Java: Switch to built-in gcd.

aschackmull · aschackmull · commit 9530eb6cdb28 · 2019-01-08T10:07:51.000+01:00
diff --git a/java/ql/src/semmle/code/java/dataflow/ModulusAnalysis.qll b/java/ql/src/semmle/code/java/dataflow/ModulusAnalysis.qll
@@ -165,21 +165,6 @@ private predicate maxPhiInputRank(SsaPhiNode phi, int rix) {
   rix = max(int r | rankedPhiInput(phi, _, _, r))
 }
 
-private int gcdLim() { result = 128 }
-
-/**
- * Gets the greatest common divisor of `x` and `y`. This is restricted to small
- * inputs and the case when `x` and `y` are not relatively prime.
- */
-private int gcd(int x, int y) {
-  result != 1 and
-  result = x.abs() and
-  y = 0 and
-  x in [-gcdLim() .. gcdLim()]
-  or
-  result = gcd(y, x % y) and y != 0 and x in [-gcdLim() .. gcdLim()]
-}
-
 /**
  * Gets the remainder of `val` modulo `mod`.
  *
@@ -203,7 +188,7 @@ private predicate phiSelfModulus(
     edge.phiInput(phi, inp) and
     phibound.getSsa() = phi and
     ssaModulus(inp, edge, phibound, v, m) and
-    mod = gcd(m, v) and
+    mod = m.gcd(v) and
     mod != 1
   )
 }
@@ -233,14 +218,14 @@ private predicate phiModulusRankStep(SsaPhiNode phi, Bound b, int val, int mod,
       rankedPhiInput(phi, inp, edge, rix) and
       phiModulusRankStep(phi, b, v1, m1, rix - 1) and
       ssaModulus(inp, edge, b, v2, m2) and
-      mod = gcd(gcd(m1, m2), v1 - v2)
+      mod = m1.gcd(m2).gcd(v1 - v2)
     )
     or
     exists(int m2 |
       rankedPhiInput(phi, inp, edge, rix) and
       phiModulusRankStep(phi, b, v1, m1, rix - 1) and
       phiSelfModulus(phi, inp, edge, m2) and
-      mod = gcd(m1, m2)
+      mod = m1.gcd(m2)
     )
   )
 }
@@ -301,15 +286,15 @@ predicate exprModulus(Expr e, Bound b, int val, int mod) {
     cond = e and
     condExprBranchModulus(cond, true, b, v1, m1) and
     condExprBranchModulus(cond, false, b, v2, m2) and
-    mod = gcd(gcd(m1, m2), v1 - v2) and
+    mod = m1.gcd(m2).gcd(v1 - v2) and
     mod != 1 and
     val = remainder(v1, mod)
   )
   or
   exists(Bound b1, Bound b2, int v1, int v2, int m1, int m2 |
     addModulus(e, true, b1, v1, m1) and
     addModulus(e, false, b2, v2, m2) and
-    mod = gcd(m1, m2) and
+    mod = m1.gcd(m2) and
     mod != 1 and
     val = remainder(v1 + v2, mod)
   |
@@ -321,7 +306,7 @@ predicate exprModulus(Expr e, Bound b, int val, int mod) {
   exists(int v1, int v2, int m1, int m2 |
     subModulus(e, true, b, v1, m1) and
     subModulus(e, false, any(ZeroBound zb), v2, m2) and
-    mod = gcd(m1, m2) and
+    mod = m1.gcd(m2) and
     mod != 1 and
     val = remainder(v1 - v2, mod)
   )
diff --git a/java/ql/src/semmle/code/java/dataflow/RangeAnalysis.qll b/java/ql/src/semmle/code/java/dataflow/RangeAnalysis.qll
@@ -151,25 +151,6 @@ private predicate boundCondition(
   )
 }
 
-private predicate gcdInput(int x, int y) {
-  exists(ComparisonExpr comp, Bound b |
-    exprModulus(comp.getLesserOperand(), b, _, x) and
-    exprModulus(comp.getGreaterOperand(), b, _, y)
-  )
-  or
-  exists(int x0, int y0 |
-    gcdInput(x0, y0) and
-    x = y0 and
-    y = x0 % y0
-  )
-}
-
-private int gcd(int x, int y) {
-  result = x.abs() and y = 0 and gcdInput(x, y)
-  or
-  result = gcd(y, x % y) and y != 0 and gcdInput(x, y)
-}
-
 /**
  * Holds if `comp` is a comparison between `x` and `y` for which `y - x` has a
  * fixed value modulo some `mod > 1`, such that the comparison can be
@@ -187,7 +168,7 @@ private predicate modulusComparison(ComparisonExpr comp, boolean testIsTrue, int
     // thus `k - 1 < y - x` with `0 <= k - 1 < mod`.
     exprModulus(comp.getLesserOperand(), b, v1, mod1) and
     exprModulus(comp.getGreaterOperand(), b, v2, mod2) and
-    mod = gcd(mod1, mod2) and
+    mod = mod1.gcd(mod2) and
     mod != 1 and
     (testIsTrue = true or testIsTrue = false) and
     (
