diff --git a/cpp/ql/lib/semmle/code/cpp/controlflow/IRGuards.qll b/cpp/ql/lib/semmle/code/cpp/controlflow/IRGuards.qll
index f06db98e91b7..59769d26c336 100644
--- a/cpp/ql/lib/semmle/code/cpp/controlflow/IRGuards.qll
+++ b/cpp/ql/lib/semmle/code/cpp/controlflow/IRGuards.qll
@@ -375,6 +375,33 @@ cached
 class IRGuardCondition extends Instruction {
   Instruction branch;
 
+  /*
+   * An `IRGuardCondition` supports reasoning about four different kinds of
+   * relations:
+   * 1. A unary equality relation of the form `e == k`
+   * 2. A binary equality relation of the form `e1 == e2 + k`
+   * 3. A unary inequality relation of the form `e < k`
+   * 4. A binary inequality relation of the form `e1 < e2 + k`
+   *
+   * where `k` is a constant.
+   *
+   * Furthermore, the unary relations (i.e., case 1 and case 3) are also
+   * inferred from `switch` statement guards: equality relations are inferred
+   * from the unique `case` statement, if any, and inequality relations are
+   * inferred from the [case range](https://gcc.gnu.org/onlinedocs/gcc/Case-Ranges.html)
+   * gcc extension.
+   *
+   * The implementation of all four follows the same structure: Each relation
+   * has a cached user-facing predicate that. For example,
+   * `GuardCondition::comparesEq` calls `compares_eq`. This predicate has
+   * several cases that recursively decompose the relation to bring it to a
+   * canonical form (i.e., a relation of the form `e1 == e2 + k`). The base
+   * case for this relation (i.e., `simple_comparison_eq`) handles
+   * `CompareEQInstruction`s and `CompareNEInstruction`, and recursive
+   * predicates (e.g., `complex_eq`) rewrites larger expressions such as
+   * `e1 + k1 == e2 + k2` into canonical the form `e1 == e2 + (k2 - k1)`.
+   */
+
   cached
   IRGuardCondition() { branch = getBranchForCondition(this) }
 
@@ -776,7 +803,9 @@ private predicate unary_compares_eq(
   Instruction test, Operand op, int k, boolean areEqual, boolean inNonZeroCase, AbstractValue value
 ) {
   /* The simple case where the test *is* the comparison so areEqual = testIsTrue xor eq. */
-  exists(AbstractValue v | unary_simple_comparison_eq(test, op, k, inNonZeroCase, v) |
+  exists(AbstractValue v |
+    unary_simple_comparison_eq(test, k, inNonZeroCase, v) and op.getDef() = test
+  |
     areEqual = true and value = v
     or
     areEqual = false and value = v.getDualValue()
@@ -821,45 +850,55 @@ private predicate simple_comparison_eq(
   value.(BooleanValue).getValue() = false
 }
 
-/**
- * Holds if `test` is an instruction that is part of test that eventually is
- * used in a conditional branch.
- */
-private predicate relevantUnaryComparison(Instruction test) {
-  not test instanceof CompareInstruction and
-  exists(IRType type, ConditionalBranchInstruction branch |
-    type instanceof IRAddressType or type instanceof IRIntegerType
-  |
-    type = test.getResultIRType() and
-    branch.getCondition() = test
-  )
-  or
-  exists(LogicalNotInstruction logicalNot |
-    relevantUnaryComparison(logicalNot) and
-    test = logicalNot.getUnary()
-  )
-}
-
 /**
  * Rearrange various simple comparisons into `op == k` form.
  */
 private predicate unary_simple_comparison_eq(
-  Instruction test, Operand op, int k, boolean inNonZeroCase, AbstractValue value
+  Instruction test, int k, boolean inNonZeroCase, AbstractValue value
 ) {
   exists(SwitchInstruction switch, CaseEdge case |
     test = switch.getExpression() and
-    op.getDef() = test and
     case = value.(MatchValue).getCase() and
     exists(switch.getSuccessor(case)) and
     case.getValue().toInt() = k and
     inNonZeroCase = false
   )
   or
-  // There's no implicit CompareInstruction in files compiled as C since C
-  // doesn't have implicit boolean conversions. So instead we check whether
-  // there's a branch on a value of pointer or integer type.
-  relevantUnaryComparison(test) and
-  op.getDef() = test and
+  // Any instruction with an integral type could potentially be part of a
+  // check for nullness when used in a guard. So we include all integral
+  // typed instructions here. However, since some of these instructions are
+  // already included as guards in other cases, we exclude those here.
+  // These are instructions that compute a binary equality or inequality
+  // relation. For example, the following:
+  // ```cpp
+  // if(a == b + 42) { ... }
+  // ```
+  // generates the following IR:
+  // ```
+  // r1(glval<int>) = VariableAddress[a]     :
+  // r2(int)        = Load[a]                : &:r1, m1
+  // r3(glval<int>) = VariableAddress[b]     :
+  // r4(int)        = Load[b]                : &:r3, m2
+  // r5(int)        = Constant[42]           :
+  // r6(int)        = Add                    : r4, r5
+  // r7(bool)       = CompareEQ              : r2, r6
+  // v1(void)       = ConditionalBranch      : r7
+  // ```
+  // and since `r7` is an integral typed instruction this predicate could
+  // include a case for when `r7` evaluates to true (in which case we would
+  // infer that `r6` was non-zero, and a case for when `r7` evaluates to false
+  // (in which case we would infer that `r6` was zero).
+  // However, since `a == b + 42` is already supported when reasoning about
+  // binary equalities we exclude those cases here.
+  not test.isGLValue() and
+  not simple_comparison_eq(test, _, _, _, _) and
+  not simple_comparison_lt(test, _, _, _) and
+  not test = any(SwitchInstruction switch).getExpression() and
+  (
+    test.getResultIRType() instanceof IRAddressType or
+    test.getResultIRType() instanceof IRIntegerType or
+    test.getResultIRType() instanceof IRBooleanType
+  ) and
   (
     k = 1 and
     value.(BooleanValue).getValue() = true and
@@ -913,7 +952,8 @@ private predicate compares_lt(
 
