Loop invariant code motion optimization does not happen for the input code

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1

Dear all,

Greetings. I have been trying to have a loop invariant code motion on the LLVM IR (.ll) file which has been generated from the following .c file.

int main()
{
int a = 5, b = 20, c = 50, d = 1, x;
while(a < b)
{
x = d + 1;
x = d + 2;
c = c + 5;
a = a + 1;
}
}
Both the first two statements should qualify for loop invariant code motion. However, loop invariant code does not move out of the loop in the generated .ll file. Note that I have applied the following commands.

clang -S -emit-llvm prog1.c -o prog1.ll
opt -licm -S prog1.ll -o prog1-opt3.ll -opt-bisect-limit=300

Can you please help in this regard ?

Thanks and regards,
Sudakshina

2

Does your function in prog1.ll have the optnone attribute on it? If so try adding “-Xclang -disable-O0-optnone” to your clang command line

3

Thanks a lot for your help.