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* [Bug c/38209] New: [avr] branch optimisation generates worse code @ 2008-11-21 6:06 k dot kosciuszkiewicz+gcc at gmail dot com 2008-11-21 6:07 ` [Bug c/38209] " k dot kosciuszkiewicz+gcc at gmail dot com 2008-11-23 18:57 ` [Bug middle-end/38209] " pinskia at gcc dot gnu dot org 0 siblings, 2 replies; 3+ messages in thread From: k dot kosciuszkiewicz+gcc at gmail dot com @ 2008-11-21 6:06 UTC (permalink / raw) To: gcc-bugs Test code: register unsigned char val asm("r4"); void negate(void) { if (val) val = ~val; else val = ~val; } Code generated with -Os .global negate .type negate, @function negate: /* prologue: function */ /* frame size = 0 */ tst r4 breq .L2 com r4 ret .L2: clr r4 dec r4 ret .size negate, .-negate In the "else" branch gcc knows that r4 is zero, and that it should change to 0xff - taking this approach generates longer code than in the first branch. The same applies when operations like !, ++, -- are used. -- Summary: [avr] branch optimisation generates worse code Product: gcc Version: 4.3.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: k dot kosciuszkiewicz+gcc at gmail dot com GCC build triplet: i386-linux-gnu GCC host triplet: i386-linux-gnu GCC target triplet: avr-*-* http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38209 ^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c/38209] [avr] branch optimisation generates worse code 2008-11-21 6:06 [Bug c/38209] New: [avr] branch optimisation generates worse code k dot kosciuszkiewicz+gcc at gmail dot com @ 2008-11-21 6:07 ` k dot kosciuszkiewicz+gcc at gmail dot com 2008-11-23 18:57 ` [Bug middle-end/38209] " pinskia at gcc dot gnu dot org 1 sibling, 0 replies; 3+ messages in thread From: k dot kosciuszkiewicz+gcc at gmail dot com @ 2008-11-21 6:07 UTC (permalink / raw) To: gcc-bugs ------- Comment #1 from k dot kosciuszkiewicz+gcc at gmail dot com 2008-11-21 06:05 ------- Created an attachment (id=16730) --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=16730&action=view) Test case. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38209 ^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/38209] branch optimisation generates worse code 2008-11-21 6:06 [Bug c/38209] New: [avr] branch optimisation generates worse code k dot kosciuszkiewicz+gcc at gmail dot com 2008-11-21 6:07 ` [Bug c/38209] " k dot kosciuszkiewicz+gcc at gmail dot com @ 2008-11-23 18:57 ` pinskia at gcc dot gnu dot org 1 sibling, 0 replies; 3+ messages in thread From: pinskia at gcc dot gnu dot org @ 2008-11-23 18:57 UTC (permalink / raw) To: gcc-bugs ------- Comment #2 from pinskia at gcc dot gnu dot org 2008-11-23 18:55 ------- This happens on i386-darwin also (-m64 -O2 -fomit-frame-pointer): movl %edi, %edx testb %dil, %dil movl $-1, %eax notl %edx cmovne %edx, %eax Using the following source: unsigned char negate(unsigned char val) { if (val) val = ~val; else val = ~val; return val; } We should be able to just do ~val but this comes down to hoisting before applying VRP/DOM. -- pinskia at gcc dot gnu dot org changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Ever Confirmed|0 |1 GCC build triplet|i386-linux-gnu | GCC host triplet|i386-linux-gnu | GCC target triplet|avr-*-* | Last reconfirmed|0000-00-00 00:00:00 |2008-11-23 18:55:59 date| | Summary|[avr] branch optimisation |branch optimisation |generates worse code |generates worse code http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38209 ^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2008-11-23 18:57 UTC | newest] Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2008-11-21 6:06 [Bug c/38209] New: [avr] branch optimisation generates worse code k dot kosciuszkiewicz+gcc at gmail dot com 2008-11-21 6:07 ` [Bug c/38209] " k dot kosciuszkiewicz+gcc at gmail dot com 2008-11-23 18:57 ` [Bug middle-end/38209] " pinskia at gcc dot gnu dot org
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