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From: "carlo at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/37810] New: Bad store sinking job Date: Sun, 12 Oct 2008 15:14:00 -0000 [thread overview] Message-ID: <bug-37810-7035@http.gcc.gnu.org/bugzilla/> (raw) The following code snippet: void g(); struct A { int n; int m; A& operator++(void) { if (__builtin_expect(n == m, false)) g(); else ++n; return *this; } A() : n(0), m(0) { } friend bool operator!=(A const& a1, A const& a2) { return a1.n != a2.n; } }; void testfunction(A& iter) { A const end; while (iter != end) ++iter; } Results in the following assembly code, using maximum optimization: movl (%rdi), %eax jmp .L6 .L4: cmpl %eax, 4(%rdi) // n == m ? je .L8 // unlikely jump addl $1, %eax // ++n movl %eax, (%rdi) // *** store result to memory *** .L6: testl %eax, %eax // iter != end ? jne .L4 // continue while loop The storing (back) of %eax to (%rdi) remains inside the inner loop no matter what I try. It could/should be moved outside the loop, since nothing inside the L4 loop is accessing (%rdi) or could possibly be accessing that memory. This loop has two exits: below the last jne .L4, and the jump to .L8. The store could be sinked to both exits. This grows the code, but it seems reasonable to do for a loop with a very small body, especially if one of the exits is marked as unlikely :p. -- Summary: Bad store sinking job Product: gcc Version: 4.4.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: carlo at gcc dot gnu dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37810
next reply other threads:[~2008-10-12 15:14 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2008-10-12 15:14 carlo at gcc dot gnu dot org [this message] 2008-10-12 15:21 ` [Bug tree-optimization/37810] " rguenth at gcc dot gnu dot org 2008-10-12 15:27 ` rguenth at gcc dot gnu dot org 2008-10-12 15:30 ` rguenth at gcc dot gnu dot org 2008-10-12 15:34 ` carlo at gcc dot gnu dot org 2009-04-03 12:34 ` rguenth at gcc dot gnu dot org
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