[LLVMbugs] [Bug 1264] NEW: converting 64-bit constant pool entry to 32-bit not necessarily beneficial

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=1264

           Summary: converting 64-bit constant pool entry to 32-bit not
                    necessarily beneficial
           Product: libraries
           Version: trunk
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Backend: X86
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: djg at cray.com


For this test case:

define double @foo(double %x) {
        %y = mul double %x, 5.000000e-01
        ret double %y
}

llc -march=x86-64 currently produces a 32-bit constant pool entry and this code:

        cvtss2sd .LCPI1_0(%rip), %xmm1
        mulsd %xmm1, %xmm0

instead of just using a 64-bit constant pool entry with this:

        mulsd .LCPI1_0(%rip), %xmm0

This is due to the code in ExpandConstantFP in LegalizeDAG.cpp. It notices that
x86-64 indeed has an instruction to load a 32-bit float from memory and convert
it into a 64-bit float in a register, however it doesn't notice that this isn't
beneficial because it prevents the load from being folded into the multiply.



------- You are receiving this mail because: -------
You are on the CC list for the bug, or are watching someone who is.