[throwaway894345]

I’ve been wondering about (1) recently—is there a way to memset the entire stack frame at the start of a function such that memcmp works as expected? Also, what are the performance implications of comparing a padded struct member-by-member vs a single big memcmp? Is member-by-member faster because you’re comparing less in total, or is memcmp faster because it’s one big contiguous compare? Or is it more complicated?

Regarding (2), sizeof(*x) doesn’t actually dereference x, right? The dereference isn’t evaluated—it’s all calculated at compile time, right?

[DSMan195276]

> is there a way to memset the entire stack frame at the start of a function such that memcmp works as expected?

I'd argue no simply because the value of the padding bytes is always unspecified. A compiler that sees such a `memset` is (IMO) perfectly free to not zero known padding bytes since it knows their value should not matter to the program. Compilers might not currently do that but you can already see this kind of behavior in other situations - C compilers will happily throw out `memset` calls if it knows the result won't be used.

But also beyond that, it probably doesn't matter anyway because there's no way to _use_ the `struct` which won't leave the padding bytes with unspecified values. `memset` might reliably zero the padding bytes for you, but writing to the struct will randomly screw up the padding bytes depending on what the compiler feels is the best way to do things, so then you're back at `memcmp` no longer working. The only real way to make `memcmp` work is to ensure you have no padding bytes to begin with.

[saagarjha]

Compiles are free to not zero padding bytes if a struct is passed to memset but there are some situations where padding is not unspecified, for example if you do partial initialization of it.

[DSMan195276]

Could you explain what you mean by partial initialization?

[saagarjha]

  = { .foo = bar };

[cmovq]

(1) If the pointers you pass to memcmp are know to the compiler, then memcmp will be treated as an intrinsic, and generally the compiler generates the optimal set of instructions for a given struct.

(2) Correct. The expression is only evaluated at compile time.

[mtlmtlmtlmtl]

If you really wanted to memcmp a padded struct, you could declare it as packed and define the padding yourself.