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[PowerPC] Check hot loop exit edge in PPCCTRLoops
PPCCTRLoops transform loops using mtctr/bdnz instructions if loop trip count is known and big enough to compensate for the cost of mtctr.
But if there is a loop exit edge which is known to be frequently taken (by builtin_expect or by PGO), we should not transform the loop to avoid the cost of mtctr instruction. Here is an example of a loop with hot exit edge:
for (unsigned i = 0; i < TripCount; i++) {
// do something
if (__builtin_expect(check(), 1))
break;
// do something
}
Differential Revision: https://reviews.llvm.org/D42637
llvm-svn: 324229
This commit is contained in:
Hiroshi Inoue
parent
06256b141d
commit
dc037f8fd3
@ -528,6 +528,27 @@ bool PPCCTRLoops::convertToCTRLoop(Loop *L) {
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SmallVector<BasicBlock*, 4> ExitingBlocks;
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L->getExitingBlocks(ExitingBlocks);
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// If there is an exit edge known to be frequently taken,
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// we should not transform this loop.
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for (auto &BB : ExitingBlocks) {
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Instruction *TI = BB->getTerminator();
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if (!TI) continue;
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if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
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uint64_t TrueWeight, FalseWeight;
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if (!BI->isConditional() ||
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!BI->extractProfMetadata(TrueWeight, FalseWeight))
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continue;
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// If the exit path is more frequent than the loop path,
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// we return here without further analysis for this loop.
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bool TrueIsExit = !L->contains(BI->getSuccessor(0));
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if (( TrueIsExit && FalseWeight < TrueWeight) ||
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(!TrueIsExit && FalseWeight > TrueWeight))
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return MadeChange;
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}
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}
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BasicBlock *CountedExitBlock = nullptr;
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const SCEV *ExitCount = nullptr;
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BranchInst *CountedExitBranch = nullptr;
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187
test/CodeGen/PowerPC/ctrloops-hot-exit.ll
Normal file
187
test/CodeGen/PowerPC/ctrloops-hot-exit.ll
Normal file
@ -0,0 +1,187 @@
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; RUN: llc -verify-machineinstrs -mtriple=powerpc64le-unknown-linux-gnu -mcpu=pwr9 < %s | FileCheck %s
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; If there is an exit edge known to be frequently taken,
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; we should not transform this loop.
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; A loop having a hot exit edge (exit in false branch)
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define signext i64 @func() {
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; CHECK: @func
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; CHECK-NOT: mtctr
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; CHECK-NOT: bdnz
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entry:
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%a = alloca [1000 x i32], align 4
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%0 = bitcast [1000 x i32]* %a to i8*
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br label %for.body
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for.body:
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%i.013 = phi i64 [ 0, %entry ], [ %inc, %if.end ]
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%b.012 = phi i64 [ 0, %entry ], [ %xor, %if.end ]
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%arrayidx = getelementptr inbounds [1000 x i32], [1000 x i32]* %a, i64 0, i64 %i.013
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%1 = load i32, i32* %arrayidx, align 4
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%tobool = icmp eq i32 %1, 0
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br i1 %tobool, label %if.end, label %cleanup, !prof !1
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if.end:
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%xor = xor i64 %i.013, %b.012
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%inc = add nuw nsw i64 %i.013, 1
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%cmp = icmp ult i64 %inc, 1000
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br i1 %cmp, label %for.body, label %cleanup
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cleanup:
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%res = phi i64 [ %b.012, %for.body ], [ %xor, %if.end ]
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ret i64 %res
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}
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; A loop having a cold exit edge (exit in false branch)
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define signext i64 @func2() {
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; CHECK: @func2
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; CHECK: mtctr
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; CHECK: bdnz
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entry:
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%a = alloca [1000 x i32], align 4
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%0 = bitcast [1000 x i32]* %a to i8*
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br label %for.body
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for.body:
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%i.013 = phi i64 [ 0, %entry ], [ %inc, %if.end ]
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%b.012 = phi i64 [ 0, %entry ], [ %xor, %if.end ]
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%arrayidx = getelementptr inbounds [1000 x i32], [1000 x i32]* %a, i64 0, i64 %i.013
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%1 = load i32, i32* %arrayidx, align 4
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%tobool = icmp eq i32 %1, 0
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br i1 %tobool, label %if.end, label %cleanup, !prof !2
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if.end:
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%xor = xor i64 %i.013, %b.012
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%inc = add nuw nsw i64 %i.013, 1
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%cmp = icmp ult i64 %inc, 1000
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br i1 %cmp, label %for.body, label %cleanup
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cleanup:
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%res = phi i64 [ %b.012, %for.body ], [ %xor, %if.end ]
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ret i64 %res
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}
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; A loop having an exit edge without profile data (exit in false branch)
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define signext i64 @func3() {
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; CHECK: @func3
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; CHECK: mtctr
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; CHECK: bdnz
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entry:
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%a = alloca [1000 x i32], align 4
