return true;
}
-/// \brief Get the maximum allowed loop scale.
-///
-/// Gives the maximum number of estimated iterations allowed for a loop. Very
-/// large numbers cause problems downstream (even within 64-bits).
-static Scaled64 getMaxLoopScale() { return Scaled64(1, 12); }
-
/// \brief Compute the loop scale for a loop.
void BlockFrequencyInfoImplBase::computeLoopScale(LoopData &Loop) {
// Compute loop scale.
DEBUG(dbgs() << "compute-loop-scale: " << getLoopName(Loop) << "\n");
+ // Infinite loops need special handling. If we give the back edge an infinite
+ // mass, they may saturate all the other scales in the function down to 1,
+ // making all the other region temperatures look exactly the same. Choose an
+ // arbitrary scale to avoid these issues.
+ //
+ // FIXME: An alternate way would be to select a symbolic scale which is later
+ // replaced to be the maximum of all computed scales plus 1. This would
+ // appropriately describe the loop as having a large scale, without skewing
+ // the final frequency computation.
+ const Scaled64 InifiniteLoopScale(1, 12);
+
// LoopScale == 1 / ExitMass
// ExitMass == HeadMass - BackedgeMass
BlockMass ExitMass = BlockMass::getFull() - Loop.BackedgeMass;
- // Block scale stores the inverse of the scale.
- Loop.Scale = ExitMass.toScaled().inverse();
+ // Block scale stores the inverse of the scale. If this is an infinite loop,
+ // its exit mass will be zero. In this case, use an arbitrary scale for the
+ // loop scale.
+ Loop.Scale =
+ ExitMass.isEmpty() ? InifiniteLoopScale : ExitMass.toScaled().inverse();
DEBUG(dbgs() << " - exit-mass = " << ExitMass << " (" << BlockMass::getFull()
<< " - " << Loop.BackedgeMass << ")\n"
<< " - scale = " << Loop.Scale << "\n");
-
- if (Loop.Scale > getMaxLoopScale()) {
- Loop.Scale = getMaxLoopScale();
- DEBUG(dbgs() << " - reduced-to-max-scale: " << getMaxLoopScale() << "\n");
- }
}
/// \brief Package up a loop.
const Scaled64 &Min, const Scaled64 &Max) {
// Scale the Factor to a size that creates integers. Ideally, integers would
// be scaled so that Max == UINT64_MAX so that they can be best
- // differentiated. However, the register allocator currently deals poorly
- // with large numbers. Instead, push Min up a little from 1 to give some
- // room to differentiate small, unequal numbers.
- //
- // TODO: fix issues downstream so that ScalingFactor can be
- // Scaled64(1,64)/Max.
- Scaled64 ScalingFactor = Min.inverse();
- if ((Max / Min).lg() < 60)
+ // differentiated. However, in the presence of large frequency values, small
+ // frequencies are scaled down to 1, making it impossible to differentiate
+ // small, unequal numbers. When the spread between Min and Max frequencies
+ // fits well within MaxBits, we make the scale be at least 8.
+ const unsigned MaxBits = 64;
+ const unsigned SpreadBits = (Max / Min).lg();
+ Scaled64 ScalingFactor;
+ if (SpreadBits <= MaxBits - 3) {
+ // If the values are small enough, make the scaling factor at least 8 to
+ // allow distinguishing small values.
+ ScalingFactor = Min.inverse();
ScalingFactor <<= 3;
+ } else {
+ // If the values need more than MaxBits to be represented, saturate small
+ // frequency values down to 1 by using a scaling factor that benefits large
+ // frequency values.
+ ScalingFactor = Scaled64(1, MaxBits) / Max;
+ }
// Translate the floats to integers.
DEBUG(dbgs() << "float-to-int: min = " << Min << ", max = " << Max
--- /dev/null
+; RUN: opt < %s -analyze -block-freq | FileCheck %s
+
+; This code contains three loops. One is triple-nested, the
+; second is double nested and the third is a single loop. At
+; runtime, all three loops execute 1,000,000 times each. We use to
+; give different frequencies to each of the loops because loop
+; scales were limited to no more than 4,096.
+;
+; This was penalizing the hotness of the second and third loops
+; because BFI was reducing the loop scale for for.cond16 and
+; for.cond26 to a max of 4,096.
+;
+; Without this restriction, all loops are now correctly given the same
+; frequency values.
+;
+; Original C code:
+;
+;
+; int g;
+; __attribute__((noinline)) void bar() {
+; g++;
+; }
+;
+; extern int printf(const char*, ...);
+;
+; int main()
+; {
+; int i, j, k;
+;
+; g = 0;
+; for (i = 0; i < 100; i++)
+; for (j = 0; j < 100; j++)
+; for (k = 0; k < 100; k++)
+; bar();
+;
+; printf ("g = %d\n", g);
+; g = 0;
+;
+; for (i = 0; i < 100; i++)
+; for (j = 0; j < 10000; j++)
+; bar();
+;
+; printf ("g = %d\n", g);
+; g = 0;
+;
+;
+; for (i = 0; i < 1000000; i++)
+; bar();
+;
+; printf ("g = %d\n", g);
+; g = 0;
+; }
+
+@g = common global i32 0, align 4
+@.str = private unnamed_addr constant [8 x i8] c"g = %d\0A\00", align 1
+
+declare void @bar()
+declare i32 @printf(i8*, ...)
