> for.body.1:                                       ; preds = %for.inc
>   %9 = load i32, i32* %c, align 4
>   %10 = load i32, i32* %d, align 4
>   %call1.1 = call i32 @add(i32 %9, i32 %10)
>   store i32 %call1.1, i32* %d, align 4
>   br label %for.inc.1
> 
> for.inc.1:                                        ; preds = %for.body.1
>   %11 = load i32, i32* %i, align 4
>   %inc.1 = add nsw i32 %11, 1
>   store i32 %inc.1, i32* %i, align 4
>   %12 = load i32, i32* %i, align 4
>   %cmp.2 = icmp slt i32 %12, 16
>   br i1 %cmp.2, label %for.body.2, label %for.end
> 
> for.body.2:                                       ; preds = %for.inc.1
>   %13 = load i32, i32* %c, align 4
>   %14 = load i32, i32* %d, align 4
>   %call1.2 = call i32 @add(i32 %13, i32 %14)
>   store i32 %call1.2, i32* %d, align 4
>   br label %for.inc.2
> 
> for.inc.2:                                        ; preds = %for.body.2
>   %15 = load i32, i32* %i, align 4
>   %inc.2 = add nsw i32 %15, 1
>   store i32 %inc.2, i32* %i, align 4
>   %16 = load i32, i32* %i, align 4
>   %cmp.3 = icmp slt i32 %16, 16
>   br i1 %cmp.3, label %for.body.3, label %for.end
> 
> for.body.3:                                       ; preds = %for.inc.2
>   %17 = load i32, i32* %c, align 4
>   %18 = load i32, i32* %d, align 4
>   %call1.3 = call i32 @add(i32 %17, i32 %18)
>   store i32 %call1.3, i32* %d, align 4
>   br label %for.inc.3
> 
> for.inc.3:                                        ; preds = %for.body.3
>   %19 = load i32, i32* %i, align 4
>   %inc.3 = add nsw i32 %19, 1
>   store i32 %inc.3, i32* %i, align 4
>   br label %for.cond, !llvm.loop !2
> }
> 
> declare dso_local i32 @printf(i8*, ...) #1
> 
> attributes #0 = { noinline nounwind uwtable
"correctly-rounded-divide-sqrt-fp-math"="false"
"disable-tail-calls"="false"
"frame-pointer"="all"
"less-precise-fpmad"="false"
"min-legal-vector-width"="0"
"no-infs-fp-math"="false"
"no-jump-tables"="false"
"no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false"
"no-trapping-math"="false"
"stack-protector-buffer-size"="8"
"target-cpu"="x86-64"
"target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87"
"unsafe-fp-math"="false"
"use-soft-float"="false" }
> attributes #1 = {
"correctly-rounded-divide-sqrt-fp-math"="false"
"disable-tail-calls"="false"
"frame-pointer"="all"
"less-precise-fpmad"="false"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false"
"no-trapping-math"="false"
"stack-protector-buffer-size"="8"
"target-cpu"="x86-64"
"target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87"
"unsafe-fp-math"="false"
"use-soft-float"="false" }
> 
> !llvm.module.flags = !{!0}
> !llvm.ident = !{!1}
> 
> !0 = !{i32 1, !"wchar_size", i32 4}
> !1 = !{!"clang version 11.0.0
(https://github.com/llvm/llvm-project.git
<https://github.com/llvm/llvm-project.git>
a3485301d4870f57590d7b69eed7959134a694ab)"}
> !2 = distinct !{!2, !3}
> !3 = !{!"llvm.loop.unroll.disable"}
> 
> 
> So my problem is:
> With unroll 4 on the loop with 16 bounds I should see one single block for
the incrementation i=i+4, then 4 instructions for each previous one instruction,
and the condition should check if i<16. This is the intuitive code. However,
the incrementation that I get is i=i+1 and there are only 4 blocks.
> 
> Do you know why this happen?
I think loop-unroll works as expected in your example, as you can see the copies
of the unrolled loop blocks (for.body.X, for.inc.X). The reason this is not
simplified to the single block you are expecting is the input for -loop-unroll:
-loop-unroll gets the IR without any optimizations (-O0).

