[LLVMbugs] [Bug 11226] New: Thumb 2: Code space: Prefer R4-R7 over IP, LR, etc.

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=11226

           Summary: Thumb 2: Code space: Prefer R4-R7 over IP, LR, etc.
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: ARM
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: xocotl at gmail.com
                CC: llvmbugs at cs.uiuc.edu


Howdy

It seems Thumb 2 code often pushes and uses R8-14 for its temporaries.
For code space, if R4-R7 are not used by arguments passed in, it would be
better if these were used instead.

For example, I noticed one function that
pushed LR (it called other functions, so this was a necessity), but presumably
to maintain 8-byte stack alignment, it also pushed IP. It then proceeded to use
it everywhere, generating 4-byte instructions left and right.

I've also seen it push FP/R11 despite not using it anywhere (alignment again, I
assume). In this case there would be no cost whatsoever to push a lower
register.

It would have been better if it had used R7 in this case, as registers R0-R7
can be used in many more two-byte instructions than R8-R15.

So, my suggestion is, unless necessary like LR, on Thumb 2 allocate from the
R0-R7 before using the later registers.

James

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