[llvm-dev] opt -instcombine interesting behavior

[Nema, Ashutosh via llvm-dev]

>   %reg = alloca i32, i32 0
Allocating 0 bytes is legal but it's an undefined behavior, please refer:
http://llvm.org/docs/LangRef.html#id184

You need to correct allocation to get the desired output, i.e.:
>   %reg = alloca i32, i32 1

Regards,
Ashutosh

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Stephan Plöger via llvm-dev
> Sent: Monday, December 19, 2016 10:11 PM
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] opt -instcombine interesting behavior
> 
> Hi all,
> 
> I've been playing around with the llvm optimizer and stumbled upon an
> interesting behavior with respect to the "instcombine" option. I tried to
> optimize the following small program (see also attachment small.ll) (only
> with "instcombine"):
> 
> clang -v
> clang version 4.0.0 (trunk 288238)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: /usr/local/bin
> Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5 Found
> candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5.4.0 Found
> candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/6 Found candidate
> GCC installation: /usr/lib/gcc/i686-linux-gnu/6.0.0 Found candidate GCC
> installation: /usr/lib/gcc/x86_64-linux-gnu/5 Found candidate GCC
> installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6 Found
> candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.0.0
> Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
> Candidate multilib: .;@m64
> Candidate multilib: 32;@m32
> Candidate multilib: x32;@mx32
> Selected multilib: .;@m64
> 
> 
> llvm-as small.ll -o small.bc
> opt -instcombine small.bc -o smallOpt.bc
> 
> 
> define i32 @main() {
>   %reg = alloca i32, i32 0
>   store i32 1, i32* %reg
>   %1 = getelementptr i32, i32* %reg, i32 0
>   %2 = bitcast i32* %1 to [4 x i8]*
>   %3 = getelementptr [4 x i8], [4 x i8]* %2, i32 0, i32 1
>   store i8 1, i8* %3
>   %4 = load i32, i32* %reg
>   ret i32 %4
> }
> 
> The program in pseudo code would/should simplified look something like
> this:
> 
> int main(){
>     reg = 1;
>     [reg+1] = 1; //Second byte of reg is set to 1
>     return reg;
> }
> 
> I assumed it would be optimized to - again pseudo code -:
> 
> int main(){
>     return 257;
> }
> 
> Instead, the program is optimized to(see also attachment
> smallOpt.ll/smallOpt.bc):
> 
> define i32 @main() {
>   ret i32 1
> }
> 
> So i verified my assumption by printing '%4' of the original program
> (smallPrint.ll/smallPrint.bc) and the output was 257.
> I also optimized smallPrint.ll/smallPrint.bc , again only with instcombine, but
> the output is changed to 1 (smallPrintOpt.bc).
> 
> I also tested it with clang 3.8.0.
> 
> What am i missing?
> 
> 
> Best regards,
> Stephan