Fewest uops (front-end bandwidth):
1 uop, latency 3c (Intel) or 1c (Zen).
Also smallest code-size, 5 bytes.
popcnt %rax, %rax # 5 bytes, 1 uop for one port
# if using a different dst, note the output dependency on Intel before ICL
On most CPUs that have it at all, it's 3c latency, 1c throughput (1 uop for one port).
Or 1c on Zen1/2/3 with 0.25c latency. (https://agner.org/optimize/ & https://uops.info/)
On Bulldozer-family before Excavator, popcnt r64 is 4c latency, 4c throughput. (32-bit operand-size has 2c throughput but still 4c latency.) Bobcat has quite slow microcoded popcnt.
Lowest latency (assuming Haswell or newer so there's no partial-register effect when writing AL and then reading EAX, or a uarch with no P6 ancestry that doesn't rename partial regs):
2 cycle latency, 2 uops, 6 bytes. Also the smallest code-size if popcnt (5B) isn't available.
test %rax, %rax # 3B, 1 uop any ALU port
setnz %al # 3B, 1 uop p06 (Haswell .. Ice Lake)
# only works in-place, with the rest of EAX already being known 0 for RAX==0
AL is the low byte of EAX, so AL=1 definitely makes EAX non-zero for any non-zero RAX.
This will cost a merging uop when reading EAX on Sandybridge/Ivy Bridge. Core2 / Nehalem will stall for a couple cycles to insert that merging uop. Earlier P6-family like Pentium-M will fully stall until the setcc retires if a later instruction reads EAX. (Why doesn't GCC use partial registers?)
Nate's neg/sbb is about the same as this on Broadwell and later, but 1 byte shorter. (And does zero the upper 32). This is better on Haswell, where sbb is 2 uops. On earlier mainstream Intel CPUs, they both require 3 uops, with this one needing a merging uop when EAX is read, or sbb (other than sbb $0 on SnB/HSW) always requiring 2 uops. neg/sbb can be used into a different register (still destroying the input), but has a false dependency on CPUs other than AMD. (K8/K10, Bulldozer-family, and Zen-family all recognize sbb same,same as depending only on CF).
If you want the upper 32 zeroed, BMI2 RORX to copy-and-shift:
2 uops, 2c latency, 8 bytes
rorx $32, %rax, %rdx # 6 bytes, 1 uop, 1c latency
or %edx, %eax # 2 bytes, 1c latency
## can produce its result in a different register without a false dependency.
rorx $32 is useful in general for horizontal SWAR reductions, e.g. for dword horizontal sum, you could movq a pair of dwords out of an XMM register and do the last shuffle+add in scalar using rorx/add instead of pshufd/paddd.
Or without BMI2 while still zeroing the upper 32:
7 bytes, 4 uops, 3c latency on Intel (where bswap r64 is 2 uops, 2c latency), otherwise 3 uops 2c latency on CPUs with efficient bswap r64 like Zen-family and Silvermont-family.
mov %eax, %edx # 2 bytes, and not on the critical path
bswap %rax # 3 bytes, vs. 4 for shr $32, %rax
or %edx, %eax # 2 bytes
## can write a different destination
A compromise using shr $32, %rax in place of bswap is
8 bytes, 3 uops, 2c latency for the above even without mov-elimination.
Running an ALU instruction on the original register instead of the mov result lets a non-eliminated MOV run in parallel with it.
Background for evaluating "best" performance:
Ideas that didn't pan out:
bsf %rax, %rax # 4 bytes. Fails for RAX=1
Leaves the destination unmodified for input=0. (AMD documents this, Intel implements it but doesn't document it as future-proof.) Unfortunately this produces RAX=0 for an input of 1.
And has the same perf as popcnt on Intel, and worse on AMD, but does save 1 byte of code size.
(Using sub $1 to set CF and then ??; Nate's usage of neg is how to make that work cleanly.)
I didn't try using a superoptimizer to brute-force check for other possible sequences, but as Nate commented this is a short enough problem to be a use-case for one.