For the expected result, you need to run a few additional passes before
-loop-unroll to promote some of the loads/stores to registers and simplify the
CFG of the input.  Running `opt -mem2reg -simplifycfg -loop-unroll
-unroll-count=4 forO0.ll -S` should give you something like

define i32 @main() #0 {
entry:
  %call = call i32 @add(i32 5, i32 15)
  br label %for.cond

for.cond:                                         ; preds = %for.body.3, %entry
  %d.0 = phi i32 [ 0, %entry ], [ %call1.3, %for.body.3 ]
  %i.0 = phi i32 [ 0, %entry ], [ %inc.3, %for.body.3 ]
  %cmp = icmp ult i32 %i.0, 16
  br i1 %cmp, label %for.body, label %for.end

for.body:                                         ; preds = %for.cond
  %call1 = call i32 @add(i32 %call, i32 %d.0)
  %inc = add nuw nsw i32 %i.0, 1
  br label %for.body.1

for.end:                                          ; preds = %for.cond
  ret i32 0

for.body.1:                                       ; preds = %for.body
  %call1.1 = call i32 @add(i32 %call, i32 %call1)
  %inc.1 = add nuw nsw i32 %inc, 1
  br label %for.body.2

for.body.2:                                       ; preds = %for.body.1
  %call1.2 = call i32 @add(i32 %call, i32 %call1.1)
  %inc.2 = add nuw nsw i32 %inc.1, 1
  br label %for.body.3

for.body.3:                                       ; preds = %for.body.2
  %call1.3 = call i32 @add(i32 %call, i32 %call1.2)
  %inc.3 = add nuw nsw i32 %inc.2, 1
  br label %for.cond, !llvm.loop !4
}

Note that there are still 4 copies of the body instead of a single one. Like
many passes in LLVM, the loop-unroll pass focuses on performing one
transformation (duplicating the loop body a number of times) and relies on other
passes to clean-up/simplify the result. To fold the 4 copies of the body into a
single block, you need another round of CFG simplifications. Running `opt
-mem2reg -simplifycfg -loop-unroll -unroll-count=4 -simplifycfg forO0.ll -S`
produces the code below, which is what you are looking for IIUC.

define i32 @main() #0 {
entry:
  %call = call i32 @add(i32 5, i32 15)
  br label %for.cond

for.cond:                                         ; preds = %for.body, %entry
  %d.0 = phi i32 [ 0, %entry ], [ %call1.3, %for.body ]
  %i.0 = phi i32 [ 0, %entry ], [ %inc.3, %for.body ]
  %cmp = icmp ult i32 %i.0, 16
  br i1 %cmp, label %for.body, label %for.end

for.body:                                         ; preds = %for.cond
  %call1 = call i32 @add(i32 %call, i32 %d.0)
  %inc = add nuw nsw i32 %i.0, 1
  %call1.1 = call i32 @add(i32 %call, i32 %call1)
  %inc.1 = add nuw nsw i32 %inc, 1
  %call1.2 = call i32 @add(i32 %call, i32 %call1.1)
  %inc.2 = add nuw nsw i32 %inc.1, 1
  %call1.3 = call i32 @add(i32 %call, i32 %call1.2)
  %inc.3 = add nuw nsw i32 %inc.2, 1
  br label %for.cond, !llvm.loop !4

for.end:                                          ; preds = %for.cond
  ret i32 0
}

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legend xx via llvm-dev

2020-May-26 17:55 UTC

head link

[llvm-dev] Loop Unroll

Awesome, thanks!

