Fix information loss in branch probability computation.

Summary:
This addresses PR 22718. When branch weights are too large, they were
being clamped to the range [1, MaxWeightForBB]. But this clamping is
only applied to edges that go outside the range, so it distorts the
relative branch probabilities.

This patch changes the weight calculation to scale every branch so the
relative probabilities are preserved. The scaling is done differently
now. First, all the branch weights are added up, and if the sum exceeds
32 bits, it computes an integer scale to bring all the weights within
the range.

The patch fixes an existing test that had slightly wrong branch
probabilities due to the previous clamping. It now gets branch weights
scaled accordingly.

Reviewers: dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9442

llvm-svn: 236750

1 files changed, 84 insertions, 0 deletions

diff --git a/llvm/test/Analysis/BranchProbabilityInfo/pr22718.ll b/llvm/test/Analysis/BranchProbabilityInfo/pr22718.ll
new file mode 100644
index 00000000000..0de4d643c9c
--- /dev/null
+++ b/llvm/test/Analysis/BranchProbabilityInfo/pr22718.ll
@@ -0,0 +1,84 @@
+; RUN: opt < %s -analyze -branch-prob | FileCheck %s
+
+; In this test, the else clause is taken about 90% of the time. This was not
+; reflected in the probability computation because the weight is larger than
+; the branch weight cap (about 2 billion).
+;
+; CHECK: edge for.body -> if.then probability is 216661881 / 2166666667 = 9.9
+; CHECK: edge for.body -> if.else probability is 1950004786 / 2166666667 = 90.0
+
+@y = common global i64 0, align 8
+@x = common global i64 0, align 8
+@.str = private unnamed_addr constant [17 x i8] c"x = %lu\0Ay = %lu\0A\00", align 1
+
+; Function Attrs: inlinehint nounwind uwtable
+define i32 @main() #0 {
+entry:
+  %retval = alloca i32, align 4
+  %i = alloca i64, align 8
+  store i32 0, i32* %retval
+  store i64 0, i64* @y, align 8
+  store i64 0, i64* @x, align 8
+  call void @srand(i32 422304) #3
+  store i64 0, i64* %i, align 8
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %0 = load i64, i64* %i, align 8
+  %cmp = icmp ult i64 %0, 13000000000
+  br i1 %cmp, label %for.body, label %for.end, !prof !1
+
+for.body:                                         ; preds = %for.cond
+  %call = call i32 @rand() #3
+  %conv = sitofp i32 %call to double
+  %mul = fmul double %conv, 1.000000e+02
+  %div = fdiv double %mul, 0x41E0000000000000
+  %cmp1 = fcmp ogt double %div, 9.000000e+01
+  br i1 %cmp1, label %if.then, label %if.else, !prof !2
+
+if.then:                                          ; preds = %for.body
+  %1 = load i64, i64* @x, align 8
+  %inc = add i64 %1, 1
+  store i64 %inc, i64* @x, align 8
+  br label %if.end
+
+if.else:                                          ; preds = %for.body
+  %2 = load i64, i64* @y, align 8
+  %inc3 = add i64 %2, 1
+  store i64 %inc3, i64* @y, align 8
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  br label %for.inc
+
+for.inc:                                          ; preds = %if.end
+  %3 = load i64, i64* %i, align 8
+  %inc4 = add i64 %3, 1
+  store i64 %inc4, i64* %i, align 8
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  %4 = load i64, i64* @x, align 8
+  %5 = load i64, i64* @y, align 8
+  %call5 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([17 x i8], [17 x i8]* @.str, i32 0, i32 0), i64 %4, i64 %5)
+  ret i32 0
+}
+
+; Function Attrs: nounwind
+declare void @srand(i32) #1
+
+; Function Attrs: nounwind
+declare i32 @rand() #1
+
+declare i32 @printf(i8*, ...) #2
+
+attributes #0 = { inlinehint nounwind uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { nounwind "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #2 = { "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #3 = { nounwind }
+
+!llvm.ident = !{!0}
+
+!0 = !{!"clang version 3.7.0 (trunk 236218) (llvm/trunk 236235)"}
+!1 = !{!"branch_weights", i32 -1044967295, i32 1}
+!2 = !{!"branch_weights", i32 433323762, i32 -394957723}