Re: duplicate cases, was Alternative C compilers on x86_64 Linux?

On 06/09/16 03:15, Kaz Kylheku wrote:
> A duplicate condition in an if-else chain is unreachable code, which is
> probably a bug.


I note the "probably", but there are several ways this can happen
legitimately:


    -- Language translation. Suppose we start with [in some other language]
something like
IF letter = 'q' THEN ...
ELIF letter IN ['a', 'e', 'i', 'o', 'u'] THEN ...
ELIF letter IN ['a' : 'z'] THEN ...
FI
and this gets turned automatically into either an "if-else" chain or a
bunch of cases in C, where 'q' and the vowels are tested twice.


    -- Similar things can happen via macro expansion. What the coder sees
is often only loosely related to what later stages of compilation see!


    -- Debugging. Eg, you don't want to deal with 'q' now, there are other
bugs you want to catch first. So you write
if (letter == 'q') { error (19); }
else if ...
in front of the code, and the "old" case 'q' is now unreachable.


Of course, deciding what constructs deserve warnings is a very
difficult problem in general, perhaps esp in languages like C where the
"coder" is often an automaton rather than a human.


--
Andy Walker,
Nottingham.

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