[LLVMbugs] [Bug 2903] New: Optimization opportunity - group similarly initialized stack slots and bulk initialize

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=2903

           Summary: Optimization opportunity - group similarly initialized
                    stack slots and bulk initialize
           Product: libraries
           Version: 2.3
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Backend: X86
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: maxburke at gmail.com
                CC: llvmbugs at cs.uiuc.edu


I've run into a case (x86) where I have a bunch of zero-initialized stack slots
(rooted GC pointers) and the initialization code for them, of the form "mov
mem, 0" is 47kb. 

Moving the immediate value to a register and using this register as the source
would save ~20% (8bytes/opcode instead of 10), ie:

xor eax, eax
mov mem, eax

But I was wondering if it'd be possible to group all similarly initialized
stack slots together so that "rep stos" could be used to initialize them all in
one go.


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