[Bug target/27827] [4.0/4.1 Regression] gcc 4 produces worse x87 code on all platforms than gcc 3

[paolo dot bonzini at lu dot unisi dot ch]

------- Comment #42 from paolo dot bonzini at lu dot unisi dot ch  2006-08-07 18:19 -------
Subject: Re:  [4.0/4.1 Regression] gcc 4 produces worse
 x87 code on all platforms than gcc 3


> We should get some idea by comparing gcc3 vs. your
> patched compiler on the various platforms, though other gcc3/4 changes will
> cloud the picture somewhat . . .
>   
That's why you should compare 4.2 before and after my patch, instead.
> If this kind of machine difference in optimality holds true for x87 as well, I
> assume a new peephole phase that looks for the scratch register could be called
> if the appropriate -march were thrown?
>   
Or you can disable the fmul[sl] instructions altogether.
> Speaking of -march issues, when I get a compiler build that gens your new code,
> I will pull the assembly trick to try it on the CoreDuo as well.  If the new
> code is worse, you can probably not call your present peephole if that -march
> is thrown?
>   
I'd find it very strange.  It is more likely that the Core Duo has a 
more powerful scheduler (maybe the micro-op fusion thing?) that does not 
dislike fmul[sl].


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27827