3 ms·

Back in the stone ages XOR ing was just 1 byte of opcode. Habbits stick. In effect XORing is no longer faster since a long time.

dragontamer 6mo ago
The XOR trick is implemented as a (malloc from register file) on modern processors, implemented in the decoder and it won't even issue a uOp to the execution pipelines. Its basically free today. Of course, mov RAX, 0 is also free and does the same thing. But CPUs have limited decoder lengths per clock tick, so the more instructions you fit in a given size, the more parallel a modern CPU can potentially execute. So.... definitely still use XOR trick today. But really, let the compiler handle it. Its pretty good at keeping track of these things in practice. ----------- I'm not sure if "sub" is hard-coded to be recognized in the decoder as a zero'd out allocation from the register file. There's only certain instructions that have been guaranteed to do this by Intel/AMD.
- mmozeiko 5mo ago
  sub is also recognized as zeroing idiom for register file. Intel documents these in "3.5.1.7 Clearing Registers and Dependency Breaking Idioms" from Optimization Reference Manual: https://www.intel.com/content/www/us/en/developer/articles/technical/intel64-and-ia32-architectures-optimization.html https://www.intel.com/content/www/us/en/developer/articles/t... Here's html version: https://zzqcn.github.io/perf/intel_opt_manual/3.html#clearing-registers-and-dependency-breaking-idioms https://zzqcn.github.io/perf/intel_opt_manual/3.html#clearin... AMD has similar list in "2.9.2 Idioms for Dependency removal" from "Software Optimization Guide for the AMD Zen5 Microarchitecture" document: https://docs.amd.com/v/u/en-US/58455_1.00 https://docs.amd.com/v/u/en-US/58455_1.00
flohofwoe 5mo ago
Depending on what's stone-age for you, a SUB with a register was also only one byte, and was the same cost as XOR, at least in the Intel/Zilog lineage all the way back to the 70s ;)
NetMageSCW 5mo ago
The article’s point is about why XOR is preferred over SUB, both being one byte. MOV is right out.

dragontamer 6mo ago

The XOR trick is implemented as a (malloc from register file) on modern processors, implemented in the decoder and it won't even issue a uOp to the execution pipelines. Its basically free today. Of course, mov RAX, 0 is also free and does the same thing. But CPUs have limited decoder lengths per clock tick, so the more instructions you fit in a given size, the more parallel a modern CPU can potentially execute. So.... definitely still use XOR trick today. But really, let the compiler handle it. Its pretty good at keeping track of these things in practice. ----------- I'm not sure if "sub" is hard-coded to be recognized in the decoder as a zero'd out allocation from the register file. There's only certain instructions that have been guaranteed to do this by Intel/AMD.

mmozeiko 5mo ago
sub is also recognized as zeroing idiom for register file. Intel documents these in "3.5.1.7 Clearing Registers and Dependency Breaking Idioms" from Optimization Reference Manual: https://www.intel.com/content/www/us/en/developer/articles/technical/intel64-and-ia32-architectures-optimization.html https://www.intel.com/content/www/us/en/developer/articles/t... Here's html version: https://zzqcn.github.io/perf/intel_opt_manual/3.html#clearing-registers-and-dependency-breaking-idioms https://zzqcn.github.io/perf/intel_opt_manual/3.html#clearin... AMD has similar list in "2.9.2 Idioms for Dependency removal" from "Software Optimization Guide for the AMD Zen5 Microarchitecture" document: https://docs.amd.com/v/u/en-US/58455_1.00 https://docs.amd.com/v/u/en-US/58455_1.00

flohofwoe 5mo ago

Depending on what's stone-age for you, a SUB with a register was also only one byte, and was the same cost as XOR, at least in the Intel/Zilog lineage all the way back to the 70s ;)

NetMageSCW 5mo ago

The article’s point is about why XOR is preferred over SUB, both being one byte. MOV is right out.