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[LVI] Add some additional comments about caching and context instructions

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Philip Reames and I had a long conversation about this, mostly because it is
not obvious why the current logic is correct. Hopefully, these comments will
prevent such confusion in the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219882 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hal Finkel 2014-10-16 00:40:05 +00:00
parent cbfd91dc20
commit 61c65b2884
1 changed files with 13 additions and 0 deletions

13
lib/Analysis/LazyValueInfo.cpp

@ -692,6 +692,9 @@ bool LazyValueInfoCache::solveBlockValuePHINode(LVILatticeVal &BBLV,
BasicBlock *PhiBB = PN->getIncomingBlock(i);
Value *PhiVal = PN->getIncomingValue(i);
LVILatticeVal EdgeResult;
// Note that we can provide PN as the context value to getEdgeValue, even
// though the results will be cached, because PN is the value being used as
// the cache key in the caller.
EdgesMissing |= !getEdgeValue(PhiVal, PhiBB, BB, EdgeResult, PN);
if (EdgesMissing)
continue;
@ -955,6 +958,8 @@ bool LazyValueInfoCache::getEdgeValue(Value *Val, BasicBlock *BBFrom,
// Try to intersect ranges of the BB and the constraint on the edge.
LVILatticeVal InBlock = getBlockValue(Val, BBFrom);
mergeAssumeBlockValueConstantRange(Val, InBlock, BBFrom->getTerminator());
// See note on the use of the CxtI with mergeAssumeBlockValueConstantRange,
// and caching, below.
mergeAssumeBlockValueConstantRange(Val, InBlock, CxtI);
if (!InBlock.isConstantRange())
return true;
@ -973,6 +978,14 @@ bool LazyValueInfoCache::getEdgeValue(Value *Val, BasicBlock *BBFrom,
// if we couldn't compute the value on the edge, use the value from the BB
Result = getBlockValue(Val, BBFrom);
mergeAssumeBlockValueConstantRange(Val, Result, BBFrom->getTerminator());
// We can use the context instruction (generically the ultimate instruction
// the calling pass is trying to simplify) here, even though the result of
// this function is generally cached when called from the solve* functions
// (and that cached result might be used with queries using a different
// context instruction), because when this function is called from the solve*
// functions, the context instruction is not provided. When called from
// LazyValueInfoCache::getValueOnEdge, the context instruction is provided,
// but then the result is not cached.
mergeAssumeBlockValueConstantRange(Val, Result, CxtI);
return true;
}