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Assume lane masks are always precise

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Allowing imprecise lane masks in case of more than 32 sub register lanes
lead to some tricky corner cases, and I need another bugfix for another
one. Instead I rather declare lane masks as precise and let tablegen
abort if we do not have enough bits.

This does not affect any in-tree target, even AMDGPU only needs 16 lanes
at the moment. If the 32 lanes turn out to be a problem in the future,
then we can easily change the LaneBitmask typedef to uint64_t.

Differential Revision: http://reviews.llvm.org/D14557

llvm-svn: 253279
This commit is contained in:
Matthias Braun 2015-11-17 00:50:55 +00:00
parent 11eb1233c6
commit 088a172bf6
4 changed files with 26 additions and 80 deletions
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22
include/llvm/Target/TargetRegisterInfo.h
View File

@ -35,24 +35,21 @@ class VirtRegMap;
class raw_ostream;
class LiveRegMatrix;
/// A bitmask representing the parts of a register are alive.
/// A bitmask representing the covering of a register with sub-registers.
///
/// This is typically used to track liveness at sub-register granularity.
/// Lane masks for sub-register indices are similar to register units for
/// physical registers. The individual bits in a lane mask can't be assigned
/// any specific meaning. They can be used to check if two sub-register
/// indices overlap.
///
/// If the target has a register such that:
/// Iff the target has a register such that:
///
/// getSubReg(Reg, A) overlaps getSubReg(Reg, B)
///
/// then:
///
/// (getSubRegIndexLaneMask(A) & getSubRegIndexLaneMask(B)) != 0
///
/// The converse is not necessarily true. If two lane masks have a common
/// bit, the corresponding sub-registers may not overlap, but it can be
/// assumed that they usually will.
typedef unsigned LaneBitmask;
class TargetRegisterClass {
@ -369,19 +366,6 @@ public:
return SubRegIndexLaneMasks[SubIdx];
}
/// Returns true if the given lane mask is imprecise.
///
/// LaneMasks as given by getSubRegIndexLaneMask() have a limited number of
/// bits, so for targets with more than 31 disjunct subregister indices there
/// may be cases where:
/// getSubReg(Reg,A) does not overlap getSubReg(Reg,B)
/// but we still have
/// (getSubRegIndexLaneMask(A) & getSubRegIndexLaneMask(B)) != 0.
/// This function returns true in those cases.
static bool isImpreciseLaneMask(LaneBitmask LaneMask) {
return LaneMask & 0x80000000u;
}
/// The lane masks returned by getSubRegIndexLaneMask() above can only be
/// used to determine if sub-registers overlap - they can't be used to
/// determine if a set of sub-registers completely cover another

