X86 NEDIR KAC BIT CODE

Inside a loop consider copying from another register instead of re-creating ones with an ALU uop, especially with AVX512.Д«ut zeroing is cheap: xor-zeroing an xmm reg inside a loop is usually as good as copying, except on some AMD CPUs (Bulldozer and Zen) which have mov-elimination for vector regs but still need an ALU uop to write zeros for xor-zeroing. SSE/AVX vpcmpeqd is dep-breaking on many (although still needs a uop to write the 1s), but AVX512 vpternlogd for ZMM regs isn't even dep-breaking. Set all bits in CPU register to 1 efficiently also covers AVX512 k0.7 mask registers. Semi-related: Fastest way to set _m256 value to all ONE bits and What is the most efficient way to clear a single or a few ZMM registers on Knights Landing? See Is vxorps-zeroing on AMD Jaguar/Bulldozer/Zen faster with xmm registers than ymm? and

X86 NEDIR KAC BIT CODE

Use AVX1 vpxor xmm0, xmm0, xmm0 even on KNL to save code size. Vpxord zmm0, zmm0, zmm0 EVEX prefix (4 bytes), and a 512-bit uop. # sub-optimal with AVX512 (even without AVX512VL) Vpxord zmm30, zmm30, zmm30 Without AVX512VL you have to use a 512-bit instruction.

# Good with AVX512 *without* AVX512VL (e.g. May be worth using only high regs to avoid needing vzeroupper in short functions. Vpxord xmm30, xmm30, xmm30 EVEX is unavoidable when zeroing zmm16.31, but still prefer XMM or YMM for fewer uops on probable future AMD. Vpxor xmm15, xmm0, xmm0 zero ZMM15 using an AVX1-encoded instruction (2-byte VEX prefix). Vpxor ymm0, ymm0, ymm0 decodes to 2 uops on AMD before Zen2 Vpxor xmm15, xmm15, xmm15 3-byte VEX prefix because of high source reg Vpxor xmm15, xmm0, xmm0 zeros X/Y/ZMM15, still only 2-byte VEX prefix Xorps xmm15, xmm15 Needs a REX prefix but that's unavoidable if you need to use high registers without AVX. Pxor xmm0, xmm0 costs an extra byte, runs on any port on Nehalem. Xorps xmm0, xmm0 smallest code size (for non-AVX) The AVX512 ZMM zeroing would require extra bytes (for the EVEX prefix), so XMM or YMM zeroing should be preferred. vpxor ymm, ymm, ymm doesn't take any extra bytes to encode, though, and runs the same on Intel, but slower on AMD before Zen2 (2 uops). Using the AVX version of a 128b vector instruction zeros the upper part of the reg as well, so vpxor xmm, xmm, xmm is a good choice for zeroing YMM(AVX1/AVX2) or ZMM(AVX512), or any future vector extension. On AMD, and pre-Nehalem P6/Core2 Intel, xorps and pxor are handled the same way (as vector-integer instructions). On SnB-family microarchitectures, neither flavour of xor-zeroing even needs an execution port. (Nehalem's 2c bypass delay latency between integer and FP is usually not relevant, because out-of-order execution can typically hide it at the start of a new dependency chain). It's one byte shorter than pxor, but xorps needs execution port 5 on Intel Nehalem, while pxor can run on any port (0/1/5). That's typically what gcc does (even before use with FP instructions).

Zeroing a vector register is usually best done with pxor xmm, xmm. Mov cl, 0 only 2 bytes, and probably better than xor cl,cl *if* you need to leave the rest of ECX/RCX unmodified Xor cl, cl false dep on some CPUs, not a zeroing idiom.

Sub eax, eax same as xor on most but not all CPUs bad on Silvermont for example. (Microbenchmark experiments might want this) Mov eax, 0 doesn't touch FLAGS, but not faster and takes more bytesДЄnd eax, 0 false dependency. Xor r10,r10 bad on Silvermont (not dep breaking), same as r10d on other CPUs because a REX prefix is still needed for r10d or r10. Xor rax,rax waste of a REX prefix, and extra slow on Silvermont small code-size alternative: cdq zero RDX if EAX is already zero GP-integer examples: xor eax, eax RAX = 0. Thus even when a REX prefix is still required because you're zeroing r8.r15, use xor r10d,r10d, not xor r10,r10. xor r64, r64 is a waste of a byte, because it needs a REX prefix.Д®ven worse than that, Silvermont only recognizes xor r32,r32 as dep-breaking, not 64-bit operand-size. In 64-bit mode, still use xor r32, r32, because writing a 32-bit reg zeros the upper 32. It's officially recommended by Intel and AMD, and what compilers do. No other method has any advantage over it, and it has at least some advantage over any other method. TL DR summary: xor same, same is the best choice for all CPUs.

X86 nedir kac bit