undefined | Better HN

0 pointsnixpulvis3y ago0 comments

I was thinking of this from the perspective of CPU pipeline pressure, but in reality it seems prosessors are indeed smart enough to avoid burdoning the ALUs execution with these kinds of special cases.

> [...] these zeroing instructions extremely efficient, with a throughput of four zeroing instructons per clock cycle.

Also, the xor instruction takes up the smallest amount of .text space (right?).

0 comments

3 comments · 1 top-level

scheme2713y ago· 2 in thread

The xor reg, reg is also special cased because it's a quick way for compilers to reinitialize a register and indicate that future uses of that register don't depend on any previous operations. It helps the cpu to parallelize any future instructions that use that register since the cpu knows that those instructions don't depend on anything that happens before the the xor.

Someone3y ago

It’s not special cased because it's a quick way for compilers to reinitialize a register and indicate that future uses of that register don't depend on any previous operations, it’s special cased to give compilers a quick way to reinitialize a register and indicate that future uses of that register don't depend on any previous operations.

https://randomascii.wordpress.com/2012/12/29/the-surprising-... is almost ten years old and thus likely dated, but still may be educational.

nixpulvisOP3y ago

Exactly! I was almost expecting to find a special instruction for this, but then again, why waste the opcodes!?

j / k navigate · click thread line to collapse