Question on missed insn combine optimization.

[Bernd Schmidt]

On 07/12/11 13:11, Georg-Johann Lay wrote:
> Not familiar with combine inerts, I'd like to know if
> it's low hanging fruit to teach insn combine to perform
> optimizations like the following.
> 
> Suppose following C code, int = HI
> 
> int y15;
> int x15;
> 
> void qmul8_xy (char c, int x, int y)
> {
>     y15 = y * c;
>     x15 = x * c;
> }
> 
> and that the target has an insn like
> 
> (set (reg:HI)
>      (mult:HI (sign_extend:HI (reg:QI))
>               (reg:HI)))
> 
> If there was just one multiplication, combine would
> combine the sign_extend into the special mul insn.
> (Provided costs are advantageous, of course)
> 
> However, with the code example from above, combine
> won't insert the sign_extend even if it was an
> improvement, i.e. the sign_extend does not disappear
> but the special mult is cheaper (or same cost) as the
> ordinary mult.
> 
> The original RTL reads:
> 
> == Stage 0 ==
> 
> (set (reg:HI 42)
>      (sign_extend:HI (reg:QI 24)))
> 
> (set (reg:HI 48)
>      (mult:HI (reg:HI 42)
>               (reg:HI 47)))
> 
> (set (reg:HI 49)
>      (mult:HI (reg:HI 42)
>               (reg:HI 46)))
> 
> == Stage 1 ==
> 
> Cheaper than Stage 0 but not performed
> 
> (set (reg:HI 42)
>      (sign_extend:HI (reg:QI 24)))
> 
> (set (reg:HI 48)
>      (mult:HI (sign_extend:HI (reg:QI 24))
>               (reg:HI 47)))
> 
> (set (reg:HI 49)
>      (mult:HI (reg:HI 42)
>               (reg:HI 46)))
> 
> == Stage 2 ==
> 
> Cheaper than Stage 0 and Stage 1, would be performed
> if we had Stage 1.
> 
> (set (reg:HI 48)
>      (mult:HI (sign_extend:HI (reg:QI 24))
>               (reg:HI 47)))
> 
> (set (reg:HI 49)
>      (mult:HI (sign_extend:HI (reg:QI 24))
>               (reg:HI 46)))

You could try the following, which allows 2->2 insn combinations
assuming the costs are right.


Bernd
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