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From: "etienne_lorrain at yahoo dot fr" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/53507] New: ia32/amd64: bsf can be used to test null memory, bsf sets zero flag Date: Mon, 28 May 2012 10:03:00 -0000 [thread overview] Message-ID: <bug-53507-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53507 Bug #: 53507 Summary: ia32/amd64: bsf can be used to test null memory, bsf sets zero flag Classification: Unclassified Product: gcc Version: 4.6.3 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: rtl-optimization AssignedTo: unassigned@gcc.gnu.org ReportedBy: etienne_lorrain@yahoo.fr Compiling: int I; void ifnull(void); void testnull(void) { if (I == 0) ifnull(); } void fct (unsigned u) { if (u != 0) I = __builtin_ffs(u) - 1; } result in for testnull(): movl I, %eax testl %eax, %eax je .L4 rep ret .L4: jmp ifnull() It would be shorter/quicker to replace the two first lines by: bsfl I, %eax je ifnull() i.e. there is no need to load the memory variable into a register with bsf/bsr. And result for fct(): movl 4(%esp), %eax testl %eax, %eax je .L5 bsfl %eax, %eax movl %eax, I .L5: rep ret It would be shorter/quicker to test if the parameter u is null by the bsf instruction: bsfl 4(%esp), %eax cmovne %eax, I .L5: rep ret
next reply other threads:[~2012-05-28 9:40 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-05-28 10:03 etienne_lorrain at yahoo dot fr [this message] 2021-11-29 9:02 ` [Bug target/53507] " pinskia at gcc dot gnu.org
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