 /** Holds if `op < k` evaluates to `isLt` given that `test` evaluates to `value`. */
 private predicate compares_lt(Instruction test, Operand op, int k, boolean isLt, AbstractValue value) {
-  simple_comparison_lt(test, op, k, isLt, value)
+  unary_simple_comparison_lt(test, k, isLt, value) and
+  op.getDef() = test
   or
   complex_lt(test, op, k, isLt, value)
   or
@@ -960,12 +1000,11 @@ private predicate simple_comparison_lt(CompareInstruction cmp, Operand left, Ope
 }
 
 /** Rearrange various simple comparisons into `op < k` form. */
-private predicate simple_comparison_lt(
-  Instruction test, Operand op, int k, boolean isLt, AbstractValue value
+private predicate unary_simple_comparison_lt(
+  Instruction test, int k, boolean isLt, AbstractValue value
 ) {
   exists(SwitchInstruction switch, CaseEdge case |
     test = switch.getExpression() and
-    op.getDef() = test and
     case = value.(MatchValue).getCase() and
     exists(switch.getSuccessor(case)) and
     case.getMaxValue() > case.getMinValue()
diff --git a/cpp/ql/test/library-tests/controlflow/guards-ir/tests.expected b/cpp/ql/test/library-tests/controlflow/guards-ir/tests.expected
index 6cf7d2b6a958..d097fa7dfa67 100644
--- a/cpp/ql/test/library-tests/controlflow/guards-ir/tests.expected
+++ b/cpp/ql/test/library-tests/controlflow/guards-ir/tests.expected
@@ -74,6 +74,8 @@ astGuardsCompare
 | 34 | j >= 10+0 when ... < ... is false |
 | 42 | 10 < j+1 when ... < ... is false |
 | 42 | 10 >= j+1 when ... < ... is true |
+| 42 | call to getABool != 0 when call to getABool is true |
+| 42 | call to getABool == 0 when call to getABool is false |
 | 42 | j < 10+0 when ... < ... is true |
 | 42 | j >= 10+0 when ... < ... is false |
 | 44 | 0 < z+0 when ... > ... is true |
@@ -537,6 +539,8 @@ astGuardsEnsure_const
 | test.cpp:31:7:31:13 | ... == ... | test.cpp:31:7:31:7 | x | != | -1 | 34 | 34 |
 | test.cpp:31:7:31:13 | ... == ... | test.cpp:31:7:31:7 | x | == | -1 | 30 | 30 |
 | test.cpp:31:7:31:13 | ... == ... | test.cpp:31:7:31:7 | x | == | -1 | 31 | 32 |
+| test.cpp:42:13:42:20 | call to getABool | test.cpp:42:13:42:20 | call to getABool | != | 0 | 43 | 45 |
+| test.cpp:42:13:42:20 | call to getABool | test.cpp:42:13:42:20 | call to getABool | == | 0 | 53 | 53 |
 irGuards
 | test.c:7:9:7:13 | CompareGT: ... > ... |
 | test.c:17:8:17:12 | CompareLT: ... < ... |
@@ -613,6 +617,8 @@ irGuardsCompare
 | 34 | j >= 10+0 when CompareLT: ... < ... is false |
 | 42 | 10 < j+1 when CompareLT: ... < ... is false |
 | 42 | 10 >= j+1 when CompareLT: ... < ... is true |
+| 42 | call to getABool != 0 when Call: call to getABool is true |
+| 42 | call to getABool == 0 when Call: call to getABool is false |
 | 42 | j < 10 when CompareLT: ... < ... is true |
 | 42 | j < 10+0 when CompareLT: ... < ... is true |
 | 42 | j >= 10 when CompareLT: ... < ... is false |
@@ -1081,3 +1087,5 @@ irGuardsEnsure_const
