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Merge pull request #20156 from MathiasVP/value-numbering-for-noop-casts

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C++: Value numbering for casts that only modify specifiers
This commit is contained in:
Mathias Vorreiter Pedersen
2025-08-11 10:33:58 +02:00
committed by GitHub
parent 56aacb1e55 65b1b7f63e
commit ccfcd90f08
7 changed files with 121 additions and 1 deletions
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4
cpp/ql/lib/change-notes/2025-08-02-gvn.md Normal file
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@@ -0,0 +1,4 @@
---
category: minorAnalysis
---
* The global value numbering library (`semmle.code.cpp.valuenumbering.GlobalValueNumbering` and `semmle.code.cpp.ir.ValueNumbering`) has been improved so more expressions are assigned the same value number.

22
cpp/ql/lib/semmle/code/cpp/ir/implementation/aliased_ssa/gvn/internal/ValueNumberingInternal.qll
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@@ -43,6 +43,23 @@ newtype TValueNumber =
} or
TUniqueValueNumber(IRFunction irFunc, Instruction instr) { uniqueValueNumber(instr, irFunc) }
/**
* A `ConvertInstruction` which converts data of type `T` to data of type `U`
* where `T` and `U` only differ in specifiers. For example, if `T` is `int`
* and `U` is `const T` this is a conversion from a non-const integer to a
* const integer.
*
* Generally, the value number of a converted value is different from the value
* number of an unconverted value, but conversions which only modify specifiers
* leave the resulting value bitwise identical to the old value.
*/
class TypePreservingConvertInstruction extends ConvertInstruction {
TypePreservingConvertInstruction() {
pragma[only_bind_out](this.getResultType().getUnspecifiedType()) =
pragma[only_bind_out](this.getUnary().getResultType().getUnspecifiedType())
}
}
/**
* A `CopyInstruction` whose source operand's value is congruent to the definition of that source
* operand.
@@ -216,6 +233,7 @@ private predicate unaryValueNumber(
not instr instanceof InheritanceConversionInstruction and
not instr instanceof CopyInstruction and
not instr instanceof FieldAddressInstruction and
not instr instanceof TypePreservingConvertInstruction and
instr.getOpcode() = opcode and
tvalueNumber(instr.getUnary()) = operand
}
@@ -351,6 +369,10 @@ private TValueNumber nonUniqueValueNumber(Instruction instr) {
or
// The value number of a copy is just the value number of its source value.
result = tvalueNumber(instr.(CongruentCopyInstruction).getSourceValue())
or
// The value number of a type-preserving conversion is just the value
// number of the unconverted value.
result = tvalueNumber(instr.(TypePreservingConvertInstruction).getUnary())
)
)
}

22
cpp/ql/lib/semmle/code/cpp/ir/implementation/raw/gvn/internal/ValueNumberingInternal.qll
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@@ -43,6 +43,23 @@ newtype TValueNumber =
} or
TUniqueValueNumber(IRFunction irFunc, Instruction instr) { uniqueValueNumber(instr, irFunc) }
/**
* A `ConvertInstruction` which converts data of type `T` to data of type `U`
* where `T` and `U` only differ in specifiers. For example, if `T` is `int`
* and `U` is `const T` this is a conversion from a non-const integer to a
* const integer.
*
* Generally, the value number of a converted value is different from the value
* number of an unconverted value, but conversions which only modify specifiers
* leave the resulting value bitwise identical to the old value.
*/
class TypePreservingConvertInstruction extends ConvertInstruction {
TypePreservingConvertInstruction() {
pragma[only_bind_out](this.getResultType().getUnspecifiedType()) =
pragma[only_bind_out](this.getUnary().getResultType().getUnspecifiedType())
}
}
/**
* A `CopyInstruction` whose source operand's value is congruent to the definition of that source
* operand.
@@ -216,6 +233,7 @@ private predicate unaryValueNumber(
not instr instanceof InheritanceConversionInstruction and
not instr instanceof CopyInstruction and
not instr instanceof FieldAddressInstruction and
not instr instanceof TypePreservingConvertInstruction and
instr.getOpcode() = opcode and
tvalueNumber(instr.getUnary()) = operand
}
@@ -351,6 +369,10 @@ private TValueNumber nonUniqueValueNumber(Instruction instr) {
or
// The value number of a copy is just the value number of its source value.
result = tvalueNumber(instr.(CongruentCopyInstruction).getSourceValue())
or
// The value number of a type-preserving conversion is just the value
// number of the unconverted value.
result = tvalueNumber(instr.(TypePreservingConvertInstruction).getUnary())
)
)
}

