C++: Value numbering for casts that only modify specifiers

[MathiasVP]

Consider the following snippet:

struct S {
  void foo() const;
  void bar();
};

void test() {
  S s;
  s.foo();
  s.bar();
}

Because foo is a const member function there will be a GlvalueConversion which converts the qualifier from a glval<S> to a glval<const S> before it's passed to foo. This conversion will generate a Convert instruction.

However, since bar isn't a const member function there won't be a Convert instruction, and so global value numbering (GVN) concludes that the two qualifiers won't necessarily evaluate to the same value at runtime (although, they obviously will).

This PR modifies GVN so that conversions that only modify specifiers receive the same global value number.

Commit-by-commit review recommended.

There was 1 query test change (see 851c498) which fixes a false positive. I went digging a bit into when that FP was introduced, and it turns out this was all the way back in #2851 when we put the IR into production (i.e., when we replaced the AST-based GVN with the IR-based GVN)!

DCA was uneventful.

[Copilot]

Pull Request Overview

This PR improves global value numbering (GVN) for C++ by treating conversions that only modify type specifiers (like const qualifiers) as preserving the same value number. This fixes false positives in static analysis by recognizing that s.foo() and s.bar() operate on the same underlying object value, even when foo requires a const conversion.

• Introduces TypePreservingConvertInstruction class to identify conversions that only change specifiers
• Updates GVN logic to assign the same value number to type-preserving conversions as their operands
• Adds test coverage for the new functionality

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
StrncpyFlippedArgs.expected	Removes a false positive test result that is now correctly handled
test.cpp	Adds test case for const member function calls to verify GVN behavior
ir_gvn.expected	Updates expected test output showing improved value numbering
ValueNumberingInternal.qll (3 files)	Implements the core logic for type-preserving conversion handling
2025-08-02-gvn.md	Documents the improvement in change notes

[jketema]

LGTM