[LLVMbugs] [Bug 5774] New: Missed optimisation from flags on SUB

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=5774

           Summary: Missed optimisation from flags on SUB
           Product: libraries
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Backend: X86
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: arplynn at gmail.com
                CC: llvmbugs at cs.uiuc.edu


Consider the following test case:

define i32 @foo1 ( i32 %a, i32 %b ) nounwind
{
entry:
        %sub = sub i32 %a, %b
        %diff = icmp slt i32 %a, %b
        %select = select i1 %diff, i32 %b, i32 %a
        %call = call i32 @bar ( i32 %sub, i32 %select )
        ret i32 %call
}

define i32 @foo2 ( i32 %a, i32 %b ) nounwind
{
entry:
        %sub = sub i32 %a, %b
        %diff = icmp slt i32 %sub, 0
        %select = select i1 %diff, i32 %b, i32 %a
        %call = call i32 @bar ( i32 %sub, i32 %select )
        ret i32 %call
}

declare i32 @bar ( i32 %a, i32 %b ) nounwind

Whilst the effect of both foo1 and foo2 are equivalent, foo2 uses the negative
flag from the result of the sub for the condition; foo1 instead generates an
extra compare instruction (on X86).


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