Gary Thomas: Code optimization bug in GCC-2.8 & EGCS

[Gary Thomas]

On 31-Jan-98 David Edelsohn wrote:
>>>>>> Gary Thomas writes:
> 
> Gary> I spent many days trying to understand the "why" of this.  I believe
> that
> Gary> in CSE, follow jumps only works properly if loops have been
> appropriately
> Gary> "marked" (which is what fails to happen in JUMP in this case).  The CSE
> then
> Gary> blindly optimizes away the first test (loop entry - above what should
> be
> Gary> LOOP_VTOP).  I found that turning off CSE follow jumps kept the problem
> Gary> from generating correct code at the expense of generating
> *non-optimal* code.
> 
> Gary> Correctly identified loops seem to work OK.  The failure here is that
> on the
> Gary> PowerPC with signed characters, the loop exit code is "just a little
> too big."
> 
>       If CSE does not handle unmarked loops properly then that is a bug
> in CSE.  For some sized loop, this always will fail. 
> 
> David

Agreed.  I was just trying to provide as much understanding as I have on
the problem.  I don't know enough of the intricacies of CSE to comment on why
it might be broken.

------------------------------------------------------------------------
Gary Thomas                              |
The Open Group / Research Institute      | "Fine wine is a necessity of
2 Avenue de Vignate                      |        life for me"
38610 Gieres - FRANCE                    |
+33 4 76 63 48 74                        |      Thomas Jefferson
email: g.thomas@opengroup.org            |
< http://www.opengroup.org/~gdt >          |
   ... opinions expressed here are mine  |
       and no one else would claim them! |
------------------------------------------------------------------------