http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48986
Jakub Jelinek <jakub at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|ASSIGNED |NEW CC| |uros at gcc dot gnu.org AssignedTo|jakub at gcc dot gnu.org |unassigned at gcc dot | |gnu.org --- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-16 11:26:51 UTC --- On: int foo (int *p) { return __sync_fetch_and_add (p, -1) == 1; } int bar (int *p) { return __sync_add_and_fetch (p, -1) == 0; } I get better generated code for the second routine if I do: --- gcc/config/i386/sync.md.jj 72010-05-21 11:46:29.000000000 +0200 +++ gcc/config/i386/sync.md 2011-05-16 13:06:13.000000000 +0200 @@ -170,7 +170,7 @@ [(match_operand:SWI 1 "memory_operand" "+m")] UNSPECV_XCHG)) (set (match_dup 1) (plus:SWI (match_dup 1) - (match_operand:SWI 2 "register_operand" "0"))) + (match_operand:SWI 2 "nonmemory_operand" "0"))) (clobber (reg:CC FLAGS_REG))] "TARGET_XADD" "lock{%;} xadd{<imodesuffix>}\t{%0, %1|%1, %0}") and for foo identical code, so maybe that change is always beneficial, allowing combiner and other early RTL passes to see there a constant instead of a REG. Unfortunately, even with this change the combiner doesn't attempt to combine this pattern with the following cmpsi_1 pattern, supposedly because sync_old_addsi pattern isn't single_set. I guess we could handle this during expansion, but it would be a mess, or some other pass (e.g. peephole2 or something similar). peephole2 might kind of too late though, by that time the constant must be loaded already into some register, so we'd need to peephole2 3 insns, where the load of the constant might often not be the first one.