[LLVMbugs] [Bug 468] NEW: [target description] Need a way to describe *preferred* target alignments

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.cs.uiuc.edu/bugs/show_bug.cgi?id=468

           Summary: [target description] Need a way to describe *preferred*
                    target alignments
           Product: libraries
           Version: 1.0
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Target Description Classes
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: sabre at nondot.org


The TargetData class currently provides interfaces to get the ABI-specified data
alignment information that backends need to lay out structures and other
important stuff.

However, some targets have ABIs that require less alignment than is good for
performance.  On X86, for example, alignof(double) is 4, but the processors
really prefer doubles to be on an 8-byte alignment boundary.

It would be great if TargetData was extended to provide 'preferred' alignment
boundaries as well as required ones.  This would allow the code generator to
align globals, stack objects, and constant pool values at an alignment that
could well lead to better performance in important cases.

-Chris



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