 | test.cpp:31:7:31:13 | CompareEQ: ... == ... | test.cpp:31:7:31:7 | Load: x | != | -1 | 34 | 34 |
 | test.cpp:31:7:31:13 | CompareEQ: ... == ... | test.cpp:31:7:31:7 | Load: x | == | -1 | 30 | 30 |
 | test.cpp:31:7:31:13 | CompareEQ: ... == ... | test.cpp:31:7:31:7 | Load: x | == | -1 | 32 | 32 |
+| test.cpp:42:13:42:20 | Call: call to getABool | test.cpp:42:13:42:20 | Call: call to getABool | != | 0 | 44 | 44 |
+| test.cpp:42:13:42:20 | Call: call to getABool | test.cpp:42:13:42:20 | Call: call to getABool | == | 0 | 53 | 53 |
diff --git a/cpp/ql/test/library-tests/controlflow/guards/Guards.expected b/cpp/ql/test/library-tests/controlflow/guards/Guards.expected
index 13d6c2b654ff..77217c663879 100644
--- a/cpp/ql/test/library-tests/controlflow/guards/Guards.expected
+++ b/cpp/ql/test/library-tests/controlflow/guards/Guards.expected
@@ -42,3 +42,6 @@
 | test.cpp:99:6:99:6 | f |
 | test.cpp:105:6:105:14 | ... != ... |
 | test.cpp:111:6:111:14 | ... != ... |
+| test.cpp:122:9:122:9 | b |
+| test.cpp:125:13:125:20 | ! ... |
+| test.cpp:125:14:125:17 | call to safe |
diff --git a/cpp/ql/test/library-tests/controlflow/guards/GuardsCompare.expected b/cpp/ql/test/library-tests/controlflow/guards/GuardsCompare.expected
index 3d32ada5f301..ee33674c4e13 100644
--- a/cpp/ql/test/library-tests/controlflow/guards/GuardsCompare.expected
+++ b/cpp/ql/test/library-tests/controlflow/guards/GuardsCompare.expected
@@ -44,6 +44,8 @@
 | 34 | j >= 10+0 when ... < ... is false |
 | 42 | 10 < j+1 when ... < ... is false |
 | 42 | 10 >= j+1 when ... < ... is true |
+| 42 | call to getABool != 0 when call to getABool is true |
+| 42 | call to getABool == 0 when call to getABool is false |
 | 42 | j < 10 when ... < ... is true |
 | 42 | j < 10+0 when ... < ... is true |
 | 42 | j >= 10 when ... < ... is false |
@@ -149,6 +151,13 @@
 | 111 | 0.0 == i+0 when ... != ... is false |
 | 111 | i != 0.0+0 when ... != ... is true |
 | 111 | i == 0.0+0 when ... != ... is false |
+| 122 | b != 0 when b is true |
+| 122 | b == 0 when b is false |
+| 125 | ! ... != 0 when ! ... is true |
+| 125 | ! ... == 0 when ! ... is false |
+| 125 | call to safe != 0 when ! ... is false |
+| 125 | call to safe != 0 when call to safe is true |
+| 125 | call to safe == 0 when call to safe is false |
 | 126 | 1 != 0 when 1 is true |
 | 126 | 1 != 0 when ... && ... is true |
 | 126 | 1 == 0 when 1 is false |
diff --git a/cpp/ql/test/library-tests/controlflow/guards/GuardsControl.expected b/cpp/ql/test/library-tests/controlflow/guards/GuardsControl.expected
index cf36a58a515b..25dca78986a5 100644
--- a/cpp/ql/test/library-tests/controlflow/guards/GuardsControl.expected
+++ b/cpp/ql/test/library-tests/controlflow/guards/GuardsControl.expected
@@ -100,3 +100,7 @@
 | test.cpp:99:6:99:6 | f | true | 99 | 100 |
 | test.cpp:105:6:105:14 | ... != ... | true | 105 | 106 |
 | test.cpp:111:6:111:14 | ... != ... | true | 111 | 112 |
