2019-01-18 05:29:34 +08:00
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; RUN: opt -S -hotcoldsplit -hotcoldsplit-threshold=0 < %s | FileCheck %s
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[HotColdSplitting] Identify larger cold regions using domtree queries
The current splitting algorithm works in three stages:
1) Identify cold blocks, then
2) Use forward/backward propagation to mark hot blocks, then
3) Grow a SESE region of blocks *outside* of the set of hot blocks and
start outlining.
While testing this pass on Apple internal frameworks I noticed that some
kinds of control flow (e.g. loops) are never outlined, even though they
unconditionally lead to / follow cold blocks. I noticed two other issues
related to how cold regions are identified:
- An inconsistency can arise in the internal state of the hotness
propagation stage, as a block may end up in both the ColdBlocks set
and the HotBlocks set. Further inconsistencies can arise as these sets
do not match what's in ProfileSummaryInfo.
- It isn't necessary to limit outlining to single-exit regions.
This patch teaches the splitting algorithm to identify maximal cold
regions and outline them. A maximal cold region is defined as the set of
blocks post-dominated by a cold sink block, or dominated by that sink
block. This approach can successfully outline loops in the cold path. As
a side benefit, it maintains less internal state than the current
approach.
Due to a limitation in CodeExtractor, blocks within the maximal cold
region which aren't dominated by a single entry point (a so-called "max
ancestor") are filtered out.
Results:
- X86 (LNT + -Os + externals): 134KB of TEXT were outlined compared to
47KB pre-patch, or a ~3x improvement. Did not see a performance impact
across two runs.
- AArch64 (LNT + -Os + externals + Apple-internal benchmarks): 149KB
of TEXT were outlined. Ditto re: performance impact.
- Outlining results improve marginally in the internal frameworks I
tested.
Follow-ups:
- Outline more than once per function, outline large single basic
blocks, & try to remove unconditional branches in outlined functions.
Differential Revision: https://reviews.llvm.org/D53627
llvm-svn: 345209
2018-10-25 06:15:41 +08:00
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; Source:
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;
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; extern void sideeffect(int);
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; extern void __attribute__((cold)) sink();
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; void foo(int cond) {
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; if (cond) { //< Start outlining here.
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; sink();
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; if (cond > 10)
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; goto exit1;
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; else
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; goto exit2;
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; }
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; exit1:
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; sideeffect(1);
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; return;
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; exit2:
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; sideeffect(2);
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; return;
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; }
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target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-apple-macosx10.14.0"
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; CHECK-LABEL: define {{.*}}@foo(
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; CHECK: br i1 {{.*}}, label %exit1, label %codeRepl
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; CHECK-LABEL: codeRepl:
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; CHECK: [[targetBlock:%.*]] = call i1 @foo.cold.1(
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; CHECK-NEXT: br i1 [[targetBlock]], label %exit1, label %[[return:.*]]
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; CHECK-LABEL: exit1:
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; CHECK: call {{.*}}@sideeffect(i32 1)
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; CHECK: [[return]]:
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; CHECK-NEXT: ret void
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define void @foo(i32 %cond) {
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entry:
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%tobool = icmp eq i32 %cond, 0
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br i1 %tobool, label %exit1, label %if.then
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if.then: ; preds = %entry
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tail call void (...) @sink()
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%cmp = icmp sgt i32 %cond, 10
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br i1 %cmp, label %exit1, label %exit2
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exit1: ; preds = %entry, %if.then
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call void @sideeffect(i32 1)
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br label %return
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exit2: ; preds = %if.then
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call void @sideeffect(i32 2)
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br label %return
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return: ; preds = %exit2, %exit1
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ret void
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}
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; CHECK-LABEL: define {{.*}}@foo.cold.1(
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; CHECK: br
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; CHECK: [[exit1Stub:.*]]:
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; CHECK-NEXT: ret i1 true
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; CHECK: [[returnStub:.*]]:
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; CHECK-NEXT: ret i1 false
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; CHECK: call {{.*}}@sink
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; CHECK-NEXT: [[cmp:%.*]] = icmp
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; CHECK-NEXT: br i1 [[cmp]], label %[[exit1Stub]], label %exit2
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; CHECK-LABEL: exit2:
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; CHECK-NEXT: call {{.*}}@sideeffect(i32 2)
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; CHECK-NEXT: br label %[[returnStub]]
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declare void @sink(...) cold
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declare void @sideeffect(i32)
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