Stripf, Timo
2009-Aug-11 08:13 UTC
[LLVMdev] Bug in optimization pass related to strcmp and big endian back-ends
Hi all, i'm working on a LLVM back-end right now and i think I found a bug in an optimization pass. When compiling the following code using llvm-gcc (the current 2.5 release) with -O2 int main(int argc, char** argv) { char* pStr = "I" + (argc > 100); printf("%d\n", strcmp(pStr, "I") == 0); } the strcmp function is replaced by a 16 bit load and compared against the integer value of 'I': define i32 @main(i32 %argc, i8** nocapture %argv) nounwind { entry: %0 = icmp sgt i32 %argc, 100 ; <i1> [#uses=1] %1 = zext i1 %0 to i32 ; <i32> [#uses=1] %2 = getelementptr [2 x i8]* @.str, i32 0, i32 %1 ; <i8*> [#uses=1] %tmp = bitcast i8* %2 to i16* ; <i16*> [#uses=1] %lhsv = load i16* %tmp, align 1 ; <i16> [#uses=1] %3 = icmp eq i16 %lhsv, 73 ; <i1> [#uses=1] %4 = zext i1 %3 to i32 ; <i32> [#uses=1] %5 = tail call i32 (i8*, ...)* @printf(i8* getelementptr ([4 x i8]* @.str1, i32 0, i32 0), i32 %4) nounwind ; <i32> [#uses=0] ret i32 undef } On little endian machines the code works correct but on big endian %lhsv must be compared against 73 << 8. Kind regards Timo Stripf -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20090811/9ec73875/attachment.html>
Eli Friedman
2009-Aug-11 08:27 UTC
[LLVMdev] Bug in optimization pass related to strcmp and big endian back-ends
On Tue, Aug 11, 2009 at 1:13 AM, Stripf, Timo<Timo.Stripf at itiv.uni-karlsruhe.de> wrote:> On little endian machines the code works correct but on big endian %lhsv > must be compared against 73 << 8.If llvm-gcc thinks it's compiling for a little-endian target, the optimizers will assume the target is little-endian... what are you trying to do? -Eli
Stripf, Timo
2009-Aug-11 09:24 UTC
[LLVMdev] Bug in optimization pass related to strcmp and bigendian back-ends
I thought the LLVM IR is target independent and that "llvm-gcc -c -emit-llvm -O2" produces target independent code. I'm working on a back-end and use llvm-gcc to first generate the bc file. Afterwards I use llc including the new back-end to produce the assembler file. -Timo -----Ursprüngliche Nachricht----- Von: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] Im Auftrag von Eli Friedman Gesendet: Dienstag, 11. August 2009 10:27 An: LLVM Developers Mailing List Betreff: Re: [LLVMdev] Bug in optimization pass related to strcmp and bigendian back-ends On Tue, Aug 11, 2009 at 1:13 AM, Stripf, Timo<Timo.Stripf at itiv.uni-karlsruhe.de> wrote:> On little endian machines the code works correct but on big endian %lhsv > must be compared against 73 << 8.If llvm-gcc thinks it's compiling for a little-endian target, the optimizers will assume the target is little-endian... what are you trying to do? -Eli _______________________________________________ LLVM Developers mailing list LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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- [LLVMdev] Bug in optimization pass related to strcmp and bigendian back-ends
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- [LLVMdev] Bug in optimization pass related to strcmp and bigendian back-ends
- [LLVMdev] Bug in optimization pass related to strcmp and big endian back-ends