N+1 iterations with loop unrolling (original) (raw)
In the following code, the loop iterates one time.
#include <stdio.h>
int main() {
volatile int sum = 0;
for (int i = 1; i < 2; i++) {
sum +=2;
}
printf("Sum is %d\n", sum);
return 0;
}
With the following commands
clang -O0 -emit-llvm -S -Xclang -disable-O0-optnone -emit-llvm <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>F</mi><mi>I</mi><mi>L</mi><mi>E</mi><mi mathvariant="normal">.</mi><mi>c</mi><mo>−</mo><mi>o</mi></mrow><annotation encoding="application/x-tex">FILE.c -o </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.7667em;vertical-align:-0.0833em;"></span><span class="mord mathnormal" style="margin-right:0.13889em;">F</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.05764em;">E</span><span class="mord">.</span><span class="mord mathnormal">c</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.4306em;"></span><span class="mord mathnormal">o</span></span></span></span>FILE.ll
opt -passes='mem2reg' -S <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>F</mi><mi>I</mi><mi>L</mi><mi>E</mi><mi mathvariant="normal">.</mi><mi>l</mi><mi>l</mi><mo>−</mo><mi>o</mi></mrow><annotation encoding="application/x-tex">FILE.ll -o </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.7778em;vertical-align:-0.0833em;"></span><span class="mord mathnormal" style="margin-right:0.13889em;">F</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.05764em;">E</span><span class="mord">.</span><span class="mord mathnormal" style="margin-right:0.01968em;">ll</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.4306em;"></span><span class="mord mathnormal">o</span></span></span></span>FILE-m2r.ll
opt -passes='loop-unroll' -unroll-count=10 -S <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>F</mi><mi>I</mi><mi>L</mi><mi>E</mi><mo>−</mo><mi>m</mi><mn>2</mn><mi>r</mi><mi mathvariant="normal">.</mi><mi>l</mi><mi>l</mi><mo>−</mo><mi>o</mi></mrow><annotation encoding="application/x-tex">FILE-m2r.ll -o </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.7667em;vertical-align:-0.0833em;"></span><span class="mord mathnormal" style="margin-right:0.13889em;">F</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.05764em;">E</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.7778em;vertical-align:-0.0833em;"></span><span class="mord mathnormal">m</span><span class="mord">2</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord">.</span><span class="mord mathnormal" style="margin-right:0.01968em;">ll</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.4306em;"></span><span class="mord mathnormal">o</span></span></span></span>FILE-unrolled.ll
when I looked at the final IR code, I see two “add” instructions.
; ModuleID = 'loop-m2r.ll'
source_filename = "loop.c"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
@.str = private unnamed_addr constant [11 x i8] c"Sum is %d\0A\00", align 1
; Function Attrs: noinline nounwind uwtable
define dso_local i32 @main() #0 {
%1 = alloca i32, align 4
store volatile i32 0, ptr %1, align 4
br label %2
2: ; preds = %0
br label %3
3: ; preds = %2
%4 = load volatile i32, ptr %1, align 4
%5 = add nsw i32 %4, 2
store volatile i32 %5, ptr %1, align 4
br label %6
6: ; preds = %3
br i1 false, label %7, label %11
7: ; preds = %6
%8 = load volatile i32, ptr %1, align 4
%9 = add nsw i32 %8, 2
store volatile i32 %9, ptr %1, align 4
br label %10
10: ; preds = %7
unreachable
11: ; preds = %6
%12 = load volatile i32, ptr %1, align 4
%13 = call i32 (ptr, ...) @printf(ptr noundef @.str, i32 noundef %12)
ret i32 0
}
declare i32 @printf(ptr noundef, ...) #1
attributes #0 = { noinline nounwind uwtable "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
attributes #1 = { "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
!llvm.module.flags = !{!0, !1, !2, !3, !4}
!llvm.ident = !{!5}
!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{i32 8, !"PIC Level", i32 2}
!2 = !{i32 7, !"PIE Level", i32 2}
!3 = !{i32 7, !"uwtable", i32 2}
!4 = !{i32 7, !"frame-pointer", i32 2}
!5 = !{!"clang version 21.0.0git (https://github.com/llvm/llvm-project d0c973a7a0149db3b71767d4c5a20a31e6a8ed5b)"}
I expected to see one add instruction. Isn’t that the case?
This line br i1 false, label %7, label %11 is a bit weird, because there is no definition for i1 in the IR file. So, I am guessing that the compiler has added one more iteration, similar to wrong path, which is controlled by a condition. But why and how that condition is controlled is not clear for me. Any idea about that?