[Bug middle-end/78809] Inline strcmp with small constant strings

[qing.zhao at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78809

--- Comment #13 from Qing Zhao <qing.zhao at oracle dot com> ---
(In reply to Richard Earnshaw from comment 12)
> That's not entirely correct.  Notionally memcmp needs to return a value
> representing the relative difference of the first different byte in the
> compared areas of memory; any later bytes are irrelevant.  Yes the compiler can
> compare multiple bytes at the same time and it does not have to worry about
> page faulting, but it does have to keep track of where the first difference
> occurs.
> 
> Of course, the compiler can see how the result is used to optimize things
> further; a simple equality test will allow the compiler to generate a simpler
> sequence that could access all bytes and accumulate the overall result.

Yes.  this should be the reason why currently in GCC, only when the result of 
memcpy is compared to zero, it is optimized by “strlen” pass and “expand” pass.