common branch conditions - could optimize better

[Jeffrey A Law]

  In message <19991216113512.A31064@horac.ta.jcu.cz>you write:
  > On Thu, Dec 16, 1999 at 02:11:45AM -0700, Jeffrey A Law wrote:
  > >   In message <Pine.LNX.4.10.9910160739260.1293-100000@first>you write:
  > >   > Hello GCC people
  > >   > 
  > >   > Consider the following code:
  > >   > 
  > >   > void f1(void), f2(void), f3(void);
  > >   > void foo (int x, int y)
  > >   > {
  > >   >     if (x*x*x<y*y)
  > >   > 	f1();
  > >   >     else
  > >   > 	f2();
  > >   >     if (x*x*x<y*y)
  > >   > 	f3();
  > >   > }
  > >   > 
  > >   > Here the condition x*x*x<y*y could be tested only once.
  > >   > GCC 2.95.1 does not optimize this - see below the asm,
  > >   > produced with gcc -O2 -S on i586-linux.
  > >   > Even x*x*x and y*y are computed twice... what is that gcse thing then
  > ?
  > > It's got to be something goofy about the ia32 target, they're removed jus
  > t
  > > fine on my PA target.
  > 
  > It is because of gcse is unable to handle parallels used by x86 to model
  > flags. I've made some progress in this path (for cprop only for now),
  > but first part (to propagate from parallels) got refused (it is not issue
  > for x86 after my mov patterns cleanups anyway) and second part (to propagat
  > e
  > to parallels) wasn't reviewed yet.
No.  That is not the cause.  Go back to gcc-2.95 and try it like the report
mentions.  Remember gcc-2.95 did not have the new ia32 backend.


jeff