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
smallPrintOpt.bc (868 Bytes)
smallPrint.bc (912 Bytes)
smallPrint.ll (486 Bytes)
smallOpt.ll (92 Bytes)
smallOpt.bc (736 Bytes)
small.bc (796 Bytes)
small.ll (273 Bytes)