Now I have another question. I have a matrix multiplication code. This is
my code:

#include <stdio.h>
#include <stdlib.h>

#define n 4

int main(int argc, char *argv[]) {
    int i, j, k;

    int A[n][n], B[n][n], C[n][n];
    for(i=0;i<n;i++){
        for(j=0;j<n;j++){
           A[i][j] = 1;
           B[i][j] = 2;
           C[i][j] = 0;
        }
    }

    for(i=0;i<n;i++){
        for(j=0;j<n;j++){
            for(k=0;k<n;k++){
                C[i][j]=(C[i][j]+(A[i][k]*B[k][j]));
            }
        }
    }

    return 0;
}


I tried over them the loop-unroll-and-jam pass. I run:

$ clang -O0 -Xclang -disable-O0-optnone -emit-llvm mult.c -S -o mult.ll
$ opt -O0 -S -mem2reg -simplifycfg -view-cfg mult.ll -o multopt.ll
$ opt -O0 -S -mem2reg -simplifycfg --loop-unroll-and-jam
--unroll-and-jam-count=4 -simplifycfg -view-cfg mult.ll -o
mult-opt00-unroll4.ll

I get the same CFG graph in two case (I attach it). Also, I tried with -O1
level (opt -01 -loop-unroll-and-jam --unroll-and-jam-count=4), but I didn't
get any differece.

Why the pass --loop-unroll-and-jam does not work?






