llvm dev - Aug 2009 - [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

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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

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
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