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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 | B⇆C | 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
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Cong Hou
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Nov 2, 2015
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122
llvm/test/CodeGen/X86/code_placement_cold_loop_blocks.ll
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; RUN: llc -mcpu=corei7 -mtriple=x86_64-linux < %s | FileCheck %s -check-prefix=CHECK | ||
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define void @foo() !prof !1 { | ||
; Test if a cold block in a loop will be placed at the end of the function | ||
; chain. | ||
; | ||
; CHECK-LABEL: foo: | ||
; CHECK: callq b | ||
; CHECK: callq c | ||
; CHECK: callq e | ||
; CHECK: callq f | ||
; CHECK: callq d | ||
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entry: | ||
br label %header | ||
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header: | ||
call void @b() | ||
%call = call zeroext i1 @a() | ||
br i1 %call, label %if.then, label %if.else, !prof !4 | ||
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if.then: | ||
call void @c() | ||
br label %if.end | ||
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if.else: | ||
call void @d() | ||
br label %if.end | ||
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if.end: | ||
call void @e() | ||
%call2 = call zeroext i1 @a() | ||
br i1 %call2, label %header, label %end, !prof !5 | ||
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end: | ||
call void @f() | ||
ret void | ||
} | ||
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define void @nested_loop_0() !prof !1 { | ||
; Test if a block that is cold in the inner loop but not cold in the outer loop | ||
; will merged to the outer loop chain. | ||
; | ||
; CHECK-LABEL: nested_loop_0: | ||
; CHECK: callq c | ||
; CHECK: callq d | ||
; CHECK: callq e | ||
; CHECK: callq b | ||
; CHECK: callq f | ||
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entry: | ||
br label %header | ||
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header: | ||
call void @b() | ||
%call4 = call zeroext i1 @a() | ||
br i1 %call4, label %header2, label %end | ||
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header2: | ||
call void @c() | ||
%call = call zeroext i1 @a() | ||
br i1 %call, label %if.then, label %if.else, !prof !2 | ||
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if.then: | ||
call void @d() | ||
%call3 = call zeroext i1 @a() | ||
br i1 %call3, label %header2, label %header, !prof !3 | ||
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if.else: | ||
call void @e() | ||
%call2 = call zeroext i1 @a() | ||
br i1 %call2, label %header2, label %header, !prof !3 | ||
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end: | ||
call void @f() | ||
ret void | ||
} | ||
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define void @nested_loop_1() !prof !1 { | ||
; Test if a cold block in an inner loop will be placed at the end of the | ||
; function chain. | ||
; | ||
; CHECK-LABEL: nested_loop_1: | ||
; CHECK: callq b | ||
; CHECK: callq c | ||
; CHECK: callq e | ||
; CHECK: callq d | ||
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entry: | ||
br label %header | ||
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header: | ||
call void @b() | ||
br label %header2 | ||
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header2: | ||
call void @c() | ||
%call = call zeroext i1 @a() | ||
br i1 %call, label %end, label %if.else, !prof !4 | ||
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if.else: | ||
call void @d() | ||
%call2 = call zeroext i1 @a() | ||
br i1 %call2, label %header2, label %header, !prof !5 | ||
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end: | ||
call void @e() | ||
ret void | ||
} | ||
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declare zeroext i1 @a() | ||
declare void @b() | ||
declare void @c() | ||
declare void @d() | ||
declare void @e() | ||
declare void @f() | ||
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!1 = !{!"function_entry_count", i64 1} | ||
!2 = !{!"branch_weights", i32 100, i32 1} | ||
!3 = !{!"branch_weights", i32 1, i32 10} | ||
!4 = !{!"branch_weights", i32 1000, i32 1} | ||
!5 = !{!"branch_weights", i32 100, i32 1} |