El dom., 24 may. 2020 a las 14:36, Florian Hahn (<florian_hahn at
apple.com>)
escribió:
>
>
> On May 23, 2020, at 17:15, legend xx <legendaryxx7slh at gmail.com>
wrote:
>
> This is my example (for.c):
>
> #include <stdio.h>
>
> int add(int a, int b) {
>     return a + b;
> }
>
> int main() {
>    int a, b, c, d;
>    a = 5;
>    b = 15;
>    c = add(a, b);
>    d = 0;
>    for(int i=0;i<16;i++)
>        d = add(c, d);
> }
>
> I run:
> $ clang -O0 -Xclang -disable-O0-optnone -emit-llvm for.c -S -o forO0.ll
> $ opt -O0 -S --loop-unroll --unroll-count=4 -view-cfg forO0.ll -o
> for-opt00-unroll4.ll
>
> And this is the LLVM IR code that I get:
>
> ; ModuleID = 'forO0.ll'
> source_filename = "for.c"
> target datalayout >
"e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-unknown-linux-gnu"
>
> ; Function Attrs: noinline nounwind uwtable
> define dso_local i32 @add(i32 %a, i32 %b) #0 {
> entry:
>   %a.addr = alloca i32, align 4
>   %b.addr = alloca i32, align 4
>   store i32 %a, i32* %a.addr, align 4
>   store i32 %b, i32* %b.addr, align 4
>   %0 = load i32, i32* %a.addr, align 4
>   %1 = load i32, i32* %b.addr, align 4
>   %add = add nsw i32 %0, %1
>   ret i32 %add
> }
>
> ; Function Attrs: noinline nounwind uwtable
> define dso_local i32 @main() #0 {
> entry:
>   %retval = alloca i32, align 4
>   %a = alloca i32, align 4
>   %b = alloca i32, align 4
>   %c = alloca i32, align 4
>   %d = alloca i32, align 4
>   %i = alloca i32, align 4
>   store i32 0, i32* %retval, align 4
>   store i32 5, i32* %a, align 4
>   store i32 15, i32* %b, align 4
>   %0 = load i32, i32* %a, align 4
>   %1 = load i32, i32* %b, align 4
>   %call = call i32 @add(i32 %0, i32 %1)
>   store i32 %call, i32* %c, align 4
>   store i32 0, i32* %d, align 4
>   store i32 0, i32* %i, align 4
>   br label %for.cond
>
> for.cond:                                         ; preds = %for.inc.3,
> %entry
>   %2 = load i32, i32* %i, align 4
>   %cmp = icmp slt i32 %2, 16
>   br i1 %cmp, label %for.body, label %for.end
>
> for.body:                                         ; preds = %for.cond
>   %3 = load i32, i32* %c, align 4
>   %4 = load i32, i32* %d, align 4
>   %call1 = call i32 @add(i32 %3, i32 %4)
>   store i32 %call1, i32* %d, align 4
>   br label %for.inc
>
> for.inc:                                          ; preds = %for.body
>   %5 = load i32, i32* %i, align 4
>   %inc = add nsw i32 %5, 1
>   store i32 %inc, i32* %i, align 4
>   %6 = load i32, i32* %i, align 4
>   %cmp.1 = icmp slt i32 %6, 16
>   br i1 %cmp.1, label %for.body.1, label %for.end
>
> for.end:                                          ; preds = %for.inc.2,
> %for.inc.1, %for.inc, %for.cond
>   %7 = load i32, i32* %d, align 4
>   %call2 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([20 x
> i8], [20 x i8]* @.str, i64 0, i64 0), i32 %7)
>   %8 = load i32, i32* %retval, align 4
>   ret i32 %8
>
> for.body.1:                                       ; preds = %for.inc
>   %9 = load i32, i32* %c, align 4
>   %10 = load i32, i32* %d, align 4
>   %call1.1 = call i32 @add(i32 %9, i32 %10)
>   store i32 %call1.1, i32* %d, align 4
>   br label %for.inc.1
>
> for.inc.1:                                        ; preds = %for.body.1
>   %11 = load i32, i32* %i, align 4
>   %inc.1 = add nsw i32 %11, 1
>   store i32 %inc.1, i32* %i, align 4
>   %12 = load i32, i32* %i, align 4
>   %cmp.2 = icmp slt i32 %12, 16
>   br i1 %cmp.2, label %for.body.2, label %for.end
>
> for.body.2:                                       ; preds = %for.inc.1
>   %13 = load i32, i32* %c, align 4
>   %14 = load i32, i32* %d, align 4
>   %call1.2 = call i32 @add(i32 %13, i32 %14)
>   store i32 %call1.2, i32* %d, align 4
>   br label %for.inc.2
>
> for.inc.2:                                        ; preds = %for.body.2
>   %15 = load i32, i32* %i, align 4
>   %inc.2 = add nsw i32 %15, 1
>   store i32 %inc.2, i32* %i, align 4
>   %16 = load i32, i32* %i, align 4
>   %cmp.3 = icmp slt i32 %16, 16
>   br i1 %cmp.3, label %for.body.3, label %for.end
>
> for.body.3:                                       ; preds = %for.inc.2
>   %17 = load i32, i32* %c, align 4
>   %18 = load i32, i32* %d, align 4
>   %call1.3 = call i32 @add(i32 %17, i32 %18)
>   store i32 %call1.3, i32* %d, align 4
>   br label %for.inc.3
>
> for.inc.3:                                        ; preds = %for.body.3
>   %19 = load i32, i32* %i, align 4
>   %inc.3 = add nsw i32 %19, 1
>   store i32 %inc.3, i32* %i, align 4
>   br label %for.cond, !llvm.loop !2
> }
>
> declare dso_local i32 @printf(i8*, ...) #1
>
> attributes #0 = { noinline nounwind uwtable
> "correctly-rounded-divide-sqrt-fp-math"="false"
> "disable-tail-calls"="false"
"frame-pointer"="all"
> "less-precise-fpmad"="false"
"min-legal-vector-width"="0"
> "no-infs-fp-math"="false"
