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Allow 0-weight branches in BranchProbabilityInfo.

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Summary:
When computing branch weights in BPI, we used to disallow branches with
weight 0. This is a minor nuisance, because a branch with weight 0 is
different to "don't have information". In the context of
instrumentation, it may mean "never executed", in the context of
sampling, it means "never or seldom executed".

In allowing 0 weight branches, I ran into issues with the switch
expansion code in selection DAG. It is currently hardwired to not handle
branches with weight 0. To maintain the current behaviour, I changed it
to use 1 when it finds 0, but perhaps the algorithm needs changes to
tolerate branches with weight zero.

Reviewers: hansw

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9533

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236617 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Diego Novillo
2015-05-06 17:55:11 +00:00
parent 5a6ea245b0
commit 26e46f2283
5 changed files with 26 additions and 20 deletions
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12
test/Transforms/SampleProfile/branch.ll
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@@ -36,8 +36,8 @@ entry:
tail call void @llvm.dbg.value(metadata i8** %argv, i64 0, metadata !14, metadata !DIExpression()), !dbg !27
%cmp = icmp slt i32 %argc, 2, !dbg !28
br i1 %cmp, label %return, label %if.end, !dbg !28
; CHECK: edge entry -> return probability is 1 / 2 = 50%
; CHECK: edge entry -> if.end probability is 1 / 2 = 50%
; CHECK: edge entry -> return probability is 0 / 1 = 0%
; CHECK: edge entry -> if.end probability is 1 / 1 = 100%
if.end: ; preds = %entry
%arrayidx = getelementptr inbounds i8*, i8** %argv, i64 1, !dbg !30
@@ -46,8 +46,8 @@ if.end: ; preds = %entry
tail call void @llvm.dbg.value(metadata i32 %call, i64 0, metadata !17, metadata !DIExpression()), !dbg !30
%cmp1 = icmp sgt i32 %call, 100, !dbg !35
br i1 %cmp1, label %for.body, label %if.end6, !dbg !35
; CHECK: edge if.end -> for.body probability is 1 / 2 = 50%
; CHECK: edge if.end -> if.end6 probability is 1 / 2 = 50%
; CHECK: edge if.end -> for.body probability is 0 / 1 = 0%
; CHECK: edge if.end -> if.end6 probability is 1 / 1 = 100%
for.body: ; preds = %if.end, %for.body
%u.016 = phi i32 [ %inc, %for.body ], [ 0, %if.end ]
@@ -65,8 +65,8 @@ for.body: ; preds = %if.end, %for.body
tail call void @llvm.dbg.value(metadata i32 %inc, i64 0, metadata !21, metadata !DIExpression()), !dbg !38
%exitcond = icmp eq i32 %inc, %call, !dbg !38
br i1 %exitcond, label %if.end6, label %for.body, !dbg !38
; CHECK: edge for.body -> if.end6 probability is 1 / 10227 = 0.00977804
; CHECK: edge for.body -> for.body probability is 10226 / 10227 = 99.9902% [HOT edge]
; CHECK: edge for.body -> if.end6 probability is 0 / 10226 = 0%
; CHECK: edge for.body -> for.body probability is 10226 / 10226 = 100% [HOT edge]
if.end6: ; preds = %for.body, %if.end
%result.0 = phi double [ 0.000000e+00, %if.end ], [ %sub, %for.body ]