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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/49095] Horrible code generation for trivial decrement with test Date: Sat, 21 May 2011 10:12:00 -0000 [thread overview] Message-ID: <bug-49095-4-dJP519Hcn4@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-49095-4@http.gcc.gnu.org/bugzilla/> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49095 Richard Guenther <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Target| |x86_64-*-*, i?86-*-* Status|UNCONFIRMED |NEW Keywords| |missed-optimization Last reconfirmed| |2011.05.21 09:52:49 Component|other |rtl-optimization Ever Confirmed|0 |1 --- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-21 09:52:49 UTC --- Confirmed (not using decq is because it is slower for some archs). On the tree level we cannot do better than D.2722_2 = *argv_1(D); D.2723_3 = D.2722_2 + -1; *argv_1(D) = D.2723_3; if (D.2723_3 == 0B) because we lack anything like direct operations on memory (and that's good). On the RTL side combine tries to do Trying 7, 8 -> 9: Failed to match this instruction: (parallel [ (set (mem/f:DI (reg/v/f:DI 63 [ argv ]) [2 *argv_1(D)+0 S8 A64]) (plus:DI (mem/f:DI (reg/v/f:DI 63 [ argv ]) [2 *argv_1(D)+0 S8 A64]) (const_int -1 [0xffffffffffffffff]))) (set (reg/f:DI 60 [ D.2723 ]) (plus:DI (mem/f:DI (reg/v/f:DI 63 [ argv ]) [2 *argv_1(D)+0 S8 A64]) (const_int -1 [0xffffffffffffffff]))) ]) because we have a use of the decrement result in the comparison. It doesn't try to combine this with the comparison though. So this case is really special ;) Without the use of the decremented value we get the desired subq $1, (%rsi). Manually sinking the store to *argv into the if and the else yields movq (%rsi), %rbx subq $1, %rbx je L4 L2: movq %rbx, (%rsi) ... L4: LCFI4: movq $0, (%rsi) movq %rsi, %rdi movq %rsi, 8(%rsp) call _fncall so at least we then can combine the decrement with the test ... As usual combine doesn't like stores.
next prev parent reply other threads:[~2011-05-21 10:09 UTC|newest] Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-05-21 3:46 [Bug other/49095] New: " torvalds@linux-foundation.org 2011-05-21 10:12 ` rguenth at gcc dot gnu.org [this message] 2011-05-21 19:22 ` [Bug rtl-optimization/49095] " torvalds@linux-foundation.org 2011-05-21 21:33 ` torvalds@linux-foundation.org 2011-05-27 10:50 ` jakub at gcc dot gnu.org 2011-05-27 12:30 ` jakub at gcc dot gnu.org 2011-05-27 14:22 ` torvalds@linux-foundation.org 2011-05-27 14:55 ` jakub at gcc dot gnu.org 2011-05-27 16:02 ` torvalds@linux-foundation.org 2011-05-27 16:36 ` jakub at gcc dot gnu.org 2011-05-27 16:52 ` torvalds@linux-foundation.org 2011-05-29 18:53 ` jakub at gcc dot gnu.org 2011-05-29 18:57 ` jakub at gcc dot gnu.org 2011-05-29 19:09 ` torvalds@linux-foundation.org
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