+| test.cpp:122:9:122:9 | b | true | 123 | 125 |
+| test.cpp:122:9:122:9 | b | true | 125 | 125 |
+| test.cpp:125:13:125:20 | ! ... | true | 125 | 125 |
+| test.cpp:125:14:125:17 | call to safe | false | 125 | 125 |
diff --git a/cpp/ql/test/library-tests/controlflow/guards/GuardsEnsure.expected b/cpp/ql/test/library-tests/controlflow/guards/GuardsEnsure.expected
index 84c416445aef..aa004e549801 100644
--- a/cpp/ql/test/library-tests/controlflow/guards/GuardsEnsure.expected
+++ b/cpp/ql/test/library-tests/controlflow/guards/GuardsEnsure.expected
@@ -257,6 +257,8 @@ unary
 | test.cpp:31:7:31:13 | ... == ... | test.cpp:31:7:31:7 | x | != | -1 | 34 | 34 |
 | test.cpp:31:7:31:13 | ... == ... | test.cpp:31:7:31:7 | x | == | -1 | 30 | 30 |
 | test.cpp:31:7:31:13 | ... == ... | test.cpp:31:7:31:7 | x | == | -1 | 31 | 32 |
+| test.cpp:42:13:42:20 | call to getABool | test.cpp:42:13:42:20 | call to getABool | != | 0 | 43 | 45 |
+| test.cpp:42:13:42:20 | call to getABool | test.cpp:42:13:42:20 | call to getABool | == | 0 | 53 | 53 |
 | test.cpp:61:10:61:10 | i | test.cpp:61:10:61:10 | i | == | 0 | 62 | 64 |
 | test.cpp:61:10:61:10 | i | test.cpp:61:10:61:10 | i | == | 1 | 65 | 66 |
 | test.cpp:74:10:74:10 | i | test.cpp:74:10:74:10 | i | < | 11 | 75 | 77 |
@@ -264,3 +266,7 @@ unary
 | test.cpp:74:10:74:10 | i | test.cpp:74:10:74:10 | i | >= | 0 | 75 | 77 |
 | test.cpp:74:10:74:10 | i | test.cpp:74:10:74:10 | i | >= | 11 | 78 | 79 |
 | test.cpp:93:6:93:6 | c | test.cpp:93:6:93:6 | c | != | 0 | 93 | 94 |
+| test.cpp:122:9:122:9 | b | test.cpp:122:9:122:9 | b | != | 0 | 123 | 125 |
+| test.cpp:122:9:122:9 | b | test.cpp:122:9:122:9 | b | != | 0 | 125 | 125 |
+| test.cpp:125:13:125:20 | ! ... | test.cpp:125:13:125:20 | ! ... | != | 0 | 125 | 125 |
+| test.cpp:125:14:125:17 | call to safe | test.cpp:125:14:125:17 | call to safe | == | 0 | 125 | 125 |
diff --git a/cpp/ql/test/library-tests/controlflow/guards/test.cpp b/cpp/ql/test/library-tests/controlflow/guards/test.cpp
index 84d02ca4efa5..eeab2f110638 100644
--- a/cpp/ql/test/library-tests/controlflow/guards/test.cpp
+++ b/cpp/ql/test/library-tests/controlflow/guards/test.cpp
@@ -112,3 +112,17 @@ void int_float_comparison(int i) {
     use(i);
   }
 }
+
+int source();
+bool safe(int);
+
+void test(bool b)
+{
+    int x;
+    if (b)
+    {
+        x = source();
+        if (!safe(x)) return;
+    }
+    use(x);
+}
\ No newline at end of file
diff --git a/cpp/ql/test/library-tests/dataflow/dataflow-tests/BarrierGuard.cpp b/cpp/ql/test/library-tests/dataflow/dataflow-tests/BarrierGuard.cpp
index 6a150af4e7b8..71d22ad4eddc 100644
--- a/cpp/ql/test/library-tests/dataflow/dataflow-tests/BarrierGuard.cpp
+++ b/cpp/ql/test/library-tests/dataflow/dataflow-tests/BarrierGuard.cpp
@@ -83,7 +83,7 @@ void test_guard_and_reassign() {
   if(!guarded(x)) {
     x = 0;
   }
-  sink(x); // $ SPURIOUS: ast,ir
+  sink(x); // $ SPURIOUS: ast
 }
 
