Mikael Egevig via llvm-dev
2019-Jul-07 23:26 UTC
[llvm-dev] Problem with the LLVM v8.0 IR 'select' instruction
Hi guys, I am doing a hobby compiler using LLVM. I have a rather peculiar problem with the 'select' instruction, as far as I can see. Given this code: ************************************************ ; Run-time Library Definitions %Exception = type { i8 } define private %Exception* @Start(i1* %$result) #0 { prologue: store i1 true, i1* %$result br label %epilogue epilogue: ret %Exception* null } ; Generated main() function. define i32 @main(i32 %argc, i8** %argv) { prologue: %0 = alloca i1 %1 = call %Exception *@Start(i1* %0) %2 = load i1, i1* %0 %3 = select i1 %2, i32 0, i32 1 ret i32 %3 } ************************************************ I'd expect the 'select' instruction in main() to yield zero, not one. But when I compile the sample on x86_64, arm7, and aarch64, I always get code that yields one. The idea is that if the program, @Start, returns false, one should be returned to the OS and vice versa. I guess I could use 'not' instead of 'select', but I don't understand the behavior of 'select'. The above code is automatically generated, except I have removed all irrelevant code generated by the compiler. I'm not sure if LLVM gets confused by me returning the result in an alloca'ed area, but I use the return value, of all functions, for the exception pointer (null = no exception). Thanks in advance! Cheers, Mikael Egevig -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190708/be3b5a79/attachment.html>
Mehdi AMINI via llvm-dev
2019-Jul-08 02:30 UTC
[llvm-dev] Problem with the LLVM v8.0 IR 'select' instruction
Hi, On Sun, Jul 7, 2019 at 4:26 PM Mikael Egevig via llvm-dev < llvm-dev at lists.llvm.org> wrote:> Hi guys, > > I am doing a hobby compiler using LLVM. I have a rather peculiar problem > with the 'select' instruction, as far as I can see. Given this code: > > ************************************************ > ; Run-time Library Definitions > %Exception = type { i8 } > > define private %Exception* @Start(i1* %$result) #0 > { > prologue: > store i1 true, i1* %$result > br label %epilogue > epilogue: > ret %Exception* null > } > > ; Generated main() function. > define i32 @main(i32 %argc, i8** %argv) > { > prologue: > %0 = alloca i1 > %1 = call %Exception *@Start(i1* %0) > %2 = load i1, i1* %0 > %3 = select i1 %2, i32 0, i32 1 > ret i32 %3 > } > ************************************************ > > I'd expect the 'select' instruction in main() to yield zero, not one. >Seems correct to me.> But when I compile the sample on x86_64, arm7, and aarch64, I always get > code that yields one. >Can you provide a reproducer? As far as I can tell the optimizer folds the code you provided above to yield 0: https://godbolt.org/z/T5wkSJ -- Mehdi> > The idea is that if the program, @Start, returns false, one should be > returned to the OS and vice versa. I guess I could use 'not' instead of > 'select', but I don't understand the behavior of 'select'. > > The above code is automatically generated, except I have removed all > irrelevant code generated by the compiler. > > I'm not sure if LLVM gets confused by me returning the result in an > alloca'ed area, but I use the return value, of all functions, for the > exception pointer (null = no exception). > > Thanks in advance! > > > Cheers, > Mikael Egevig > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190707/edadef2f/attachment.html>
Mikael Egevig via llvm-dev
2019-Jul-08 02:54 UTC
[llvm-dev] Problem with the LLVM v8.0 IR 'select' instruction
Hi again, It is probably me doing something wrong, but I have attached a trimmed down sample that shows the issue and the output that Clang 8.0.0 makes (using the binary release from llvm.org). I invoke clang as follows: clang -S -O3 -o Evergrowing.s -Wall Evergrowing.ll I tried using -O1, -O2, and -O3, but all where the same. Please notice that I renamed 'main()' to 'mainfunc()' to be sure Clang isn't picking up 'main()' as a reserved, special function. This on a Windows 10 x64 PC, but I got exactly the same behavior on arm7hf and aarch64. Cheers, Mikael Egevig Den man. 8. jul. 2019 kl. 04.30 skrev Mehdi AMINI <joker.eph at gmail.com>:> Hi, > > > > On Sun, Jul 7, 2019 at 4:26 PM Mikael Egevig via llvm-dev < > llvm-dev at lists.llvm.org> wrote: > >> Hi guys, >> >> I am doing a hobby compiler using LLVM. I have a rather peculiar problem >> with the 'select' instruction, as far as I can see. Given this code: >> >> ************************************************ >> ; Run-time Library Definitions >> %Exception = type { i8 } >> >> define private %Exception* @Start(i1* %$result) #0 >> { >> prologue: >> store i1 true, i1* %$result >> br label %epilogue >> epilogue: >> ret %Exception* null >> } >> >> ; Generated main() function. >> define i32 @main(i32 %argc, i8** %argv) >> { >> prologue: >> %0 = alloca i1 >> %1 = call %Exception *@Start(i1* %0) >> %2 = load i1, i1* %0 >> %3 = select i1 %2, i32 0, i32 1 >> ret i32 %3 >> } >> ************************************************ >> >> I'd expect the 'select' instruction in main() to yield zero, not one. >> > > Seems correct to me. > > >> But when I compile the sample on x86_64, arm7, and aarch64, I always >> get code that yields one. >> > > Can you provide a reproducer? > As far as I can tell the optimizer folds the code you provided above to > yield 0: https://godbolt.org/z/T5wkSJ > > -- > Mehdi > > > >> >> The idea is that if the program, @Start, returns false, one should be >> returned to the OS and vice versa. I guess I could use 'not' instead of >> 'select', but I don't understand the behavior of 'select'. >> >> The above code is automatically generated, except I have removed all >> irrelevant code generated by the compiler. >> >> I'm not sure if LLVM gets confused by me returning the result in an >> alloca'ed area, but I use the return value, of all functions, for the >> exception pointer (null = no exception). >> >> Thanks in advance! >> >> >> Cheers, >> Mikael Egevig >> >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190708/b5e697b8/attachment.html> -------------- next part -------------- A non-text attachment was scrubbed... Name: Evergrowing.ll Type: application/octet-stream Size: 1385 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190708/b5e697b8/attachment.obj> -------------- next part -------------- A non-text attachment was scrubbed... Name: Evergrowing.s Type: application/octet-stream Size: 427 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190708/b5e697b8/attachment-0001.obj>
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