+
+; CHECK: Printing analysis {{.*}} for function 'main':
+; CHECK-NEXT: block-frequency-info: main
+define i32 @main() {
+entry:
+ %retval = alloca i32, align 4
+ %i = alloca i32, align 4
+ %j = alloca i32, align 4
+ %k = alloca i32, align 4
+ store i32 0, i32* %retval
+ store i32 0, i32* @g, align 4
+ store i32 0, i32* %i, align 4
+ br label %for.cond
+
+for.cond: ; preds = %for.inc10, %entry
+ %0 = load i32, i32* %i, align 4
+ %cmp = icmp slt i32 %0, 100
+ br i1 %cmp, label %for.body, label %for.end12, !prof !1
+
+for.body: ; preds = %for.cond
+ store i32 0, i32* %j, align 4
+ br label %for.cond1
+
+for.cond1: ; preds = %for.inc7, %for.body
+ %1 = load i32, i32* %j, align 4
+ %cmp2 = icmp slt i32 %1, 100
+ br i1 %cmp2, label %for.body3, label %for.end9, !prof !2
+
+for.body3: ; preds = %for.cond1
+ store i32 0, i32* %k, align 4
+ br label %for.cond4
+
+for.cond4: ; preds = %for.inc, %for.body3
+ %2 = load i32, i32* %k, align 4
+ %cmp5 = icmp slt i32 %2, 100
+ br i1 %cmp5, label %for.body6, label %for.end, !prof !3
+
+; CHECK: - for.body6: float = 500000.5, int = 4000003
+for.body6: ; preds = %for.cond4
+ call void @bar()
+ br label %for.inc
+
+for.inc: ; preds = %for.body6
+ %3 = load i32, i32* %k, align 4
+ %inc = add nsw i32 %3, 1
+ store i32 %inc, i32* %k, align 4
+ br label %for.cond4
+
+for.end: ; preds = %for.cond4
+ br label %for.inc7
+
+for.inc7: ; preds = %for.end
+ %4 = load i32, i32* %j, align 4
+ %inc8 = add nsw i32 %4, 1
+ store i32 %inc8, i32* %j, align 4
+ br label %for.cond1
+
+for.end9: ; preds = %for.cond1
+ br label %for.inc10
+
+for.inc10: ; preds = %for.end9
+ %5 = load i32, i32* %i, align 4
+ %inc11 = add nsw i32 %5, 1
+ store i32 %inc11, i32* %i, align 4
+ br label %for.cond
+
+for.end12: ; preds = %for.cond
+ %6 = load i32, i32* @g, align 4
+ %call = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([8 x i8], [8 x i8]* @.str, i32 0, i32 0), i32 %6)
+ store i32 0, i32* @g, align 4
+ store i32 0, i32* %i, align 4
+ br label %for.cond13
+
+for.cond13: ; preds = %for.inc22, %for.end12
+ %7 = load i32, i32* %i, align 4
+ %cmp14 = icmp slt i32 %7, 100
+ br i1 %cmp14, label %for.body15, label %for.end24, !prof !1
+
+for.body15: ; preds = %for.cond13
+ store i32 0, i32* %j, align 4
+ br label %for.cond16
+
+for.cond16: ; preds = %for.inc19, %for.body15
+ %8 = load i32, i32* %j, align 4
+ %cmp17 = icmp slt i32 %8, 10000
+ br i1 %cmp17, label %for.body18, label %for.end21, !prof !4
+
+; CHECK: - for.body18: float = 500000.5, int = 4000003
+for.body18: ; preds = %for.cond16
+ call void @bar()
+ br label %for.inc19
+
+for.inc19: ; preds = %for.body18
+ %9 = load i32, i32* %j, align 4
+ %inc20 = add nsw i32 %9, 1
+ store i32 %inc20, i32* %j, align 4
+ br label %for.cond16
+
+for.end21: ; preds = %for.cond16
+ br label %for.inc22
+
+for.inc22: ; preds = %for.end21
+ %10 = load i32, i32* %i, align 4
+ %inc23 = add nsw i32 %10, 1
+ store i32 %inc23, i32* %i, align 4
+ br label %for.cond13
+
+for.end24: ; preds = %for.cond13
+ %11 = load i32, i32* @g, align 4
+ %call25 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([8 x i8], [8 x i8]* @.str, i32 0, i32 0), i32 %11)
+ store i32 0, i32* @g, align 4
+ store i32 0, i32* %i, align 4
+ br label %for.cond26
+
+for.cond26: ; preds = %for.inc29, %for.end24
+ %12 = load i32, i32* %i, align 4
+ %cmp27 = icmp slt i32 %12, 1000000
+ br i1 %cmp27, label %for.body28, label %for.end31, !prof !5
+
+; CHECK: - for.body28: float = 500000.5, int = 4000003
+for.body28: ; preds = %for.cond26
+ call void @bar()
+ br label %for.inc29
+
+for.inc29: ; preds = %for.body28
+ %13 = load i32, i32* %i, align 4
+ %inc30 = add nsw i32 %13, 1
+ store i32 %inc30, i32* %i, align 4
+ br label %for.cond26
+
+for.end31: ; preds = %for.cond26
+ %14 = load i32, i32* @g, align 4
+ %call32 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([8 x i8], [8 x i8]* @.str, i32 0, i32 0), i32 %14)
+ store i32 0, i32* @g, align 4
+ %15 = load i32, i32* %retval
+ ret i32 %15
+}
+
+!llvm.ident = !{!0}
+
+!0 = !{!"clang version 3.7.0 (trunk 232635) (llvm/trunk 232636)"}
+!1 = !{!"branch_weights", i32 101, i32 2}
+!2 = !{!"branch_weights", i32 10001, i32 101}
+!3 = !{!"branch_weights", i32 1000001, i32 10001}
+!4 = !{!"branch_weights", i32 1000001, i32 101}
+!5 = !{!"branch_weights", i32 1000001, i32 2}
projects / oota-llvm.git / commitdiff