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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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8
test/Transforms/SampleProfile/propagate.ll
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@@ -73,8 +73,8 @@ for.cond: ; preds = %for.inc16, %if.else
%5 = load i64, i64* %N.addr, align 8, !dbg !15
%cmp1 = icmp slt i64 %4, %5, !dbg !15
br i1 %cmp1, label %for.body, label %for.end18, !dbg !15
; CHECK: edge for.cond -> for.body probability is 10 / 11 = 90.9091% [HOT edge]
; CHECK: edge for.cond -> for.end18 probability is 1 / 11 = 9.09091%
; CHECK: edge for.cond -> for.body probability is 10 / 10 = 100% [HOT edge]
; CHECK: edge for.cond -> for.end18 probability is 0 / 10 = 0%
for.body: ; preds = %for.cond
%6 = load i64, i64* %i, align 8, !dbg !18
@@ -119,8 +119,8 @@ for.cond8: ; preds = %for.inc, %if.else7
%14 = load i64, i64* %i, align 8, !dbg !28
%cmp10 = icmp slt i64 %conv9, %14, !dbg !28
br i1 %cmp10, label %for.body11, label %for.end, !dbg !28
; CHECK: edge for.cond8 -> for.body11 probability is 16191 / 16192 = 99.9938% [HOT edge]
; CHECK: edge for.cond8 -> for.end probability is 1 / 16192 = 0.00617589%
; CHECK: edge for.cond8 -> for.body11 probability is 16191 / 16191 = 100% [HOT edge]
; CHECK: edge for.cond8 -> for.end probability is 0 / 16191 = 0%
for.body11: ; preds = %for.cond8
%15 = load i32, i32* %j, align 4, !dbg !31