[Bug target/43892] PowerPC suboptimal "add with carry" optimization

[segher at kernel dot crashing dot org]

------- Comment #14 from segher at kernel dot crashing dot org  2010-05-26 16:46 -------
(In reply to comment #13)
> > Please see -mcpu= .
> 
> Almost forgot, but how do I specify that at gcc build/configure ?

You can configure with --with-cpu= to set a default for -mcpu= .

> Also, I haven't seen any progress on this issue

You have no patience, now do you?

> even though it sounded that the
> initial fix was easy(add<mode>cc expander)

The fix will be a few thousand lines of patch.  Literally.

In order to fix this problem (and a whole host of way more important
missed optimisation opportunities) we need to expose the CA bit to
the compiler as an actual register.  Currently, whenever GCC uses the
carry bit it does so by having the consumer and producer in a canned
asm sequence; this is suboptimal for many reasons.

Fixing it properly will take a while.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43892