LLVM Chapter 5 reductions

1. Hacker’s Delight popcount network not canonicalized to ctpop

File: popcount_hd_to_ctpop.ll

Issue:

LLVM does not canonicalize the classic Hacker’s Delight parallel popcount network to llvm.ctpop.
On x86_64 with POPCNT available, this misses a direct lowering to popcntl/popcntq.

Observed current-head behavior:

Expected x86_64 shape:

popcntl %edi, %eax
retq

and

popcntq %rdi, %rax
retq

Suggested bug report title:

InstCombine misses ctpop canonicalization for Hacker's Delight parallel popcount idiom

File: popcount_loop_to_ctpop.ll

Issue:

LLVM partially strength-reduces the loop body to blsr, but does not replace the whole counting loop with ctpop.

Observed current-head behavior:

xorl  %eax, %eax
testl %edi, %edi
je    ...
.Lloop:
  incl  %eax
  blsrl %edi, %edi
  jne   .Lloop
retq

Caveat:

This is a weaker bug-report candidate than the parallel popcount network.
Loop idiom recognition here may be considered optional unless there is an established canonicalization policy for this specific pattern.