[llvm] r231186 - RegisterCoalescer: Gracefully continue if subrange merging fails.
Matthias Braun
matze at braunis.de
Tue Mar 3 16:43:50 PST 2015
Author: matze
Date: Tue Mar 3 18:43:50 2015
New Revision: 231186
URL: http://llvm.org/viewvc/llvm-project?rev=231186&view=rev
Log:
RegisterCoalescer: Gracefully continue if subrange merging fails.
There is a known bug where the register coalescer fails to merge
subranges when multiple ranges end up in the "overflow" bit 32 of the
lanemasks. A proper fix for this is complicated so for now this is a
workaround which lets the register coalescer drop the subregister
liveness information (we just loose some precision by that) and
continue.
Modified:
llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp
Modified: llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp?rev=231186&r1=231185&r2=231186&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp (original)
+++ llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp Tue Mar 3 18:43:50 2015
@@ -160,12 +160,14 @@ namespace {
/// LaneMask are split as necessary. @p LaneMask are the lanes that
/// @p ToMerge will occupy in the coalescer register. @p LI has its subrange
/// lanemasks already adjusted to the coalesced register.
- void mergeSubRangeInto(LiveInterval &LI, const LiveRange &ToMerge,
+ /// @returns false if live range conflicts couldn't get resolved.
+ bool mergeSubRangeInto(LiveInterval &LI, const LiveRange &ToMerge,
unsigned LaneMask, CoalescerPair &CP);
/// Join the liveranges of two subregisters. Joins @p RRange into
/// @p LRange, @p RRange may be invalid afterwards.
- void joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
+ /// @returns false if live range conflicts couldn't get resolved.
+ bool joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
unsigned LaneMask, const CoalescerPair &CP);
/// We found a non-trivially-coalescable copy. If the source value number is
@@ -2382,7 +2384,7 @@ void JoinVals::eraseInstrs(SmallPtrSetIm
}
}
-void RegisterCoalescer::joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
+bool RegisterCoalescer::joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
unsigned LaneMask,
const CoalescerPair &CP) {
SmallVector<VNInfo*, 16> NewVNInfo;
@@ -2392,12 +2394,19 @@ void RegisterCoalescer::joinSubRegRanges
NewVNInfo, CP, LIS, TRI, true, true);
// Compute NewVNInfo and resolve conflicts (see also joinVirtRegs())
- // Conflicts should already be resolved so the mapping/resolution should
- // always succeed.
- if (!LHSVals.mapValues(RHSVals) || !RHSVals.mapValues(LHSVals))
- llvm_unreachable("Can't join subrange although main ranges are compatible");
- if (!LHSVals.resolveConflicts(RHSVals) || !RHSVals.resolveConflicts(LHSVals))
- llvm_unreachable("Can't join subrange although main ranges are compatible");
+ // We should be able to resolve all conflicts here as we could successfully do
+ // it on the mainrange already. There is however a problem when multiple
+ // ranges get mapped to the "overflow" lane mask bit which creates unexpected
+ // interferences.
+ if (!LHSVals.mapValues(RHSVals) || !RHSVals.mapValues(LHSVals)) {
+ DEBUG(dbgs() << "*** Couldn't join subrange!\n");
+ return false;
+ }
+ if (!LHSVals.resolveConflicts(RHSVals) ||
+ !RHSVals.resolveConflicts(LHSVals)) {
+ DEBUG(dbgs() << "*** Couldn't join subrange!\n");
+ return false;
+ }
// The merging algorithm in LiveInterval::join() can't handle conflicting
// value mappings, so we need to remove any live ranges that overlap a
@@ -2416,16 +2425,17 @@ void RegisterCoalescer::joinSubRegRanges
DEBUG(dbgs() << "\t\tjoined lanes: " << LRange << "\n");
if (EndPoints.empty())
- return;
+ return true;
// Recompute the parts of the live range we had to remove because of
// CR_Replace conflicts.
DEBUG(dbgs() << "\t\trestoring liveness to " << EndPoints.size()
<< " points: " << LRange << '\n');
LIS->extendToIndices(LRange, EndPoints);
+ return true;
}
-void RegisterCoalescer::mergeSubRangeInto(LiveInterval &LI,
+bool RegisterCoalescer::mergeSubRangeInto(LiveInterval &LI,
const LiveRange &ToMerge,
unsigned LaneMask, CoalescerPair &CP) {
BumpPtrAllocator &Allocator = LIS->getVNInfoAllocator();
@@ -2453,7 +2463,8 @@ void RegisterCoalescer::mergeSubRangeInt
CommonRange = &R;
}
LiveRange RangeCopy(ToMerge, Allocator);
- joinSubRegRanges(*CommonRange, RangeCopy, Common, CP);
+ if (!joinSubRegRanges(*CommonRange, RangeCopy, Common, CP))
+ return false;
LaneMask &= ~RMask;
}
@@ -2461,6 +2472,7 @@ void RegisterCoalescer::mergeSubRangeInt
DEBUG(dbgs() << format("\t\tNew Lane %04X\n", LaneMask));
LI.createSubRangeFrom(Allocator, LaneMask, ToMerge);
}
+ return true;
}
bool RegisterCoalescer::joinVirtRegs(CoalescerPair &CP) {
@@ -2511,22 +2523,40 @@ bool RegisterCoalescer::joinVirtRegs(Coa
// Determine lanemasks of RHS in the coalesced register and merge subranges.
unsigned SrcIdx = CP.getSrcIdx();
+ bool Abort = false;
if (!RHS.hasSubRanges()) {
unsigned Mask = SrcIdx == 0 ? CP.getNewRC()->getLaneMask()
: TRI->getSubRegIndexLaneMask(SrcIdx);
- mergeSubRangeInto(LHS, RHS, Mask, CP);
+ if (!mergeSubRangeInto(LHS, RHS, Mask, CP))
+ Abort = true;
} else {
// Pair up subranges and merge.
for (LiveInterval::SubRange &R : RHS.subranges()) {
unsigned Mask = TRI->composeSubRegIndexLaneMask(SrcIdx, R.LaneMask);
- mergeSubRangeInto(LHS, R, Mask, CP);
+ if (!mergeSubRangeInto(LHS, R, Mask, CP)) {
+ Abort = true;
+ break;
+ }
}
}
+ if (Abort) {
+ // This shouldn't have happened :-(
+ // However we are aware of at least one existing problem where we
+ // can't merge subranges when multiple ranges end up in the
+ // "overflow bit" 32. As a workaround we drop all subregister ranges
+ // which means we loose some precision but are back to a well defined
+ // state.
+ assert((CP.getNewRC()->getLaneMask() & 0x80000000u)
+ && "SubRange merge should only fail when merging into bit 32.");
+ DEBUG(dbgs() << "\tSubrange join aborted!\n");
+ LHS.clearSubRanges();
+ RHS.clearSubRanges();
+ } else {
+ DEBUG(dbgs() << "\tJoined SubRanges " << LHS << "\n");
- DEBUG(dbgs() << "\tJoined SubRanges " << LHS << "\n");
-
- LHSVals.pruneSubRegValues(LHS, ShrinkMask);
- RHSVals.pruneSubRegValues(LHS, ShrinkMask);
+ LHSVals.pruneSubRegValues(LHS, ShrinkMask);
+ RHSVals.pruneSubRegValues(LHS, ShrinkMask);
+ }
}
// The merging algorithm in LiveInterval::join() can't handle conflicting
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