[llvm-dev] loop unrolling introduces conditional branch

Xiangyang Guo via llvm-dev
Sat, 22 Aug 2015 14:27:06 -0000

Hi, Mehdi,

For example, I have this very simple source code:
void foo( int n, int array_x[])
{
    for (int i=0; i < n; i++)
   array_x[i] = i;
}

After I use "clang -emit-llvm -o bc_from_clang.bc -c try.cc", I get
bc_from_clang.bc. With my code (using LLVM IRbuilder API), I get
bc_from_api.bc. Attachment please find thse two files. I also past the IR
here.
******************************** Clang Generate IR Start
***********************************************************
; ModuleID = 'bc_from_clang.bc'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: nounwind uwtable
define void @_Z3fooiPi(i32 %n, i32* %array_x) #0 {
  %1 = alloca i32, align 4
  %2 = alloca i32*, align 8
  %i = alloca i32, align 4
  store i32 %n, i32* %1, align 4
  store i32* %array_x, i32** %2, align 8
  store i32 0, i32* %i, align 4
  br label %3

; :3 ; preds = %13, %0
  %4 = load i32, i32* %i, align 4
  %5 = load i32, i32* %1, align 4
  %6 = icmp slt i32 %4, %5
  br i1 %6, label %7, label %16

; :7 ; preds = %3
  %8 = load i32, i32* %i, align 4
  %9 = load i32, i32* %i, align 4
  %10 = sext i32 %9 to i64
  %11 = load i32*, i32** %2, align 8
  %12 = getelementptr inbounds i32, i32* %11, i64 %10
  store i32 %8, i32* %12, align 4
  br label %13

; :13 ; preds = %7
  %14 = load i32, i32* %i, align 4
  %15 = add nsw i32 %14, 1
  store i32 %15, i32* %i, align 4
  br label %3

; :16 ; preds = %3
  ret void
}

attributes #0 = { nounwind uwtable
"disable-tail-calls"="false"
"less-precise-fpmad"="false"
"no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"stack-protector-buffer-size"="8"
"target-cpu"="x86-64"
"target-features"="+sse,+sse2"
"unsafe-fp-math"="false"
"use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.8.0 (trunk 245730) (llvm/trunk 245727)"}

******************************** Clang Generate IR End
 ***********************************************************

******************************** API Generate IR Start
***********************************************************
; ModuleID = 'bc_from_api.bc'
target triple = "x86_64-unkown-linux-gnu"

; Function Attrs: nounwind
define void @_Z3fooiPi(i32 %n, i32* %array_x) #0 {
entry:
  %n.addr = alloca i32, align 4
  %array_x.addr = alloca i32*, align 8
  %i = alloca i32, align 4
  store i32 %n, i32* %n.addr, align 4
  store i32* %array_x, i32** %array_x.addr, align 8
  store i32 0, i32* %i, align 4
  br label %for.cond

for.cond:                                         ; preds = %for.inc, %entry
  %0 = load i32, i32* %i, align 4
  %1 = load i32, i32* %n.addr, align 4
  %cmp = icmp slt i32 %0, %1
  br i1 %cmp, label %for.body, label %for.end

for.body:                                         ; preds = %for.cond
  %2 = load i32, i32* %i, align 4
  %3 = load i32, i32* %i, align 4
  %idxprom = sext i32 %3 to i64
  %4 = load i32*, i32** %array_x.addr, align 8
  %arrayidx = getelementptr inbounds i32, i32* %4, i64 %idxprom
  store i32 %2, i32* %arrayidx, align 4
  br label %for.inc

for.inc:                                          ; preds = %for.body
  %5 = load i32, i32* %i, align 4
  %inc = add i32 %5, 1
  store i32 %inc, i32* %i, align 4
  br label %for.cond

