Writing C software without the standard library

Re:Writing C software without the standard library by SonOfLilit on Tuesday November 29, 2016 @03:34AM (#13061704)

Probably yes, because Intel knows this is the code every compiler outputs for zeroing a register.

Also, the reason it is "faster" is that the encoding is 1 byte, vs. 9 bytes (in 64 bit) for "mov rbp, 0" - roughly, 1 for "mov rbp,", 8 more for a 64 bit "0".

[ Parent | on HN ]

Re:Writing C software without the standard library by bonzini on Tuesday November 29, 2016 @04:18AM (#13061864)

Technically you could get by with 5 bytes for "mov ebp, 0".

Another reason why it was faster was that the processor recognized it and avoided partial flags stalls after an "inc". But in 64-bit code you rarely have "inc" at all, so it matters less. On the other hand, a few years ago XOR had a false dependency on the register you're clearing; I'm not sure it is still that way on more recent processors.

[ Parent | on HN ]

Re:Writing C software without the standard library by SonOfLilit on Tuesday November 29, 2016 @06:49AM (#13062434)

I tip my hat to you, your analysis is far more interesting than mine.

[ Parent | on HN ]

Re:Writing C software without the standard library by bonzini on Wednesday November 30, 2016 @02:15AM (#13069054)

Wrong too, it's partial register stalls not partial flags stalls.

[ Parent | on HN ]