 void test_phi_read_guard(bool b) {
@@ -98,7 +98,7 @@ void test_phi_read_guard(bool b) {
       return;
   }
   
-  sink(x); // $ SPURIOUS: ast,ir
+  sink(x); // $ SPURIOUS: ast
 }
 
 bool unsafe(int);
diff --git a/cpp/ql/test/library-tests/dataflow/dataflow-tests/TestBase.qll b/cpp/ql/test/library-tests/dataflow/dataflow-tests/TestBase.qll
index 71db02642878..426098084ca4 100644
--- a/cpp/ql/test/library-tests/dataflow/dataflow-tests/TestBase.qll
+++ b/cpp/ql/test/library-tests/dataflow/dataflow-tests/TestBase.qll
@@ -7,14 +7,17 @@ module AstTest {
    * S in `if (guarded(x)) S`.
    */
   // This is tested in `BarrierGuard.cpp`.
-  predicate testBarrierGuard(GuardCondition g, Expr checked, boolean isTrue) {
-    g.(FunctionCall).getTarget().getName() = "guarded" and
-    checked = g.(FunctionCall).getArgument(0) and
-    isTrue = true
-    or
-    g.(FunctionCall).getTarget().getName() = "unsafe" and
-    checked = g.(FunctionCall).getArgument(0) and
-    isTrue = false
+  predicate testBarrierGuard(GuardCondition g, Expr checked, boolean branch) {
+    exists(Call call, boolean b |
+      checked = call.getArgument(0) and
+      g.comparesEq(call, 0, b, any(BooleanValue bv | bv.getValue() = branch))
+    |
+      call.getTarget().hasName("guarded") and
+      b = false
+      or
+      call.getTarget().hasName("unsafe") and
+      b = true
+    )
   }
 
   /** Common data flow configuration to be used by tests. */
@@ -106,16 +109,16 @@ module IRTest {
    * S in `if (guarded(x)) S`.
    */
   // This is tested in `BarrierGuard.cpp`.
-  predicate testBarrierGuard(IRGuardCondition g, Expr checked, boolean isTrue) {
-    exists(Call call |
-      call = g.getUnconvertedResultExpression() and
-      checked = call.getArgument(0)
+  predicate testBarrierGuard(IRGuardCondition g, Expr checked, boolean branch) {
+    exists(CallInstruction call, boolean b |
+      checked = call.getArgument(0).getUnconvertedResultExpression() and
+      g.comparesEq(call.getAUse(), 0, b, any(BooleanValue bv | bv.getValue() = branch))
     |
-      call.getTarget().hasName("guarded") and
-      isTrue = true
+      call.getStaticCallTarget().hasName("guarded") and
+      b = false
       or
-      call.getTarget().hasName("unsafe") and
-      isTrue = false
+      call.getStaticCallTarget().hasName("unsafe") and
+      b = true
     )
   }
 
diff --git a/cpp/ql/test/library-tests/dataflow/dataflow-tests/test-source-sink.expected b/cpp/ql/test/library-tests/dataflow/dataflow-tests/test-source-sink.expected
index a136e94e8e94..42144e098148 100644
--- a/cpp/ql/test/library-tests/dataflow/dataflow-tests/test-source-sink.expected
+++ b/cpp/ql/test/library-tests/dataflow/dataflow-tests/test-source-sink.expected
@@ -145,8 +145,6 @@ irFlow
 | BarrierGuard.cpp:49:10:49:15 | call to source | BarrierGuard.cpp:55:13:55:13 | x |
 | BarrierGuard.cpp:60:11:60:16 | call to source | BarrierGuard.cpp:64:14:64:14 | x |
 | BarrierGuard.cpp:60:11:60:16 | call to source | BarrierGuard.cpp:66:14:66:14 | x |
-| BarrierGuard.cpp:81:11:81:16 | call to source | BarrierGuard.cpp:86:8:86:8 | x |
-| BarrierGuard.cpp:90:11:90:16 | call to source | BarrierGuard.cpp:101:8:101:8 | x |
 | acrossLinkTargets.cpp:19:27:19:32 | call to source | acrossLinkTargets.cpp:12:8:12:8 | x |
 | clang.cpp:12:9:12:20 | sourceArray1 | clang.cpp:18:8:18:19 | sourceArray1 |
 | clang.cpp:12:9:12:20 | sourceArray1 | clang.cpp:23:17:23:29 | *& ... |