[LLVMbugs] [Bug 893] NEW: Using the address of a global isn't necessarily cheap.

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=893

           Summary: Using the address of a global isn't necessarily cheap.
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Loop Optimizer
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: alenhar2 at uiuc.edu


On some architectures (and PIC modes) using the address of a global variable is
not cheap (sometimes involving a load).  Thus the global should be hoisted out
of loops.  This is probably true of large constants that the target will wind up
spilling to the constant pool also.
Thus LICM (and probably instcombine and such) should be aware of this (informed
by target data I'm sure).

consider:
%GV = internal global int 0

...
cond_true:
%b = call foo(int* %GV,...)
br bool %b, %cond_true, %cond_false

here the passing of GV causes an extra load each iteration of the loop.  This is
bad.



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