[Bug target/93006] New: Better code with a-- == 0 rather than --a == -1

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93006

            Bug ID: 93006
           Summary: Better code with a-- == 0 rather than --a == -1
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

unsigned int a;

unsigned
foo (unsigned b, unsigned c)
{
  return b + c + (--a == -1);
}

unsigned
bar (unsigned b, unsigned c)
{
  return b + c + (a-- == 0);
}

unsigned
baz (unsigned b, unsigned c)
{
  return b + c + (--a != -1);
}

unsigned
qux (unsigned b, unsigned c)
{
  return b + c + (a-- != 0);
}

where foo is equivalent to bar and baz to qux generates better code (just 6
instructions) in bar/qux and 9 in foo/baz on x86_64-linux.