51
lib/CodeGen/RegisterCoalescer.cpp
View File

@ -163,14 +163,12 @@ 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.
/// @returns false if live range conflicts couldn't get resolved.
bool mergeSubRangeInto(LiveInterval &LI, const LiveRange &ToMerge,
void mergeSubRangeInto(LiveInterval &LI, const LiveRange &ToMerge,
LaneBitmask LaneMask, CoalescerPair &CP);
/// Join the liveranges of two subregisters. Joins @p RRange into
/// @p LRange, @p RRange may be invalid afterwards.
/// @returns false if live range conflicts couldn't get resolved.
bool joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
void joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
LaneBitmask LaneMask, const CoalescerPair &CP);
/// We found a non-trivially-coalescable copy. If the source value number is
@ -2469,7 +2467,7 @@ void JoinVals::eraseInstrs(SmallPtrSetImpl<MachineInstr*> &ErasedInstrs,
}
}
bool RegisterCoalescer::joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
void RegisterCoalescer::joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
LaneBitmask LaneMask,
const CoalescerPair &CP) {
SmallVector<VNInfo*, 16> NewVNInfo;
@ -2484,13 +2482,15 @@ bool RegisterCoalescer::joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
// 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;
// We already determined that it is legal to merge the intervals, so this
// should never fail.
llvm_unreachable("*** Couldn't join subrange!\n");
}
if (!LHSVals.resolveConflicts(RHSVals) ||
!RHSVals.resolveConflicts(LHSVals)) {
DEBUG(dbgs() << "*** Couldn't join subrange!\n");
return false;
// We already determined that it is legal to merge the intervals, so this
// should never fail.
llvm_unreachable("*** Couldn't join subrange!\n");
}
// The merging algorithm in LiveInterval::join() can't handle conflicting
@ -2513,17 +2513,16 @@ bool RegisterCoalescer::joinSubRegRanges(LiveRange &LRange, LiveRange &RRange,
DEBUG(dbgs() << "\t\tjoined lanes: " << LRange << "\n");
if (EndPoints.empty())
return true;
return;
// 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;
}
bool RegisterCoalescer::mergeSubRangeInto(LiveInterval &LI,
void RegisterCoalescer::mergeSubRangeInto(LiveInterval &LI,
const LiveRange &ToMerge,
LaneBitmask LaneMask,
CoalescerPair &CP) {
@ -2553,8 +2552,7 @@ bool RegisterCoalescer::mergeSubRangeInto(LiveInterval &LI,
CommonRange = &R;
}
LiveRange RangeCopy(ToMerge, Allocator);
if (!joinSubRegRanges(*CommonRange, RangeCopy, Common, CP))
return false;
joinSubRegRanges(*CommonRange, RangeCopy, Common, CP);
LaneMask &= ~RMask;
}
@ -2562,7 +2560,6 @@ bool RegisterCoalescer::mergeSubRangeInto(LiveInterval &LI,
DEBUG(dbgs() << "\t\tNew Lane " << PrintLaneMask(LaneMask) << '\n');
LI.createSubRangeFrom(Allocator, LaneMask, ToMerge);
}
return true;
}
bool RegisterCoalescer::joinVirtRegs(CoalescerPair &CP) {
@ -2613,42 +2610,22 @@ bool RegisterCoalescer::joinVirtRegs(CoalescerPair &CP) {
// Determine lanemasks of RHS in the coalesced register and merge subranges.
unsigned SrcIdx = CP.getSrcIdx();
bool Abort = false;
if (!RHS.hasSubRanges()) {
LaneBitmask Mask = SrcIdx == 0 ? CP.getNewRC()->getLaneMask()
: TRI->getSubRegIndexLaneMask(SrcIdx);
if (!mergeSubRangeInto(LHS, RHS, Mask, CP))
Abort = true;
mergeSubRangeInto(LHS, RHS, Mask, CP);
} else {
// Pair up subranges and merge.
for (LiveInterval::SubRange &R : RHS.subranges()) {
LaneBitmask Mask = TRI->composeSubRegIndexLaneMask(SrcIdx, R.LaneMask);
if (!mergeSubRangeInto(LHS, R, Mask, CP)) {
Abort = true;
break;
mergeSubRangeInto(LHS, R, Mask, CP);
}
}
}
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(TargetRegisterInfo::isImpreciseLaneMask(
CP.getNewRC()->getLaneMask())
&& "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");
LHSVals.pruneSubRegValues(LHS, ShrinkMask);
RHSVals.pruneSubRegValues(LHS, ShrinkMask);
}
}
// The merging algorithm in LiveInterval::join() can't handle conflicting
// value mappings, so we need to remove any live ranges that overlap a

8
lib/CodeGen/VirtRegMap.cpp
View File

@ -400,14 +400,6 @@ void VirtRegRewriter::rewrite() {
MO.setIsUndef(true);
} else if (!MO.isDead()) {
assert(MO.isDef());
// Things get tricky when we ran out of lane mask bits and
// merged multiple lanes into the overflow bit: In this case
// our subregister liveness tracking isn't precise and we can't
// know what subregister parts are undefined, fall back to the
// implicit super-register def then.
LaneBitmask LaneMask = TRI->getSubRegIndexLaneMask(SubReg);
if (TargetRegisterInfo::isImpreciseLaneMask(LaneMask))
SuperDefs.push_back(PhysReg);
}
}

17
utils/TableGen/CodeGenRegisters.cpp
View File

@ -1171,20 +1171,13 @@ void CodeGenRegBank::computeSubRegLaneMasks() {
CoveringLanes = ~0u;
for (auto &Idx : SubRegIndices) {
if (Idx.getComposites().empty()) {
if (Bit > 32) {
PrintFatalError(
Twine("Ran out of lanemask bits to represent subregister ")
+ Idx.getName());
}
Idx.LaneMask = 1u << Bit;
// Share bit 31 in the unlikely case there are more than 32 leafs.
//
// Sharing bits is harmless; it allows graceful degradation in targets
// with more than 32 vector lanes. They simply get a limited resolution
// view of lanes beyond the 32nd.
//
// See also the comment for getSubRegIndexLaneMask().
if (Bit < 31)
++Bit;
else
// Once bit 31 is shared among multiple leafs, the 'lane' it represents
// is no longer covering its registers.
CoveringLanes &= ~(1u << Bit);
} else {
Idx.LaneMask = 0;
}