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

attributes #0 = { nounwind }

******************************** API Generate IR End
 ***********************************************************

Then I use "opt file.bc -mem2reg -loops -loop-simplify -loop-rotate -lcssa
-indvars -loop-unroll -unroll-count=4 -irce -simplifycfg -S" to run both
.bc files.
The first .bc file give me this:

***************************** Clang Generate IR with LoopUnrolling
Start**********************************************
; ModuleID = 'bc_from_clang.bc'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: nounwind uwtable
define void @_Z3fooiPi(i32 %n, i32* %array_x) #0 {
  %1 = icmp slt i32 0, %n
  br i1 %1, label %.lr.ph, label %._crit_edge

.lr.ph:                                           ; preds = %0
  %2 = add i32 %n, -1
  %xtraiter = and i32 %n, 3
  %lcmp.mod = icmp ne i32 %xtraiter, 0
  br i1 %lcmp.mod, label %3, label %.lr.ph.split

; :3 ; preds = %3, %.lr.ph
  %indvars.iv.prol = phi i64 [ 0, %.lr.ph ], [ %indvars.iv.next.prol, %3 ]
  %prol.iter = phi i32 [ %xtraiter, %.lr.ph ], [ %prol.iter.sub, %3 ]
  %4 = getelementptr inbounds i32, i32* %array_x, i64 %indvars.iv.prol
  %5 = trunc i64 %indvars.iv.prol to i32
  store i32 %5, i32* %4, align 4
  %indvars.iv.next.prol = add nuw nsw i64 %indvars.iv.prol, 1
  %lftr.wideiv.prol = trunc i64 %indvars.iv.next.prol to i32
  %exitcond.prol = icmp ne i32 %lftr.wideiv.prol, %n
  %prol.iter.sub = sub i32 %prol.iter, 1
  %prol.iter.cmp = icmp ne i32 %prol.iter.sub, 0
  br i1 %prol.iter.cmp, label %3, label %.lr.ph.split, !llvm.loop !1

.lr.ph.split:                                     ; preds = %3, %.lr.ph
  %indvars.iv.unr = phi i64 [ 0, %.lr.ph ], [ %indvars.iv.next.prol, %3 ]
  %6 = icmp ult i32 %2, 3
  br i1 %6, label %._crit_edge, label %.lr.ph.split.split

.lr.ph.split.split:                               ; preds = %.lr.ph.split,
%.lr.ph.split.split
  %indvars.iv = phi i64 [ %indvars.iv.next.3, %.lr.ph.split.split ], [
%indvars.iv.unr, %.lr.ph.split ]
  %7 = getelementptr inbounds i32, i32* %array_x, i64 %indvars.iv
  %8 = trunc i64 %indvars.iv to i32
  store i32 %8, i32* %7, align 4
  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
  %9 = getelementptr inbounds i32, i32* %array_x, i64 %indvars.iv.next
  %10 = trunc i64 %indvars.iv.next to i32
  store i32 %10, i32* %9, align 4
  %indvars.iv.next.1 = add nuw nsw i64 %indvars.iv.next, 1
  %lftr.wideiv.1 = trunc i64 %indvars.iv.next.1 to i32
  %11 = getelementptr inbounds i32, i32* %array_x, i64 %indvars.iv.next.1
  %12 = trunc i64 %indvars.iv.next.1 to i32
  store i32 %12, i32* %11, align 4
  %indvars.iv.next.2 = add nuw nsw i64 %indvars.iv.next.1, 1
  %lftr.wideiv.2 = trunc i64 %indvars.iv.next.2 to i32
  %13 = getelementptr inbounds i32, i32* %array_x, i64 %indvars.iv.next.2
  %14 = trunc i64 %indvars.iv.next.2 to i32
  store i32 %14, i32* %13, align 4
  %indvars.iv.next.3 = add nuw nsw i64 %indvars.iv.next.2, 1
  %lftr.wideiv.3 = trunc i64 %indvars.iv.next.3 to i32
  %exitcond.3 = icmp ne i32 %lftr.wideiv.3, %n
  br i1 %exitcond.3, label %.lr.ph.split.split, label %._crit_edge