22
cpp/ql/lib/semmle/code/cpp/ir/implementation/unaliased_ssa/gvn/internal/ValueNumberingInternal.qll
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@@ -43,6 +43,23 @@ newtype TValueNumber =
} or
TUniqueValueNumber(IRFunction irFunc, Instruction instr) { uniqueValueNumber(instr, irFunc) }
/**
* A `ConvertInstruction` which converts data of type `T` to data of type `U`
* where `T` and `U` only differ in specifiers. For example, if `T` is `int`
* and `U` is `const T` this is a conversion from a non-const integer to a
* const integer.
*
* Generally, the value number of a converted value is different from the value
* number of an unconverted value, but conversions which only modify specifiers
* leave the resulting value bitwise identical to the old value.
*/
class TypePreservingConvertInstruction extends ConvertInstruction {
TypePreservingConvertInstruction() {
pragma[only_bind_out](this.getResultType().getUnspecifiedType()) =
pragma[only_bind_out](this.getUnary().getResultType().getUnspecifiedType())
}
}
/**
* A `CopyInstruction` whose source operand's value is congruent to the definition of that source
* operand.
@@ -216,6 +233,7 @@ private predicate unaryValueNumber(
not instr instanceof InheritanceConversionInstruction and
not instr instanceof CopyInstruction and
not instr instanceof FieldAddressInstruction and
not instr instanceof TypePreservingConvertInstruction and
instr.getOpcode() = opcode and
tvalueNumber(instr.getUnary()) = operand
}
@@ -351,6 +369,10 @@ private TValueNumber nonUniqueValueNumber(Instruction instr) {
or
// The value number of a copy is just the value number of its source value.
result = tvalueNumber(instr.(CongruentCopyInstruction).getSourceValue())
or
// The value number of a type-preserving conversion is just the value
// number of the unconverted value.
result = tvalueNumber(instr.(TypePreservingConvertInstruction).getUnary())
)
)
}

40
cpp/ql/test/library-tests/valuenumbering/GlobalValueNumbering/ir_gvn.expected
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@@ -1145,3 +1145,43 @@ test.cpp:
# 152| v152_7(void) = ReturnVoid :
# 152| v152_8(void) = AliasedUse : ~m156_7
# 152| v152_9(void) = ExitFunction :
# 166| void test_constMemberFunction()
# 166| Block 0
# 166| v166_1(void) = EnterFunction :
# 166| m166_2(unknown) = AliasedDefinition :
# 166| valnum = unique
# 166| m166_3(unknown) = InitializeNonLocal :
# 166| valnum = unique
# 166| m166_4(unknown) = Chi : total:m166_2, partial:m166_3
# 166| valnum = unique
# 167| r167_1(glval<StructWithConstMemberFunction>) = VariableAddress[s] :
# 167| valnum = r167_1, r168_2, r169_1, r169_2
# 167| m167_2(StructWithConstMemberFunction) = Uninitialized[s] : &:r167_1
# 167| valnum = m167_2, m168_4, r168_3
# 167| m167_3(unknown) = Chi : total:m166_4, partial:m167_2
# 167| valnum = unique
# 168| r168_1(glval<StructWithConstMemberFunction>) = VariableAddress[s2] :
# 168| valnum = unique
# 168| r168_2(glval<StructWithConstMemberFunction>) = VariableAddress[s] :
# 168| valnum = r167_1, r168_2, r169_1, r169_2
# 168| r168_3(StructWithConstMemberFunction) = Load[s] : &:r168_2, m167_2
# 168| valnum = m167_2, m168_4, r168_3
# 168| m168_4(StructWithConstMemberFunction) = Store[s2] : &:r168_1, r168_3
# 168| valnum = m167_2, m168_4, r168_3
# 169| r169_1(glval<StructWithConstMemberFunction>) = VariableAddress[s] :
# 169| valnum = r167_1, r168_2, r169_1, r169_2
# 169| r169_2(glval<StructWithConstMemberFunction>) = Convert : r169_1
# 169| valnum = r167_1, r168_2, r169_1, r169_2
# 169| r169_3(glval<unknown>) = FunctionAddress[constMemberFunction] :
# 169| valnum = unique
# 169| v169_4(void) = Call[constMemberFunction] : func:r169_3, this:r169_2
# 169| m169_5(unknown) = ^CallSideEffect : ~m167_3
# 169| valnum = unique
# 169| m169_6(unknown) = Chi : total:m167_3, partial:m169_5
# 169| valnum = unique
# 169| v169_7(void) = ^IndirectReadSideEffect[-1] : &:r169_2, ~m169_6
# 170| v170_1(void) = NoOp :
# 166| v166_5(void) = ReturnVoid :
# 166| v166_6(void) = AliasedUse : ~m169_6
# 166| v166_7(void) = ExitFunction :

11
cpp/ql/test/library-tests/valuenumbering/GlobalValueNumbering/test.cpp
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@@ -156,4 +156,15 @@ void test_read_global_different(int n) {
global_a->y = n;
int d = global_a->x;
}
struct StructWithConstMemberFunction {
int x;
void constMemberFunction() const;
};
void test_constMemberFunction() {
StructWithConstMemberFunction s;
StructWithConstMemberFunction s2 = s;
s.constMemberFunction();
}

1
cpp/ql/test/query-tests/Likely Bugs/Memory Management/StrncpyFlippedArgs/StrncpyFlippedArgs.expected
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@@ -13,7 +13,6 @@
| test.cpp:49:2:49:9 | call to strcpy_s | Potentially unsafe call to strcpy_s; second argument should be size of destination. |
| test.cpp:62:3:62:9 | call to strncpy | Potentially unsafe call to strncpy; third argument should be size of destination. |
| test.cpp:65:3:65:9 | call to strncpy | Potentially unsafe call to strncpy; third argument should be size of destination. |
| test.cpp:70:2:70:8 | call to strncpy | Potentially unsafe call to strncpy; third argument should be size of destination. |
| test.cpp:81:3:81:9 | call to strncpy | Potentially unsafe call to strncpy; third argument should be size of destination. |
| test.cpp:84:3:84:9 | call to strncpy | Potentially unsafe call to strncpy; third argument should be size of destination. |
| test.cpp:105:2:105:10 | call to wcsxfrm_l | Potentially unsafe call to wcsxfrm_l; third argument should be size of destination. |