"no-jump-tables"="false"
> "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false"
> "no-trapping-math"="false"
"stack-protector-buffer-size"="8"
> "target-cpu"="x86-64"
"target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87"
> "unsafe-fp-math"="false"
"use-soft-float"="false" }
> attributes #1 = {
"correctly-rounded-divide-sqrt-fp-math"="false"
> "disable-tail-calls"="false"
"frame-pointer"="all"
> "less-precise-fpmad"="false"
"no-infs-fp-math"="false"
> "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false"
> "no-trapping-math"="false"
"stack-protector-buffer-size"="8"
> "target-cpu"="x86-64"
"target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87"
> "unsafe-fp-math"="false"
"use-soft-float"="false" }
>
> !llvm.module.flags = !{!0}
> !llvm.ident = !{!1}
>
> !0 = !{i32 1, !"wchar_size", i32 4}
> !1 = !{!"clang version 11.0.0
(https://github.com/llvm/llvm-project.git
> a3485301d4870f57590d7b69eed7959134a694ab)"}
> !2 = distinct !{!2, !3}
> !3 = !{!"llvm.loop.unroll.disable"}
>
>
> So my problem is:
> With unroll 4 on the loop with 16 bounds I should see one single block for
> the incrementation i=i+4, then 4 instructions for each previous one
> instruction, and the condition should check if i<16. This is the
intuitive
> code. However, the incrementation that I get is i=i+1 and there are only 4
> blocks.
>
> Do you know why this happen?
>
>
> I think loop-unroll works as expected in your example, as you can see the
> copies of the unrolled loop blocks (for.body.X, for.inc.X). The reason this
> is not simplified to the single block you are expecting is the input for
> -loop-unroll: -loop-unroll gets the IR without any optimizations (-O0).
>
> For the expected result, you need to run a few additional passes before
> -loop-unroll to promote some of the loads/stores to registers and simplify
> the CFG of the input.  Running `opt -mem2reg -simplifycfg -loop-unroll
> -unroll-count=4 forO0.ll -S` should give you something like
>
> define i32 @main() #0 {
> entry:
>   %call = call i32 @add(i32 5, i32 15)
>   br label %for.cond
>
> for.cond:                                         ; preds = %for.body.3,
> %entry
>   %d.0 = phi i32 [ 0, %entry ], [ %call1.3, %for.body.3 ]
>   %i.0 = phi i32 [ 0, %entry ], [ %inc.3, %for.body.3 ]
>   %cmp = icmp ult i32 %i.0, 16
>   br i1 %cmp, label %for.body, label %for.end
>
> for.body:                                         ; preds = %for.cond
>   %call1 = call i32 @add(i32 %call, i32 %d.0)
>   %inc = add nuw nsw i32 %i.0, 1
>   br label %for.body.1
>
> for.end:                                          ; preds = %for.cond
>   ret i32 0
>
> for.body.1:                                       ; preds = %for.body
>   %call1.1 = call i32 @add(i32 %call, i32 %call1)
>   %inc.1 = add nuw nsw i32 %inc, 1
>   br label %for.body.2
>
> for.body.2:                                       ; preds = %for.body.1
>   %call1.2 = call i32 @add(i32 %call, i32 %call1.1)
>   %inc.2 = add nuw nsw i32 %inc.1, 1
>   br label %for.body.3
>
> for.body.3:                                       ; preds = %for.body.2
>   %call1.3 = call i32 @add(i32 %call, i32 %call1.2)
>   %inc.3 = add nuw nsw i32 %inc.2, 1
>   br label %for.cond, !llvm.loop !4
> }
>
> Note that there are still 4 copies of the body instead of a single one.
> Like many passes in LLVM, the loop-unroll pass focuses on performing one
> transformation (duplicating the loop body a number of times) and relies on
> other passes to clean-up/simplify the result. To fold the 4 copies of the
> body into a single block, you need another round of CFG simplifications.
> Running `opt -mem2reg -simplifycfg -loop-unroll -unroll-count=4
> -simplifycfg forO0.ll -S` produces the code below, which is what you are
> looking for IIUC.
>
> define i32 @main() #0 {
> entry:
>   %call = call i32 @add(i32 5, i32 15)
>   br label %for.cond
>
> for.cond:                                         ; preds = %for.body,
> %entry
>   %d.0 = phi i32 [ 0, %entry ], [ %call1.3, %for.body ]
>   %i.0 = phi i32 [ 0, %entry ], [ %inc.3, %for.body ]
>   %cmp = icmp ult i32 %i.0, 16
>   br i1 %cmp, label %for.body, label %for.end
>
> for.body:                                         ; preds = %for.cond
>   %call1 = call i32 @add(i32 %call, i32 %d.0)
>   %inc = add nuw nsw i32 %i.0, 1
>   %call1.1 = call i32 @add(i32 %call, i32 %call1)
>   %inc.1 = add nuw nsw i32 %inc, 1
>   %call1.2 = call i32 @add(i32 %call, i32 %call1.1)
>   %inc.2 = add nuw nsw i32 %inc.1, 1
>   %call1.3 = call i32 @add(i32 %call, i32 %call1.2)
>   %inc.3 = add nuw nsw i32 %inc.2, 1
>   br label %for.cond, !llvm.loop !4
>
> for.end:                                          ; preds = %for.cond
>   ret i32 0
> }
>
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