[LLVMbugs] [Bug 986] NEW: Extremely ugly code is generated for integer division in some cases.

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=986

           Summary: Extremely ugly code is generated for integer division in
                    some cases.
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Backend: X86
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: asl at math.spbu.ru


Consider the folowing code:

uint %test_div_func7(uint %u) {
entry:
        %tmp1 = udiv uint %u, 7
        ret uint %tmp1
}

llvm currently generates the following code (well, actually it was generated via
llvm-gcc from C source with -O3):

test_div_func7:
        subl $4, %esp
        movl %esi, (%esp)
        movl $613566757, %ecx
        movl 8(%esp), %esi
        movl %esi, %eax
        mull %ecx
        subl %edx, %esi
        shrl %esi
        addl %edx, %esi
        shrl $2, %esi
        movl %esi, %eax
        movl (%esp), %esi
        addl $4, %esp
        ret

This is really ugly:

1. "General" algorithm for division-via-multiplication is ok, but it can be
tweaked more for x86 platform.
2. Codegenerator transforms LLVM code to something not so good.



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