diff options
Diffstat (limited to 'llvm/lib/Analysis')
| -rw-r--r-- | llvm/lib/Analysis/LazyValueInfo.cpp | 19 |
1 files changed, 17 insertions, 2 deletions
diff --git a/llvm/lib/Analysis/LazyValueInfo.cpp b/llvm/lib/Analysis/LazyValueInfo.cpp index 9f919f7644a..281ff89bfe4 100644 --- a/llvm/lib/Analysis/LazyValueInfo.cpp +++ b/llvm/lib/Analysis/LazyValueInfo.cpp @@ -500,8 +500,23 @@ bool LazyValueInfoCache::solveBlockValue(Value *Val, BasicBlock *BB) { // cache needs updating, i.e. if we have solve a new value or not. OverDefinedCacheUpdater ODCacheUpdater(Val, BB, BBLV, this); - // If we've already computed this block's value, return it. - if (!BBLV.isUndefined()) { + // Once this BB is encountered, Val's value for this BB will not be Undefined + // any longer. When we encounter this BB again, if Val's value is Overdefined, + // we need to compute its value again. + // + // For example, considering this control flow, + // BB1->BB2, BB1->BB3, BB2->BB3, BB2->BB4 + // + // Suppose we have "icmp slt %v, 0" in BB1, and "icmp sgt %v, 0" in BB3. At + // the very beginning, when analyzing edge BB2->BB3, we don't know %v's value + // in BB2, and the data flow algorithm tries to compute BB2's predecessors, so + // then we know %v has negative value on edge BB1->BB2. And then we return to + // check BB2 again, and at this moment BB2 has Overdefined value for %v in + // BB2. So we should have to follow data flow propagation algorithm to get the + // value on edge BB1->BB2 propagated to BB2, and finally %v on BB2 has a + // constant range describing a negative value. + + if (!BBLV.isUndefined() && !BBLV.isOverdefined()) { DEBUG(dbgs() << " reuse BB '" << BB->getName() << "' val=" << BBLV <<'\n'); // Since we're reusing a cached value here, we don't need to update the |
