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From: "b.grayson at samsung dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/58622] New: With -fomit-frame-pointer, A64 does not generate post-decrement stores Date: Fri, 04 Oct 2013 18:24:00 -0000 [thread overview] Message-ID: <bug-58622-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58622 Bug ID: 58622 Summary: With -fomit-frame-pointer, A64 does not generate post-decrement stores Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: b.grayson at samsung dot com Target: AArch64 Build: 4.9.0 20130602 In A64, if one compiles a simple program under -O3, one gets code like this: int bar(int i); int foo() { return bar(5)+4; } A64 -O3 assembly: foo: stp x29, x30, [sp, -16]! add x29, sp, 0 mov w0, 5 bl bar add w0, w0, 4 ldp x29, x30, [sp], 16 ret Note the use of update-form loads and stores for the SP. But if one uses -O3 -fomit-frame-pointer, the following is obtained: foo: sub sp, sp, #16 mov w0, 5 str x30, [sp] bl bar add w0, w0, 4 ldr x30, [sp] add sp, sp, 16 ret The sub and str could be merged into str x30, [sp, #-16]!, and the ldr/add could be merged into ldr x30, [sp], #16 (if I have my assembly correct), as they were in the with-frame-pointer case. On some ARM implementations, the updates are "for free", so one would get better performance with the merged load/store instructions, not to mention better instruction-cache density. Note that under A32, identical code (using update/post-decrement stores) is generated regardless of omit-frame-pointer settings.
next reply other threads:[~2013-10-04 18:24 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-10-04 18:24 b.grayson at samsung dot com [this message] 2014-01-28 15:23 ` [Bug target/58622] " rearnsha at gcc dot gnu.org 2014-03-13 3:24 ` kuganv at linaro dot org 2014-03-20 19:18 ` ramana at gcc dot gnu.org 2014-07-24 14:54 ` mshawcroft at gcc dot gnu.org 2014-07-24 14:54 ` mshawcroft at gcc dot gnu.org
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