Gcc16 or 17: when is the pointed type relevant for emitted assembler code?
Basile asks if using `void*` and explicit casts affects assembly code generation in GCC, sparking a discussion on type-based alias analysis.
Basile Starynkevitch asks whether using void* for all pointers and casting them at each dereference impacts the generated assembly code in GCC 16 or 17. Richard Biener responds that using specific pointer types enables type-based alias analysis (TBAA), which helps the compiler disambiguate memory accesses. When all pointers are void*, this optimization is not possible.
- proposer
Asks whether using only `void*` pointers with explicit casts affects assembly code generation.
“If every generated pointer was void* and at every dereferencing occurrence explicitly casted to the actual data pointer, does that make any difference in generated assembler code?”
- reviewer
Explains that different pointer types enable type-based alias analysis (TBAA) for memory access disambiguation, which is not possible with `void*`.
“when *p1 and *p2 have different pointer types we can disambiguate the two memory accesses. If they are both void *, we can't.”
Technical Tradeoffs
- Using void* can reduce type safety and potentially hinder compiler optimizations like type-based alias analysis.
- Using specific pointer types enables more aggressive optimizations but requires careful type management.
In Details
Type-based alias analysis (TBAA) is a compiler optimization technique that uses type information to determine whether two pointers can point to the same memory location. This enables the compiler to make assumptions about memory accesses and perform optimizations such as reordering or eliminating loads and stores. This is related to the -fstrict-aliasing flag.
For Context
In C and C++, pointers are variables that store the memory address of other variables. Type-based alias analysis is a compiler optimization that analyzes pointer types to determine if two pointers might point to the same memory location (aliasing). If the compiler can determine that two pointers cannot alias, it can perform optimizations to improve performance. Using void* disables this optimization.