Compiling the following function with -O3 gives the following assembly output:
void spin(int volatile* ptr) {
while(*ptr);
return;
}
spin:
.LLFB1:
.register %g2, #scratch
lduw [%o0], %g1 ! 8 *zero_extendsidi2_insn_sp64/2 [length
= 1]
cmp %g1, 0 ! 9 *cmpsi_insn [length = 1]
be,pn %icc, .LL3 ! 10 *normal_branch [length = 1]
mov 0, %g1 ! 17 *movdi_insn_sp64/1 [length = 1]
.LL6:
lduw [%o0], %g2 ! 20 *zero_extendsidi2_insn_sp64/2 [length
= 1]
cmp %g2, 0 ! 22 *cmpsi_insn [length = 1]
bne,pt %icc, .LL6 ! 23 *normal_branch [length = 1]
add %g1, 1, %g1 ! 19 *adddi3_sp64/1 [length = 1]
.LL3:
jmp %o7+8 ! 55 *return_internal [length = 1]
mov %g1, %o0 ! 30 *movdi_insn_sp64/1 [length = 1]
Manually replacing the cmp/b* pairs with br* instructions gives 10-11% more
iterations/sec on my machine:
.global spin_brz
spin_brz:
.register %g2, #scratch
ld [%o0], %g1
brz,pn %g1, spin_brz_done
clr %g1
spin_brz_again:
ld [%o0], %g2
brnz,pt %g2, spin_brz_again
add %g1, 0x1, %g1
spin_brz_done:
retl
mov %g1, %o0
.size spin_brz, .- spin_brz
--
Summary: gcc should use brz(brnz) instead of cmp/be(bne) when
possible
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: scovich at gmail dot com
GCC target triplet: sparc-sun-solaris2.10
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40067
