Re: java/8204: gcj -O2 to native reorders certain instructions improperly.

[Casey Marshall <rsdio@metastatic.org>]

The following reply was made to PR java/8204; it has been noted by GNATS.

From: Casey Marshall <rsdio@metastatic.org>
To: bangerth@dealii.org,  gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: java/8204: gcj -O2 to native reorders certain instructions improperly.
Date: Sat, 22 Mar 2003 16:00:40 -0800

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 bangerth@dealii.org wrote:
 | Synopsis: gcj -O2 to native reorders certain instructions improperly.
 |
 | State-Changed-From-To: open->feedback
 | State-Changed-By: bangerth
 | State-Changed-When: Sat Mar 22 18:09:51 2003
 | State-Changed-Why:
 |     In your code, you are modifying i twice in the same line:
 |       (f(s.charAt(i++)) << 4) | (f(s.charAt(i++))))
 |     I don't know enough about Java, but in C/C++ this will invoke
 |     undefined behavior, since the standard doesn't prescribe
 |     which of the two function calls happen first, and with
 |     which value of i. Is this different in Java, i.e. does
 |     the Java standard give guarantees as to the order in which
 |     the sub-statements are executed?
 |
 
 I haven't gone through the JLS very carefully, but it does seem that a
 construction of this sort *should* produce the behavior I'd expect.
 Specifically, the JLS says that when evaluating expressions (chapter 15):
 
 1. The left operand of a binary operator is evaluated first.
 2. Respect is paid to operator precedence and parentheses.
 3. Operands are evaluated fully before operation.
 
 The problem appeared to be that when compiled with -O2 the right
 expression was evaluated first, rather than the left (at least this is
 what the result *looked* like). It would seem out of character for some
 behavior to be undefined in Java anyway ;)
 
 That said, this type of statement *is* unnecessary, and I can do without it.
 
 Cheers,
 
 - --
 Casey Marshall || rsdio@metastatic.org
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