[coverage] Filter out singleton ranges that alias full ranges
Block coverage is based on a system of ranges that can either have both a start and end position, or only a start position (so-called singleton ranges). When formatting coverage information, singletons are expanded until the end of the immediate full parent range. E.g. in: {0, 10} // Full range. {5, -1} // Singleton range. the singleton range is expanded to {5, 10}. Singletons are produced mostly for continuation counters that track whether we execute past a specific language construct. Unfortunately, continuation counters can turn up in spots that confuse our post-processing. For example: if (true) { ... block1 ... } else { ... block2 ... } If block1 produces a continuation counter, it could end up with the same start position as the else-branch counter. Since we merge identical blocks, the else-branch could incorrectly end up with an execution count of one. We need to avoid merging such cases. A full range should always take precedence over a singleton range; a singleton range should never expand to completely fill a full range. An additional post-processing pass ensures this. Bug: v8:8237 Change-Id: Idb3ec7b2feddc0585313810b9c8be1e9f4ec64bf Reviewed-on: https://chromium-review.googlesource.com/c/1273095 Reviewed-by: Georg Neis <neis@chromium.org> Reviewed-by: Yang Guo <yangguo@chromium.org> Commit-Queue: Jakob Gruber <jgruber@chromium.org> Cr-Commit-Position: refs/heads/master@{#56531}
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@ -171,6 +171,12 @@ class CoverageBlockIterator final {
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return function_->blocks[read_index_ + 1];
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}
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CoverageBlock& GetPreviousBlock() {
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DCHECK(IsActive());
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DCHECK_GT(read_index_, 0);
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return function_->blocks[read_index_ - 1];
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}
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CoverageBlock& GetParent() {
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DCHECK(IsActive());
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return nesting_stack_.back();
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@ -325,6 +331,30 @@ void MergeNestedRanges(CoverageFunction* function) {
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}
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}
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void FilterAliasedSingletons(CoverageFunction* function) {
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CoverageBlockIterator iter(function);
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iter.Next(); // Advance once since we reference the previous block later.
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while (iter.Next()) {
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CoverageBlock& previous_block = iter.GetPreviousBlock();
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CoverageBlock& block = iter.GetBlock();
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bool is_singleton = block.end == kNoSourcePosition;
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bool aliases_start = block.start == previous_block.start;
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if (is_singleton && aliases_start) {
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// The previous block must have a full range since duplicate singletons
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// have already been merged.
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DCHECK_NE(previous_block.end, kNoSourcePosition);
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// Likewise, the next block must have another start position since
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// singletons are sorted to the end.
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DCHECK_IMPLIES(iter.HasNext(), iter.GetNextBlock().start != block.start);
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iter.DeleteBlock();
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}
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}
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}
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void FilterUncoveredRanges(CoverageFunction* function) {
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CoverageBlockIterator iter(function);
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@ -397,6 +427,15 @@ void CollectBlockCoverage(CoverageFunction* function, SharedFunctionInfo* info,
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// Remove duplicate singleton ranges, keeping the max count.
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MergeDuplicateSingletons(function);
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// Remove singleton ranges with the same start position as a full range and
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// throw away their counts.
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// Singleton ranges are only intended to split existing full ranges and should
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// never expand into a full range. Consider 'if (cond) { ... } else { ... }'
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// as a problematic example; if the then-block produces a continuation
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// singleton, it would incorrectly expand into the else range.
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// For more context, see https://crbug.com/v8/8237.
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FilterAliasedSingletons(function);
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// Rewrite all singletons (created e.g. by continuations and unconditional
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// control flow) to ranges.
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RewritePositionSingletonsToRanges(function);
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@ -847,7 +847,49 @@ Util.escape("foo.bar"); // 0400
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[{"start":0,"end":449,"count":1},
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{"start":64,"end":351,"count":1},
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{"start":112,"end":203,"count":0},
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{"start":303,"end":350,"count":0}]
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{"start":268,"end":350,"count":0}]
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);
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TestCoverage(
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"https://crbug.com/v8/8237",
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`
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!function() { // 0000
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if (true) // 0050
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while (false) return; else nop(); // 0100
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}(); // 0150
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!function() { // 0200
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if (true) l0: { break l0; } else // 0250
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if (nop()) { } // 0300
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}(); // 0350
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!function() { // 0400
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if (true) { if (false) { return; } // 0450
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} else if (nop()) { } }(); // 0500
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!function(){ // 0550
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if(true)while(false)return;else nop() // 0600
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}(); // 0650
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!function(){ // 0700
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if(true) l0:{break l0}else if (nop()){} // 0750
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}(); // 0800
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!function(){ // 0850
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if(true){if(false){return}}else // 0900
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if(nop()){} // 0950
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}(); // 1000
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`,
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[{"start":0,"end":1049,"count":1},
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{"start":1,"end":151,"count":1},
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{"start":118,"end":137,"count":0},
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{"start":201,"end":351,"count":1},
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{"start":277,"end":318,"count":0},
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{"start":401,"end":525,"count":1},
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{"start":475,"end":486,"count":0},
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{"start":503,"end":523,"count":0},
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{"start":551,"end":651,"count":1},
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{"start":622,"end":639,"count":0},
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{"start":701,"end":801,"count":1},
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{"start":773,"end":791,"count":0},
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{"start":851,"end":1001,"count":1},
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{"start":920,"end":928,"count":0},
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{"start":929,"end":965,"count":0}]
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);
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%DebugToggleBlockCoverage(false);
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