._crit_edge:                                      ; preds = %.lr.ph.split,
%.lr.ph.split.split, %0
  ret void
}

attributes #0 = { nounwind uwtable
"disable-tail-calls"="false"
"less-precise-fpmad"="false"
"no-frame-pointer-elim"="true"
"no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="false"
"no-nans-fp-math"="false"
"stack-protector-buffer-size"="8"
"target-cpu"="x86-64"
"target-features"="+sse,+sse2"
"unsafe-fp-math"="false"
"use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.8.0 (trunk 245730) (llvm/trunk 245727)"}
!1 = distinct !{!1, !2}
!2 = !{!"llvm.loop.unroll.disable"}

******************************Clang Generate IR with LoopUnrolling
End***********************************************

The second .bc file gives me this:
******************************API Generate IR with LoopUnrolling
Start*************************************************
; ModuleID = 'bc_from_api.bc'
target triple = "x86_64-unkown-linux-gnu"

; Function Attrs: nounwind
define void @_Z3fooiPi(i32 %n, i32* %array_x) #0 {
entry:
  %cmp.1 = icmp slt i32 0, %n
  br i1 %cmp.1, label %for.body, label %for.end

for.body:                                         ; preds = %entry,
%for.body.3
  %i.02 = phi i32 [ %inc.3, %for.body.3 ], [ 0, %entry ]
  %idxprom = sext i32 %i.02 to i64
  %arrayidx = getelementptr inbounds i32, i32* %array_x, i64 %idxprom
  store i32 %i.02, i32* %arrayidx, align 4
  %inc = add nuw nsw i32 %i.02, 1
  %cmp = icmp slt i32 %inc, %n
  br i1 %cmp, label %for.body.1, label %for.end

for.end:                                          ; preds = %for.body,
%for.body.1, %for.body.2, %for.body.3, %entry
  ret void

for.body.1:                                       ; preds = %for.body
  %idxprom.1 = sext i32 %inc to i64
  %arrayidx.1 = getelementptr inbounds i32, i32* %array_x, i64 %idxprom.1
  store i32 %inc, i32* %arrayidx.1, align 4
  %inc.1 = add nuw nsw i32 %inc, 1
  %cmp.1.3 = icmp slt i32 %inc.1, %n
  br i1 %cmp.1.3, label %for.body.2, label %for.end

for.body.2:                                       ; preds = %for.body.1
  %idxprom.2 = sext i32 %inc.1 to i64
  %arrayidx.2 = getelementptr inbounds i32, i32* %array_x, i64 %idxprom.2
  store i32 %inc.1, i32* %arrayidx.2, align 4
  %inc.2 = add nuw nsw i32 %inc.1, 1
  %cmp.2 = icmp slt i32 %inc.2, %n
  br i1 %cmp.2, label %for.body.3, label %for.end

for.body.3:                                       ; preds = %for.body.2
  %idxprom.3 = sext i32 %inc.2 to i64
  %arrayidx.3 = getelementptr inbounds i32, i32* %array_x, i64 %idxprom.3
  store i32 %inc.2, i32* %arrayidx.3, align 4
  %inc.3 = add nuw nsw i32 %inc.2, 1
  %cmp.3 = icmp slt i32 %inc.3, %n
  br i1 %cmp.3, label %for.body, label %for.end
}

attributes #0 = { nounwind }
******************************API Generate IR with LoopUnrolling
End**************************************************

Sorry I post two many code here. Can you give me any suggestion? Thanks
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150822/fd12e18f/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: bc_from_clang.bc
Type: application/octet-stream
Size: 1260 bytes
Desc: not available
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150822/fd12e18f/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: bc_from_api.bc
Type: application/octet-stream
Size: 788 bytes
Desc: not available
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150822/fd12e18f/attachment-0001.obj>