https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114835
Bug ID: 114835
Summary: AVR popcountqi2 is not fast&small as can be
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: WolfgaNg.Hospital at arcor dot de
Target Milestone: ---
Created attachment 58025
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58025&action=edit
AVR assembly code for __popcountqi2
For 8-bit AVRs, the current 16-instruction __popcountqi2 uses 6 ADC
instructions to establish the number of bits set in a byte.
In "Bit Twiddling Hacks", Sean E. Anderson presents several approaches to this;
"parallel/sideways addition" can be coded with one instruction&cycle less.
When establishing the "popcount" of an uint8_t, I've seen GCC to widen the
value to "half int" and use __popcountqi2 twice.
