[Bug middle-end/19912] GCC does not disclose parentheses

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From joseph at codesourcery dot com  2005-02-11 21:34 -------
Subject: Re:  New: GCC does not disclose parentheses

On Fri, 11 Feb 2005, l_belev at yahoo dot com wrote:

> GCC generates different codes for the both functions:
>   int f1(int k) { return k*k+k; }
>   int f2(int k) { return k*(k+1); }
> 
> also the generated code for, say '(k*k+k) + (k*(k+1))'
> calculates the two parts independently and then sums them,
> whereas the code for '(k*k+k) + (k*k+k)' just calculates
> k*k+k once and then doubles it.

Note that the first expression can involve undefined behavior when the 
second doesn't, so transformations between these forms would be invalid 
with -ftrapv and until we have internal overflow flags on expressions we 
could only transform in one direction without -fwrapv.

(Consider k == -46341 with 32-bit int.  The first expression involves 
signed integer overflow, undefined behavior in ISO C which needs a trap 
with -ftrapv, while the second doesn't.)



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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19912