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Rename isEarlierInSameTrace to isUsefulDominator.

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In very rare cases caused by irreducible control flow, the dominating
block can have the same trace head without actually being part of the
trace.

As long as such a dominator still has valid instruction depths, it is OK
to use it for computing instruction depths.

Rename the function to avoid lying, and add a check that instruction
depths are computed for the dominator.

llvm-svn: 176668
This commit is contained in:
Jakob Stoklund Olesen 2013-03-07 23:55:49 +00:00
parent 88061183a0
commit b83588cd0b
2 changed files with 20 additions and 7 deletions
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23
include/llvm/CodeGen/MachineTraceMetrics.h
View File

@ -165,12 +165,25 @@ public:
/// Invalidate height resources when a block below this one has changed.
void invalidateHeight() { InstrHeight = ~0u; HasValidInstrHeights = false; }
/// Determine if this block belongs to the same trace as TBI and comes
/// before it in the trace.
/// Assuming that this is a dominator of TBI, determine if it contains
/// useful instruction depths. A dominating block can be above the current
/// trace head, and any dependencies from such a far away dominator are not
/// expected to affect the critical path.
///
/// Also returns true when TBI == this.
bool isEarlierInSameTrace(const TraceBlockInfo &TBI) const {
return hasValidDepth() && TBI.hasValidDepth() &&
Head == TBI.Head && InstrDepth <= TBI.InstrDepth;
bool isUsefulDominator(const TraceBlockInfo &TBI) const {
// The trace for TBI may not even be calculated yet.
if (!hasValidDepth() || !TBI.hasValidDepth())
return false;
// Instruction depths are only comparable if the traces share a head.
if (Head != TBI.Head)
return false;
// It is almost always the case that TBI belongs to the same trace as
// this block, but rare convoluted cases involving irreducible control
// flow, a dominator may share a trace head without actually being on the
// same trace as TBI. This is not a big problem as long as it doesn't
// increase the instruction depth.
return HasValidInstrDepths && InstrDepth <= TBI.InstrDepth;
}
// Data-dependency-related information. Per-instruction depth and height

4
lib/CodeGen/MachineTraceMetrics.cpp
View File

@ -677,7 +677,7 @@ computeCrossBlockCriticalPath(const TraceBlockInfo &TBI) {
const MachineInstr *DefMI = MTM.MRI->getVRegDef(LIR.Reg);
// Ignore dependencies outside the current trace.
const TraceBlockInfo &DefTBI = BlockInfo[DefMI->getParent()->getNumber()];
if (!DefTBI.isEarlierInSameTrace(TBI))
if (!DefTBI.isUsefulDominator(TBI))
continue;
unsigned Len = LIR.Height + Cycles[DefMI].Depth;
MaxLen = std::max(MaxLen, Len);
@ -740,7 +740,7 @@ computeInstrDepths(const MachineBasicBlock *MBB) {
const TraceBlockInfo&DepTBI =
BlockInfo[Dep.DefMI->getParent()->getNumber()];
// Ignore dependencies from outside the current trace.
if (!DepTBI.isEarlierInSameTrace(TBI))
if (!DepTBI.isUsefulDominator(TBI))
continue;
assert(DepTBI.HasValidInstrDepths && "Inconsistent dependency");
unsigned DepCycle = Cycles.lookup(Dep.DefMI).Depth;