Re: Global constant propagation

[Erik Corry <erik at arbat dot com>]

On Sun, Apr 04, 1999 at 05:47:47AM -0600, Jeffrey A Law wrote:
> 
>   In message <19990404133100.A3351@daimi.au.dk>you write:
>   > 
>   > Here is a small C fn:
>   > 
>   > int testfn(int a, int b)
>   > {
>   >    a = 1;
>   >    if(b == 1)
>   >    {
>   >      b = 2;
>   >    }
>   >    else
>   >    {
>   >      b = 3;
>   >    }
>   >    return a*b;
>   > }
> 
> If the if-then-else is confusing cprop, then that would be a bug.

It turns out it isn't.  The problem is that on MIPS the
multiplication is inside a PARALLEL rtl (due to the clobbers
of the HI and LO registers as far as I can see), which means
the code in gcse.c can't recognise it and doesn't propagate
the a=1 into it (see cprop_insn).  Looks like this:

(insn 34 51 36 (parallel[ 
            (set (reg:SI 85)
                (mult:SI (reg/v:SI 81)
                    (reg/v:SI 82)))
            (clobber (scratch:SI))
            (clobber (scratch:SI))
            (clobber (scratch:SI))
        ] ) 57 {mulsi3_r4000} (nil)
    (nil))

If the a=1 and the multiplication are in the same basic block
then it is spotted by the code in combine, which understands
parralels better (it seems to be doing a rather complete
constant propagation of its own, though not much across basic
blocks).

A solution might be to get gcse.c to recognise parallels that
consist only of a 'set' and some clobbers and treat them  like
'set's.  I'm not really into this enough to do that...

-- 
Erik Corry erik@arbat.com     Ceterum censeo, Microsoftem esse delendam!