optimization/10823: 3.3 - wrong code after first cse pass

optimization/10823: 3.3 - wrong code after first cse pass

[gawrilow]

>Number:         10823
>Category:       optimization
>Synopsis:       3.3 - wrong code after first cse pass
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Fri May 16 16:56:01 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Ewgenij Gawrilow
>Release:        3.3
>Organization:
>Environment:
RedHat 7.3 on an Athlon PC

../gcc-3.3/configure --enable-languages=c++ --enable-shared --with-cpu=athlon --enable-version-specific-runtime-libs --with-gxx-include-dir=/usr/site-local/share/include/c++/3.3 --prefix=/usr/site-local/share --exec-prefix=/usr/site-local
>Description:
The first cse pass garbles the class LV constructor code inlined into the fetch() function.  Compare the insn 405, 406 in the 08.null and 09.cse dumps.  The _start field of the target is copied from the _size field of the source, which is obviuosly wrong.

I hope, this time the test case is sufficiently short.  The original program that crashed due to this bug has over 80K lines after cpp.  Further reducing makes the bug disappear, especially removing the explicitly defined copy constructor of class Gen (if this helps...)

The parts embraced in #ifdef DEMO are useful only for the bug demonstration, e.g. in a test suite, and can be safely ignored when debugging.

BTW, as I analyzed the assembler output, some other optimization deficiences have been revealed. If you mean, I should submit them as separate bugs, or they are no bugs at all, please let me know. Here they are:

1. The placement operator new is not required to check his argument for being non-null. Neither the constructors are. Nevertheless, gcc emits a test and a branch.

2. Even if this test would be required, it should have been eliminated by the previous optimization pass (null pointer elimination), as the same pointer is used to access the 'exists' field prior to the test.  Or, alternatively, the null pointer elimination should be repeated after cse, if this failure is caused by the expressions looking too different at the beginning.

3. The constructor argument in the placement new expression is a temporary object returned by the inlined function get(). It would be much better for the run-time efficiency if the target address (area) would be passed directly to get(). Currently the temporary object is created separately and then copied into the area.  I think, the Standard allows to eliminate such temporaries together with copy constructor calls, doesn't it?
>How-To-Repeat:
Compile the attached program with additional -DDEMO option, then run it.  If compiled with -O1, it produces the correct output string '0 3 0 3'.  When compiled with -O2 or higher, the output will be '0 3 3 3'.
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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