[llvm-dev] loop unrolling introduces conditional branch

Mehdi Amini via llvm-dev
Sat, 22 Aug 2015 15:36:39 -0000

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
<https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=9pD2qNHBiPJscAs-ZzWT7q61NeZNcwsY8Io52JnaI3E&s=OaVJWcn8TI03qp9lgqeWMIVMySoyBIPowkHDEyR7lvQ&e=>,
label %._crit_edge
>
> .lr.ph
<https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=9pD2qNHBiPJscAs-ZzWT7q61NeZNcwsY8Io52JnaI3E&s=OaVJWcn8TI03qp9lgqeWMIVMySoyBIPowkHDEyR7lvQ&e=>:
; 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
<https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=9pD2qNHBiPJscAs-ZzWT7q61NeZNcwsY8Io52JnaI3E&s=OaVJWcn8TI03qp9lgqeWMIVMySoyBIPowkHDEyR7lvQ&e=>
>   %indvars.iv.prol = phi i64 [ 0, %.lr.ph
<https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=9pD2qNHBiPJscAs-ZzWT7q61NeZNcwsY8Io52JnaI3E&s=OaVJWcn8TI03qp9lgqeWMIVMySoyBIPowkHDEyR7lvQ&e=>
], [ %indvars.iv.next.prol, %3 ]
>   %prol.iter = phi i32 [ %xtraiter, %.lr.ph
<https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=9pD2qNHBiPJscAs-ZzWT7q61NeZNcwsY8Io52JnaI3E&s=OaVJWcn8TI03qp9lgqeWMIVMySoyBIPowkHDEyR7lvQ&e=>
], [ %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
<https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=9pD2qNHBiPJscAs-ZzWT7q61NeZNcwsY8Io52JnaI3E&s=OaVJWcn8TI03qp9lgqeWMIVMySoyBIPowkHDEyR7lvQ&e=>
>   %indvars.iv.unr = phi i64 [ 0, %.lr.ph
<https://urldefense.proofpoint.com/v2/url?u=http-3A__lr.ph&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=v-ruWq0KCv2O3thJZiK6naxuXK8mQHZUmGq5FBtAmZ4&m=9pD2qNHBiPJscAs-ZzWT7q61NeZNcwsY8Io52JnaI3E&s=OaVJWcn8TI03qp9lgqeWMIVMySoyBIPowkHDEyR7lvQ&e=>
], [ %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?

Yes, use an online service like pastebin :)

You don’t have defined the DataLayout in the API cases, it should help to do so.

—
Mehdi

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