[llvm-dev] loop unrolling introduces conditional branch
Mehdi Amini via llvm-dev
llvm-dev at lists.llvm.org
Sat Aug 22 08:36:39 PDT 2015
> On Aug 22, 2015, at 7:27 AM, Xiangyang Guo <xguo6 at ncsu.edu> wrote:
>
> 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
>
> ; <label>: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
>
> ; <label>: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
>
> ; <label>: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
>
> ; <label>: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
>
> ; <label>: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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