2016-04-20 07:51:52 +08:00
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; RUN: llc -mcpu=corei7 -mtriple=x86_64-linux < %s | FileCheck %s
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In MachineBlockPlacement, filter cold blocks off the loop chain when profile data is available.
In the current BB placement algorithm, a loop chain always contains all loop blocks. This has a drawback that cold blocks in the loop may be inserted on a hot function path, hence increasing branch cost and also reducing icache locality.
Consider a simple example shown below:
A
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B⇆C
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D
When B->C is quite cold, the best BB-layout should be A,B,D,C. But the current implementation produces A,C,B,D.
This patch filters those cold blocks off from the loop chain by comparing the ratio:
LoopBBFreq / LoopFreq
to 20%: if it is less than 20%, we don't include this BB to the loop chain. Here LoopFreq is the frequency of the loop when we reduce the loop into a single node. In general we have more cold blocks when the loop has few iterations. And vice versa.
Differential revision: http://reviews.llvm.org/D11662
llvm-svn: 251833
2015-11-03 05:24:00 +08:00
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define void @foo() !prof !1 {
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; Test if a cold block in a loop will be placed at the end of the function
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; chain.
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;
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; CHECK-LABEL: foo:
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; CHECK: callq b
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; CHECK: callq c
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; CHECK: callq e
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; CHECK: callq f
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; CHECK: callq d
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entry:
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br label %header
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header:
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call void @b()
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%call = call zeroext i1 @a()
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br i1 %call, label %if.then, label %if.else, !prof !4
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if.then:
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call void @c()
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br label %if.end
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if.else:
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call void @d()
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br label %if.end
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if.end:
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call void @e()
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%call2 = call zeroext i1 @a()
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br i1 %call2, label %header, label %end, !prof !5
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end:
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call void @f()
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ret void
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}
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define void @nested_loop_0() !prof !1 {
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; Test if a block that is cold in the inner loop but not cold in the outer loop
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; will merged to the outer loop chain.
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;
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; CHECK-LABEL: nested_loop_0:
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; CHECK: callq c
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; CHECK: callq d
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; CHECK: callq e
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; CHECK: callq b
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; CHECK: callq f
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entry:
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br label %header
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header:
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call void @b()
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%call4 = call zeroext i1 @a()
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br i1 %call4, label %header2, label %end
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header2:
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call void @c()
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%call = call zeroext i1 @a()
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br i1 %call, label %if.then, label %if.else, !prof !2
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if.then:
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call void @d()
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%call3 = call zeroext i1 @a()
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br i1 %call3, label %header2, label %header, !prof !3
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if.else:
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call void @e()
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%call2 = call zeroext i1 @a()
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br i1 %call2, label %header2, label %header, !prof !3
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end:
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call void @f()
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ret void
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}
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define void @nested_loop_1() !prof !1 {
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; Test if a cold block in an inner loop will be placed at the end of the
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; function chain.
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;
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; CHECK-LABEL: nested_loop_1:
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; CHECK: callq b
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; CHECK: callq c
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; CHECK: callq e
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; CHECK: callq d
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entry:
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br label %header
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header:
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call void @b()
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br label %header2
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header2:
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call void @c()
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%call = call zeroext i1 @a()
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br i1 %call, label %end, label %if.else, !prof !4
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if.else:
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call void @d()
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%call2 = call zeroext i1 @a()
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br i1 %call2, label %header2, label %header, !prof !5
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end:
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call void @e()
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ret void
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}
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declare zeroext i1 @a()
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declare void @b()
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declare void @c()
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declare void @d()
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declare void @e()
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declare void @f()
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!1 = !{!"function_entry_count", i64 1}
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!2 = !{!"branch_weights", i32 100, i32 1}
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!3 = !{!"branch_weights", i32 1, i32 10}
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!4 = !{!"branch_weights", i32 1000, i32 1}
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!5 = !{!"branch_weights", i32 100, i32 1}
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