String comparison

[certik]

Currently string comparison such as assert x = "abc" doesn't work properly yet, I think we should implement the operation in LPython in the runtime library and the frontend should insert the call to it.

[certik]

Here is how to do it:

def _lpython_str_equal(a: str, b: str) -> bool:
    if len(a) != len(b):
        return False
    i: i32
    for i in range(len(a)):
        if a[i] != b[i]:
            return False
    return True

def f():
    assert _lpython_str_equal("a", "a")
    assert not _lpython_str_equal("a2", "a")
    assert not _lpython_str_equal("a", "a123")
    assert not _lpython_str_equal("a23", "a24")
    assert _lpython_str_equal("a24", "a24")
    assert _lpython_str_equal("abcdefg", "abcdefg")
    assert not _lpython_str_equal("abcdef3", "abcdefg")

f()

Currently fails in the LLVM backend.

[certik]

With #198 the above snippet now works. All that remains is to now call it from the frontend when full string comparison is being done.

[namannimmo10]

I tried the following:

def main0():
  s1: str
  s1 = "abc"
  s2: str
  s2 = "ab"
  assert s1 == s2

main0()

And it worked. So it seems like it is only comparing the first character of the strings,

lpython/src/libasr/codegen/asr_to_llvm.cpp

Lines 2630 to 2641 in 1d7ba2e

	// TODO: For now we only compare the first character of the strings
	left = CreateLoad(left);
	right = CreateLoad(right);
	switch (x.m_op) {
	case (ASR::cmpopType::Eq) : {
	tmp = builder->CreateICmpEQ(left, right);
	break;
	}
	case (ASR::cmpopType::NotEq) : {
	tmp = builder->CreateICmpNE(left, right);
	break;
	}

[certik]

Yes, we have to fix it, I think the best way is for the front end to implement s1 == s2 by inserting lpython_str_compare(s1, s2). It can be implemented in LPython itself. This is one of those functions that are "intrinsic" and the LLVM backend can choose to implement it differently if there is some more efficient implementation depending on the string implementation in the backend. See https://github.com/lcompilers/lpython/wiki/Intrinsic-Functions-Design for more info.