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From: "drepper.fsp at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/53243] New: Use vector comparisons for if cascades Date: Sat, 05 May 2012 04:06:00 -0000 [thread overview] Message-ID: <bug-53243-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53243 Bug #: 53243 Summary: Use vector comparisons for if cascades Classification: Unclassified Product: gcc Version: 4.8.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: tree-optimization AssignedTo: unassigned@gcc.gnu.org ReportedBy: drepper.fsp@gmail.com Target: x86_64-linux Created attachment 27312 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27312 Test program (compile with and without -DOLD) The vector units can compare multiple comparisons concurrently but this is not used automatically in gcc in situations where it can lead to better performance. Assume a function like this: void f(float a) { if (a < 1.0) cb(1); else if (a < 2.0) cb(2); else if (a < 3.0) cb(3); else if (a < 4.0) cb(4); else if (a < 5.0) cb(5); else if (a < 6.0) cb(6); else if (a < 7.0) cb(7); else if (a < 8.0) cb(8); else ++o; } In this case the first or second if is not marked with __builtin_expect as likely, otherwise the following *might* not apply. The routine can be rewritten for AVX machines like this: void f(float a) { const __m256 fv = _mm256_set_ps(8.0,7.0,6.0,5.0,4.0,3.0,2.0,1.0); __m256 r = _mm256_cmp_ps(fv, _mm256_set1_ps(a), _CMP_LT_OS); int i = _mm256_movemask_ps(r); asm goto ("bsr %0, %0; jz %l[less1]; .pushsection .rodata; 1: .quad %l2, %l3, %l4, %l5, %l6, %l7, %l8, %l9; .popsection; jmp *1b(,%0,8)" : : "r" (i) : : less1, less2, less3, less4, less5, less6, less7, less8, gt8); __builtin_unreachable (); less1: cb(1); return; less2: cb(2); return; less3: cb(3); return; less4: cb(4); return; less5: cb(5); return; less6: cb(6); return; less7: cb(7); return; less8: cb(8); return; gt8: ++o; } This might not generate the absolute best code but it runs for the test program which I attach 20% faster. The same technique can be applied to integer comparisons. More complex if cascades can also be simplified a lot by masking the integer bsr result accordingly. This should still be faster.
next reply other threads:[~2012-05-05 4:06 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-05-05 4:06 drepper.fsp at gmail dot com [this message] 2012-05-07 9:33 ` [Bug tree-optimization/53243] " rguenth at gcc dot gnu.org
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