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%0 = bitcast [1000 x i32]* %a to i8*
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br label %for.body
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for.body:
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%i.013 = phi i64 [ 0, %entry ], [ %inc, %if.end ]
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%b.012 = phi i64 [ 0, %entry ], [ %xor, %if.end ]
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%arrayidx = getelementptr inbounds [1000 x i32], [1000 x i32]* %a, i64 0, i64 %i.013
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%1 = load i32, i32* %arrayidx, align 4
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%tobool = icmp eq i32 %1, 0
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br i1 %tobool, label %if.end, label %cleanup
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if.end:
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%xor = xor i64 %i.013, %b.012
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%inc = add nuw nsw i64 %i.013, 1
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%cmp = icmp ult i64 %inc, 1000
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br i1 %cmp, label %for.body, label %cleanup
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cleanup:
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%res = phi i64 [ %b.012, %for.body ], [ %xor, %if.end ]
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ret i64 %res
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}
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; A loop having a hot exit edge (exit in true branch)
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define signext i64 @func4() {
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; CHECK: @func4
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; CHECK-NOT: mtctr
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; CHECK-NOT: bdnz
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entry:
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%a = alloca [1000 x i32], align 4
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%0 = bitcast [1000 x i32]* %a to i8*
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br label %for.body
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for.body:
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%i.013 = phi i64 [ 0, %entry ], [ %inc, %if.end ]
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%b.012 = phi i64 [ 0, %entry ], [ %xor, %if.end ]
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%arrayidx = getelementptr inbounds [1000 x i32], [1000 x i32]* %a, i64 0, i64 %i.013
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%1 = load i32, i32* %arrayidx, align 4
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%tobool = icmp ne i32 %1, 0
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br i1 %tobool, label %cleanup, label %if.end, !prof !2
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if.end:
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%xor = xor i64 %i.013, %b.012
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%inc = add nuw nsw i64 %i.013, 1
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%cmp = icmp ult i64 %inc, 1000
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br i1 %cmp, label %for.body, label %cleanup
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cleanup:
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%res = phi i64 [ %b.012, %for.body ], [ %xor, %if.end ]
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ret i64 %res
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}
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; A loop having a cold exit edge (exit in true branch)
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define signext i64 @func5() {
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; CHECK: @func5
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; CHECK: mtctr
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; CHECK: bdnz
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entry:
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%a = alloca [1000 x i32], align 4
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%0 = bitcast [1000 x i32]* %a to i8*
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br label %for.body
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for.body:
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%i.013 = phi i64 [ 0, %entry ], [ %inc, %if.end ]
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%b.012 = phi i64 [ 0, %entry ], [ %xor, %if.end ]
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%arrayidx = getelementptr inbounds [1000 x i32], [1000 x i32]* %a, i64 0, i64 %i.013
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%1 = load i32, i32* %arrayidx, align 4
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%tobool = icmp ne i32 %1, 0
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br i1 %tobool, label %cleanup, label %if.end, !prof !1
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if.end:
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%xor = xor i64 %i.013, %b.012
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%inc = add nuw nsw i64 %i.013, 1
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%cmp = icmp ult i64 %inc, 1000
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br i1 %cmp, label %for.body, label %cleanup
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cleanup:
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%res = phi i64 [ %b.012, %for.body ], [ %xor, %if.end ]
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ret i64 %res
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}
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; A loop having an exit edge without profile data (exit in true branch)
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define signext i64 @func6() {
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; CHECK: @func6
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; CHECK: mtctr
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; CHECK: bdnz
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entry:
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%a = alloca [1000 x i32], align 4
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%0 = bitcast [1000 x i32]* %a to i8*
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br label %for.body
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for.body:
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%i.013 = phi i64 [ 0, %entry ], [ %inc, %if.end ]
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%b.012 = phi i64 [ 0, %entry ], [ %xor, %if.end ]
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%arrayidx = getelementptr inbounds [1000 x i32], [1000 x i32]* %a, i64 0, i64 %i.013
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%1 = load i32, i32* %arrayidx, align 4
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%tobool = icmp ne i32 %1, 0
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br i1 %tobool, label %cleanup, label %if.end
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if.end:
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%xor = xor i64 %i.013, %b.012
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%inc = add nuw nsw i64 %i.013, 1
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%cmp = icmp ult i64 %inc, 1000
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br i1 %cmp, label %for.body, label %cleanup
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cleanup:
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%res = phi i64 [ %b.012, %for.body ], [ %xor, %if.end ]
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ret i64 %res
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}
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!1 = !{!"branch_weights", i32 1, i32 2000}
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!2 = !{!"branch_weights", i32 2000, i32 1}
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