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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal>Hi All, <o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>I had a question on how clang frontend treats unreachable code. I am using llvm-2.9 and clang++ and the flags used to compile the code are –O0 –g –Wall.<o:p></o:p></p><p class=MsoNormal>The code that I compiled and the corresponding bitcode is given below.<o:p></o:p></p><p class=MsoNormal>int main ()<o:p></o:p></p><p class=MsoNormal>{<o:p></o:p></p><p class=MsoNormal> int a=100;<o:p></o:p></p><p class=MsoNormal> return 1;<o:p></o:p></p><p class=MsoNormal> a++;<o:p></o:p></p><p class=MsoNormal> return a;<o:p></o:p></p><p class=MsoNormal>}<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>define i32 @main() nounwind {<o:p></o:p></p><p class=MsoNormal>bb:<o:p></o:p></p><p class=MsoNormal> %tmp = alloca i32, align 4<o:p></o:p></p><p class=MsoNormal> %a = alloca i32, align 4<o:p></o:p></p><p class=MsoNormal> store i32 0, i32* %tmp<o:p></o:p></p><p class=MsoNormal> call void @llvm.dbg.declare(metadata !{i32* %a}, metadata !6), !dbg !8<o:p></o:p></p><p class=MsoNormal> store i32 100, i32* %a, align 4, !dbg !9<o:p></o:p></p><p class=MsoNormal> ret i32 1, !dbg !10<o:p></o:p></p><p class=MsoNormal>}<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Note that the code corresponding to a++; and return a; statements are not generated by frontend. Looks like the frontend detected that a++ and return a; to be dead code. My question is under what circumstances does the frontend do this optimization and is it possible to tell the frontend to NOT do the optimization?.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Sreekumar<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>PS: I modified the code to the following to add an IF statement as shown below. <o:p></o:p></p><p class=MsoNormal>int main ()<o:p></o:p></p><p class=MsoNormal>{<o:p></o:p></p><p class=MsoNormal> int a=100;<o:p></o:p></p><p class=MsoNormal> if(a) return 1;<o:p></o:p></p><p class=MsoNormal> a++;<o:p></o:p></p><p class=MsoNormal> return a;<o:p></o:p></p><p class=MsoNormal>}<o:p></o:p></p><p class=MsoNormal>The resulting code was still logically the same as the code given above. For this code, clang++ did generate the code corresponding to the dead code. <o:p></o:p></p></div></body></html>