[Bug other/16795] New: PowerPC - Divide sequences by negative power of two could be more efficient

["gcc-bugzilla at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

Description:
A non-optimal code sequence is illustraded.  Dividing by a negative power of two could be more efficiently done.  Duplicate using gcc 3.5 and command line:

gcc -O3 -m64 -c test.c

Testcase:
int x;

void foo1() {

  x /= -0x2;

}

void foo2() {

  x /= -0x4;

}

Assembly:
Code generated for "foo1" looks like this:

.foo:
	ld 9,.LC0@toc(2)
	lwz 5,0(9)
	srwi 11,5,31
	add 4,5,11
	srawi 3,4,1
	neg 0,3
	stw 0,0(9)
	blr

A better alternative would be this:

.foo:
	ld 9,.LC0@toc(2)
	lwz 5,0(9)
	srawi 0,5,1
	addze 3,0
	neg 0,3
	stw 0,0(9)
	blr

Code generated for "foo2" looks like this:

.foo:
	ld 9,.LC0@toc(2)
	lwz 5,0(9)
	srawi 6,5,31
	srwi 11,6,30
	add 4,5,11
	srawi 3,4,2
	neg 0,3
	stw 0,0(9)
	blr

More optimal sequence would be:

.foo:
	ld 9,.LC0@toc(2)
	lwz 5,0(9)
	srawi 0,5,2
	addze 3,0
	neg 0,3
	stw 0,0(9)
	blr



-- 
           Summary: PowerPC - Divide sequences by negative power of two
                    could be more efficient
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P1
         Component: other
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: steinmtz at us dot ibm dot com
                CC: gcc-bugs at gcc dot gnu dot org,steinmtz at us dot ibm
                    dot com
 GCC build triplet: powerpc64-linux
  GCC host triplet: powerpc64-linux
GCC target triplet: powerpc64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16795