bpf, x86: Shorten the cast to a user address

The cast of an arena pointer to its user address zero extended the low
half, shifted it up, or'ed the user base in, rotated it back down and
replaced the result with zero from a cleared r11 through a cmove for a
NULL pointer: seven instructions with a dependency chain of six. Or the
user base into r11 instead and pick it with a cmovne when the low half
is not zero, five instructions with a chain of three. LLVM emits the
cast for every arena pointer handed back to user space.

Before, for r1 = addr_space_cast(r7, 1, 0) with a user base of
0x7f1200000000:

  44 89 ef                       movl %r13d,%edi
  48 c1 e7 20                    shlq $0x20,%rdi
  48 81 cf 12 7f 00 00           orq $0x7f12,%rdi
  48 c1 c7 20                    rolq $0x20,%rdi
  4d 31 db                       xorq %r11,%r11
  85 ff                          testl %edi,%edi
  49 0f 44 fb                    cmoveq %r11,%rdi

After:

  44 89 ef                       movl %r13d,%edi
  49 bb 00 00 00 00 12 7f 00 00  movabsq $0x7f1200000000,%r11
  49 09 fb                       orq %rdi,%r11
  85 ff                          testl %edi,%edi
  49 0f 45 fb                    cmovneq %r11,%rdi

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20261013171230.523677-17-daniel@iogearbox.net
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

0 files changed, 0 insertions, 0 deletions