C++: Improve join orders with QL CFG

jbj · jbj · commit eed24f193313 · 2019-10-01T14:33:28.000+02:00
Size estimates are slightly different when we enable the QL CFG, and
this caused bad join orders in these predicates.
diff --git a/cpp/ql/src/semmle/code/cpp/dataflow/internal/FlowVar.qll b/cpp/ql/src/semmle/code/cpp/dataflow/internal/FlowVar.qll
@@ -490,7 +490,7 @@ module FlowVar_internal {
       exists(VariableAccess va |
         va.getTarget() = result and
         readAccess(va) and
-        bbNotInLoop(va.getBasicBlock())
+        exists(BasicBlock bb | bb = va.getBasicBlock() | not this.bbInLoop(bb))
       )
     }
 
@@ -679,10 +679,11 @@ module FlowVar_internal {
   predicate dominatedByOverwrite(UninitializedLocalVariable v, VariableAccess va) {
     exists(BasicBlock bb, int vaIndex |
       va = bb.getNode(vaIndex) and
-      va.getTarget() = v
-    |
+      va.getTarget() = v and
       vaIndex > indexOfFirstOverwriteInBB(v, bb)
       or
+      va = bb.getNode(vaIndex) and
+      va.getTarget() = v and
       bbStrictlyDominates(getAnOverwritingBB(v), bb)
     )
   }

Original file line number	Diff line number	Diff line change
`@@ -490,7 +490,7 @@ module FlowVar_internal {`
`490`	`490`	`exists(VariableAccess va \|`
`491`	`491`	`va.getTarget() = result and`
`492`	`492`	`readAccess(va) and`
`493`		`- bbNotInLoop(va.getBasicBlock())`
	`493`	`+ exists(BasicBlock bb \| bb = va.getBasicBlock() \| not this.bbInLoop(bb))`
`494`	`494`	`)`
`495`	`495`	`}`
`496`	`496`
`@@ -679,10 +679,11 @@ module FlowVar_internal {`
`679`	`679`	`predicate dominatedByOverwrite(UninitializedLocalVariable v, VariableAccess va) {`
`680`	`680`	`exists(BasicBlock bb, int vaIndex \|`
`681`	`681`	`va = bb.getNode(vaIndex) and`
`682`		`- va.getTarget() = v`
`683`		`- \|`
	`682`	`+ va.getTarget() = v and`
`684`	`683`	`vaIndex > indexOfFirstOverwriteInBB(v, bb)`
`685`	`684`	`or`
	`685`	`+ va = bb.getNode(vaIndex) and`
	`686`	`+ va.getTarget() = v and`
`686`	`687`	`bbStrictlyDominates(getAnOverwritingBB(v), bb)`
`687`	`688`	`)`
`688`	`689`	`}`